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APPLICATION INFORMATIONBUCK CONTROLLER IMPLEMENTATION USING A80803
WITH SLEW-RATE CONTROL FOR 48 V AUTOMOTIVE LED
HEADLIGHT APPLICATIONS
By Vashist Bist and Shashank Wekhande, Systems Engineers
Allegro MicroSystems

ABSTRACTINTRODUCTION

The need for more-power-efficient systems in automotive
applications has triggered a trend-shift from the 12 V battery
to the 48 V battery. This shift enables hybridization with better
fuel economy and reduced emissions at low cost. In applications such as headlights, taillights POS/daytime running lights(DRLs), this shift necessitates high-voltage LED controllers tominimize the power losses that result from adapting the 48 V
source to 12 V applications.

The A80803 is a constant-current DC-DC switching controller
for high-power automotive lighting applications. The controller
is based on a programmable fixed-frequency, peak-currentmode control architecture and can be configured in multiple
different switching topologies to suit different application
requirements. The device includes both low-side and high-sidegate drivers to control the external power MOSFETs. Moreover,
two additional gate drivers are integrated to enable/disable
part of the LED string to simplify high-/low-beam applications.

This application note considers automotive high-power/
high-voltage applications where a smaller number of LEDs are
driven at a battery voltage of 48 V (with maximum operating
voltage at 56 V, supporting 70 V load dump) using the Allegro
multi-topology synchronous LED controller (A80803). A design
procedure is also detailed, including device behavior during
steady-state operation, transient analysis, slew-rate control invarious modes, and behavior with various protection features.

Diagnostics can be reported through SPI or via the two fault
pins. The SPI interface can also be used to control many configuration options of the A80803. Alternatively, these options
can be factory-programmed and stored in EEPROM to remove
the need for a local microcontroller.
LED brightness can be controlled by a pulse-width-modulated(PWM) signal on the EN/PWM pin or by an internal PWM sig-AN296294
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APPLICATION INFORMATION

nal configured via SPI. An internal driver controls a MOSFET inseries with the LED string to optimize dimming. The PWMOUT
driver strength is programmable via SPI to optimize LED current
during PWM transitions. LED current foldback is provided for
low-input voltage and thermal events.

High-beam or low-beam operation can be achieved byeither turning on all LEDs (six LEDs) in a string or turning on
only a few LEDs (four LEDs) and short-circuiting the rest. The
low-beam operation is achieved by turning on the bypassMOSFET (SWl) controlled with the LBG node. Due to thelimited response time of the power-stage controller, LEDs
may experience a large deviation in the current during high-/
low-beam transitions. It is important to minimize these LED
current deviations to eliminate any undesirable flicker. LED
current deviations during high/low-beam transitions can be
minimized by slowing the turn-on and turn-off rates of the
bypass MOSFET (SWlb). The slew-rate control is achieved
using the SLEW pin, the input of which is supplied by a differential amplifier (LED string voltage is input to the differential
amplifier) and two slew components (RSLEW and CSLEW) that
determine the rate of transition, as discussed in the Design
Procedure section.

The buck converter implementation with the A80803
device with slew-rate control is shown in Figure 1. The
buck converter is implemented using a low-side switch(SW1) and a free-wheeling synchronous switch (SW2). Two
PMOS switches are used for the PWM dimming (SWp) andhigh-beam/low-beam operation (SWlb). A slew-rate control
is implemented using a differential amplifier (Op1) which
provides a scaled output of the LED voltage (i.e., voltage
between anode and cathode).

SW2

HSG

VLED

C

+

- Co

Lsw

Vbat
R LED

Rln

SWp
Rp
A

LN

LP
8 V LDO

VIN
Cr

Control andConfiguration

SPI
Communication

CT

SWlb

PWMOUT

CLBG

LBS
LBG

GND
EN/PWM
DIMn
LBEAMnBINSEL
COMP
FSET
NTC
HBG

BOOT
SW
LSG
SP
HSG
CT

SCLK
MOSI
MISO
SSn

Regulators

VCC
VDRV
VREG

Diagnostics

FF1
FF2

LED-

R G_LBG
Cbt

R LSG

SW1

R SP

R SEN
HSG
CT
C
Rs4

CSLEW
SLEW

A80803

R SLEW

Op1

+
-

+
Cb
-

Rs2

Rs3

C

Rs1

A

Slew Control

Figure 1: High-voltage buck configuration using A80803.

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APPLICATION INFORMATIONOPERATION
The simplified power stage of a buck converter in continuous-conduction mode (CCM) is shown in Figure 2. The waveform of the buck converter in CCM is shown over a complete
switching cycle in Figure 3. Buck-converter operation duringswitch-on and switch-off is as follows:

Considering the input voltage (VIN), LED voltage (VLED,
output voltage), and switch turn-on and turn-off times (tONand tOFF, respectively), the volts-per-second balance acrossinductors (LSW) in continuous-conduction mode as:

Mode 1 (switch turn-on): During the switch (SW) turn-on
period (tON), energy is stored in the inductor (LSW) while supplying the LED current, as shown in Figure 2 (b). The inductor current (ILsw) increases, and the output voltage (VLED)
changes depending on the direction of the output-capacitor
current, as shown in Figure 3.

( VIN − VLED ) × tON = VLED × t OFF ,Mode 2 (switch turn-off): During the switch turn-off period(tOFF), the diode (DSW) conducts, and the inductor (LSW) provides the required energy to the LEDs to maintain a continuous current, as shown in Figure 2 (c). The inductor current(ILsw) reduces, as shown in Figure 3. The output capacitor(Cout) helps in the reduction of ripple current.

Equation 1:

which can be rearranged as:
Equation 2:
VIN × t ON = V LED × ( t ON + tOFF ).
On- and off-time can be replaced as a function of duty (D) as:

Equation 3:

VLED =t ON
× V = D × VIN( t ON + t OFF ) IN.

This is a typical buck-converter equation showing how output
voltage (VLED) is linked with the input voltage (VIN).

DSW
Cout
+

VLED

LSW

+
V
- IN

SW

VLSG

(a)

DSW

ILsw

Cout
+ VLED

ΔILsw

LSW

ΔIsw

Isw

+
V
- IN

SW

ΔIDsw

(b)

IDsw

DSW
Cout
+ VLED

+
V
- IN

ICout
LSW
SW

VLED

(c)

Figure 2: Buck converter: (a) simplified power stage, (b) operation during theswitch-on period, and (c) operation during the switch-off period.

∆Vo
Turn-onTurn-off

1/fsw

Figure 3: Waveforms of the operation of the buck converter in continuousconduction mode (CCM).

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APPLICATION INFORMATIONDESIGN PROCEDURE
Power Components

Slew-Rate Control

Considering the on-time period as shown in in Figure 2 (b),the voltage across inductor, VLon, is given as:

This section presents the design procedure of the slew-rate
control circuitry for the A80803.

Equation 4:

Due to buck configuration, the shorting of MOSFET is to be
carried out by the LBG MOSFET because this configuration
requires a floating gate driver or a PMOS. For further details
about the slew-rate control, refer to the A80803 datasheet.

d
VLon = L( I LSW ) = (VIN − VLED )dtON.
Substitution of Equation 3 in Equation 4 results in:Equation 5:
VLon = LSW⇒ LSWd( I LSW ) = (VIN − D × VIN )dtON∆I LSW= VIN (1 − D )Dt.

Using Equation 5, the value of the inductor is expressed as:
Equation 6:

LSW =VIN D (1 − D )∆I LSW f SW=

VLED (1 − D )∆I LSW f SW.

Moreover, the output capacitor is designed as:
Equation 7:

COUT =∆I LSW
8 × f SW × ∆VCout ,where, ΔILSW is the peak-to-peak ripple current in the inductor, ΔVCout is the peak-to-peak voltage ripple across theoutput capacitor, and fsw is the PWM switching frequency.
Both the inductor (LSW) and the output capacitor (COUT) must
be designed at the maximum input voltage.

The operating input voltage of the A80803 driver is limited
to 37 V; it cannot be powered directly from the 48 V battery.
Moreover, the primary condition for the LBG MOSFET to slew
requires the input voltage (VIN) to be lower than the voltage at
the CT terminal (VCT). [1]The voltage at the CT terminal (VCT) is calculated as the difference between the minimum supply voltage and the voltage
across the high-beam LED string:
Equation 8:
VCT = VIN – VLEDHB,
where VLEDHB is the voltage across the LED string from anodeto CT node.
Hence, to power the input-voltage (VIN) pin of the A80803—
which maintains the input voltage (VIN) of the A80803 suchthat it is always lower than the voltage at VCT for the slew-rate
control of the LBG MOSFET, and such that it meets the maximum supply-voltage requirement for the A80803 device—a
low-dropout regulator (LDO) is connected.
The slew-rate circuitry of the buck converter, which comprises a differential amplifier and a differentiator, is shownin Figure 4. Because the voltage at the CT terminal (VCT) is
almost constant during transitions between low- and-highbeam operation, a differential amplifier is able to provide an
inverted, scaled differential output of the LED string voltage(from anode to cathode). Therefore, a differential amplifier
is used to provide feedback of the LED voltage that needs
to be slewed. Finally, as a feedback signal to the A80803
device, the RC differentiator provides the rate-transitioned
output of the SLEW pin.
Rs4

SLEW
Pin

CSLEW
RSLEW

VD

Differentiator

+
-

VSLEW

Op1

IRs4
V-

IC
Rs3

(Cathode)
VC

Rs1

VA
(Anode)

V+

Rs2

IA

Differential
AmplifierFigure 4: Slew-rate control circuitry

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The design of various components of the differential amplifier
is as follows.

The values of Rs1 = Rs3 = 100 kΩ, and Rs2 = Rs4 = 15 kΩ canthen be substituted into Equation 15 to produce:

The voltage at the noninverting input (V+) is expressed as:

Equation 16:
VD = Ad × (VA − VC ) = 0.15 × (VA − VC ) .

Equation 9:

 RS 2 
V+ = VA  RS 1 + RS 2  .
The amplifier output (VD(V–)) in the inverting-amplifier configuration, when VA = 0,
Equation 10:

R 
VD (V − ) = −  S 4  VC
 RS 3  .
Considering the amplifier output (VD(V+)) in the noninverting
amplifier configuration, when VC = 0:
Equation 11:

 R 
VD (V + ) = 1 + S 4  V+
 RS 3  .
The value of (V+) obtained in Equation 11 can then be substituted into Equation 4 to obtain:
Equation 12: R  RS 2 
VD (V + ) = 1 + S 4 
 VA
 RS 3  RS 1 + RS 2  .
The superposition theorem can then be used to express thedifferential amplifier output as:
Equation 13:

VD = VD (V + ) + VD (V − ) .
Hence, Equation 10 and Equation 11 can be substituted into
Equation 13 to produce:
Equation 14:

 R  RS 2  RS 4 
VD = 1 + S 4 
 VA −  VC
 RS 3  RS 1 + RS 2  RS 3  .

Considering six LEDs (~19.8 V typical LED voltage) in thehigh-beam operation and four LEDs (~13.2 V typical LED voltage) in the low-beam operation, with anode voltage as 48 V,
the cathode voltage is calculated as 28.2 V and 34.8 V for
high-beam and low-beam operation, respectively.
Considering, the case such that Rs1 = Rs3 and Rs2 = Rs4, Equation 14 can be rearranged as:
Equation 15:

R 
VD =  S 4  (VA − VC ) = Ad × (VA − VC ) , RS 3 
where Ad is the gain of the differential amplifier.

Using Equation 16, the gain of the differential amplifier (Ad) is
calculated as 0.15.
Also using Equation 16, the differential voltage in high-beam(VD_HB) and low-beam (VD_LB) operation is calculated as:
Equation 17:
VD _ HB = 0.15 × ( 48 − 28.2 ) = 2.97 V ; andEquation 18:

VD _ LB = 0.15 × ( 48 − 34.8 ) = 1.98V .

As shown in Equation 17 and Equation 18, the output voltageof the differential amplifier is higher than the SLEW pin threshold of 250 mV and there is sufficient margin for LED voltage
variation. Also, the output voltage swing (i.e., VD_HB – VD_LB)
is approximately 1 V, which is sufficient for effective slew-rate
detection. For different system configurations, select thedifferential amplifier gain (AD) such that VD(V+) is less than the
supply voltage to the differential comparator and maximizes
the differential output voltage swing.
Moreover, the SLEW pin is designed such that the rate ofslew control is defined by the RC differentiator as:
Equation 19:
250 ( mV )dV(V / ms ) =dt
RSLEW (k Ω ) ×CSLEW (nF ) ,where RSLEW and CSLEW are components shown in Figure 4
and 250 mV is the internal voltage threshold of the SLEW pin.
Because LED voltage is fed through a differential amplifier,the differential amplifier gain is used to calculate the actual
slew as:Equation 20:
250 ( mV )dVLED
(V / ms ) =AD × RSLEW (k Ω )× CSLEW (nF ) .
dt
Using Equation 20, the slew rate is calculated for RSLEW of
5 kΩ and CSLEW of 47 nF and 100 nF as:
Equation 21:
dVLED
250
= 7.09 V / ms
( @ 47nF ) =dt
0.15 × 5 × 47
; andEquation 22:
dVLED
250
= 3.33V / ms
( @100nF ) =dt
0.15 × 5 × 100.
Hence, by adjusting the component values of the differentiator, the slew rate can be adjusted.
Design and selection of other components and configurationof various features is available in the datasheet. [1]955 PERIMETER ROAD • MANCHESTER, NH 03103 • USA+1-603-626-2300 • FAX: +1-603-641-5336 • ALLEGROMICRO.COM

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APPLICATION INFORMATIONSTEADY-STATE OPERATIONThis section presents the steady-state operation of the buckcontroller implemented using the A80803 device.
The efficiency and LED current plots with respect to the input
voltage are shown in Figure 5 and Figure 6, respectively. The
system depicts high efficiency and good LED current regulation at varying input voltage.
88

High Beam
Low Beam

86
Efficiency (%)

The steady-state operation of the buck controller using theA80803 device in high-beam operation at the input voltageof 48 V is shown in Figure 7. As shown, the LED voltage—
which is the difference between the anode voltage and thecathode voltage—is measured to be approximately 17.43 V
for six LEDs. The A80803 device operates at a switchingfrequency ~350 kHz. Similarly, the operation of the buckcontroller in high-beam operation at the input voltage of 40 V
and 56 V is shown in Figure 8 and Figure 9, respectively.
V ANODE84

V CATHODE

82

I LED

VANODE = 47.10 V

80
78
76

SW Node

40

42

44

46
48
50
Input Voltage (V)

52

54

56

Figure 5: Efficiency vs. input voltage for high-beam and low-beam operationof a buck controller using the A80803 device.

Figure 7: Steady-state operating waveform for six LEDs (high-beam operation)at an input voltage of 48 V.

V ANODE
VLED = 17.34 V

900
898
LED Current (mA)

VLED = 17.43 V

V CATHODE
I LED

896

VANODE = 39.13 V

894
892
SW Node890
888
High Beam
Low Beam

886
884

40

42

44

46
48
50
52
Input Voltage (V)54

Figure 8: Steady-state operating waveform for six LEDs (high-beam operation)at an input voltage of 40 V.

56

VLED = 17.38 V

V ANODE

Figure 6: LED current vs. input voltage for high-beam and low-beam operation of a buck controller using the A80803 device.

V CATHODEI LED

VANODE = 55.09 V

SW NodeFigure 9: Steady-state operating waveform for six LEDs (low-beam operation)at an input voltage of 56 V.

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APPLICATION INFORMATIONThe steady-state operation of the buck controller using theA80803 device in low-beam operation is shown in Figure 10,
Figure 11, and Figure 12, at the input voltage of 48 V, 40 V
and 56 V, respectively. As shown in these figures, the LED
voltage drop of ~12 V (typical) is observed in low-beam
operation for four LEDs in series, and the cathode voltage
is adjusted by the buck controller to meet the LED voltage
demand.
V ANODE

VLED = 11.96 V

V CATHODE

VANODE = 47.15 V

I LED

Dimming
This section presents the steady-state operation of the buckcontroller during LED dimming using the internal PWM generator of the A80803 device.
The variation of the output power versus the duty ratio of
the PWMOUT pin for high-beam and low-beam operation isshown in Figure 13. As shown, output power varies linearlywith duty. The variation of the efficiency of the overall systemwith the variation of the duty of the PWMOUT pin is shown in
Figure 14. Efficiency increases at the higher duty ratio.
Operation of the A80803 buck controller is shown in
Figure 15 and Figure 16 at a PWM duty of 70% and 30%,
respectively, with at a dimming frequency if 250 Hz.
16
14

SW NodeFigure 10: Steady-state operating waveform for four LEDs (low-beam operation) at an input voltage of 48 V.

V ANODE
V CATHODEI LED

VLED = 11.88 V

Output Power (W)

12
10
8
6
4
High Beam
Low Beam

20

VANODE = 39.20 V

20

30

40

50
60
70
80
PWMOUT Duty (%)

90

100

Figure 13: Output power vs. PWM duty for high-beam and low-beam operation of a buck controller using the A80803 device.

90

SW Node85

V ANODE
V CATHODE

VLED = 11.84 V

I LED

VANODE = 55.15 V

Efficiency (%)

80
Figure 11: Steady-state operating waveform for four LEDs (low-beam operation) at an input voltage of 40 V.

75
70
65
60
50

SW NodeHigh Beam
Low Beam

55
20

30

40

50
60
70
80
PWMOUT Duty (%)

90

100

Figure 14: Efficiency vs. PWM duty for high-beam and low-beam operation of
a buck controller using the A80803 device.

Figure 12: Steady-state operating waveform for four LEDs (low-beam operation) at an input voltage of 56 V.

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APPLICATION INFORMATION

Thermals
Thermal images of the buck-controller board at a rated
input voltage of 48 V driving six LEDs is shown in Figure 17
and driving four LEDs is shown in Figure 18. As shown,the maximum temperature on the board is approximately
67.9°C/63.8°C at room temperature with high-beam/lowbeam operation.

LDO

70% PWM

V ANODE250 Hz PWM

Switching
MOSFETsV CATHODE

A80803I LED

PWMOUT
MOSFET

Figure 15: Dimming operation at 70% PWM duty for six LEDs (high-beam
operation) at an input voltage of 48 V.

Figure 17: Thermal performance of high-beam operation (six LEDs) with a 48 V
input at room temperature.

LDO

30% PWM

V ANODE250 Hz PWM

Switching
MOSFETsV CATHODE

A80803I LED

PWMOUT
MOSFET
Figure 16: Dimming operation at 30% PWM duty for six LEDs (high-beam
operation) at an input voltage of 48 V.

Figure 18: Thermal performance for high-beam operation (four LEDs) with a
48 V input at room temperature.

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APPLICATION INFORMATION

TRANSIENT OPERATION
Slew Control
This section presents operation of the A80803 buck controller with slew-rate control during the high-beam to low-beamtransition and the low-beam to high-beam transition.
As previously discussed, the components RSLEW and CSLEW
on the SLEW pin determine the rate of slew control of the
LBG MOSFET, which controls the high-beam/low-beam
operation. Operation of the A80803 buck controller with
CSLEW of 47 nF and 100 nF (RSLEW fixed at 5 kΩ) during thetransition from high-beam to low-beam operation is shown in
Figure 19 and Figure 20, respectively. As shown, the cathodevoltage increases gradually with a slew time of ~900 µs and1800 µs for a cathode voltage rise of ~6.6 V. This corresponds to measured slew rates of 7.33 V/ms and 3.66 V/msfor the 47 nF and 100 nF capacitors (CSLEW), per designed
values, as shown in Equation 21 and Equation 22.
Similarly, operation of the A80803 buck controller with
CSLEW of 47 nF and 100 nF (RSLEW fixed at 5 kΩ) during thetransition from low-beam operation to high-beam operationis shown in Figure 21 and Figure 22, respectively. As shown,
the slew rate achieved closely matches the designed values
with good slew-rate control of the cathode voltage.

Operation of the slew-rate control in Figure 23 shows thevariation of the cathode voltage (pink), voltage at the SLEW
pin (blue), and gate voltage of the slew (SWlb) MOSFET connected to the LBG node (green) with high-beam to low-beamtransition signal (LBEAMn, yellow). As shown, once the SLEW
pin threshold of 250 mV is achieved, the controller starts
to operate in the slew-control mode. In this mode, the gateof the slew control (SWlb) operates in the Miller region, andthe cathode voltage slews during this period. This timing iseffectively controlled by the values of RSLEW and CSLEW.
V ANODE
V CATHODE

Slew Time ~900 µs
Peak-to-Peak
Transient ~200 mA

LBEAMnFigure 21: Transient behavior during low-beam to high-beam transition with
CSLEW as 47 nF.

V ANODE
V CATHODE

V ANODE
V CATHODE

Slew Time ~900 µs

I LED

I LED

I LED
~
Peak-to-Peak
Transient ~200 mA

Peak-to-Peak
Transient ~220 mA

Slew Time ~1800 µs

LBEAMnLBEAMnFigure 19: Transient behavior during the transition from high-beam operation
to low-beam operation, with CSLEW of 47 nF.

Figure 22: Transient behavior during the transition from low-beam operation
to high-beam operation, with CSLEW of 100 nF.

V ANODE
V CATHODE

250 mV Threshold
VDS Slew inMiller Region

I LED
Peak-to-Peak
Transient ~180 mA

Slew Time ~1800 µs

V CATHODESLEW_Pin
LBG

LBEAMnFigure 20: Transient behavior during the transition from high-beam operation
to low-beam operation, with CSLEW of 100 nF.

LBEAMnFigure 23: Transient behavior showing intermediate signals during the transition from high-beam operation to low-beam operation (CSLEW = 100 nF).

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APPLICATION INFORMATION

Simple Slew Control
This section presents operation of the A80803 buck controller with simple slew-rate control during the transitions fromhigh-beam to low-beam operation and from low-beam tohigh-beam operation.
The circuit configuration of the A80803 buck controller withsimple slew-rate control is shown in Figure 24. As shown, todisable slew-rate control, components on the SLEW pin are
removed, and the SLEW pin is connected directly to ground.
Moreover, for an additional cost benefit when there is norequirement for configuration of slew-rate control, the LDO
regulator and the differential amplifier can also be eliminatedin the circuitry.
SW2

HSG

- Co

+

+
Cb
-

Rln

8 V LDO

VIN
Cr

Control andConfigurationCT

PWMOUT

SWlb

V ANODE
V CATHODE

C
Lsw

I LED

SWp
Rp
A

LN

LP

Slew Time~ 500 µs

VLED

Vbat
R LED

Transient operation of the A80803 buck controller withsimple slew-rate control for the high-beam to low-beamtransition and the low-beam to high-beam transition is shownin Figure 25 and Figure 26, respectively. The period of overshoot/undershoot of LED current increases from <100 µs
typically (Figure 19 through Figure 22) to >400 µs (Figure 25
and Figure 26). Moreover, this undershoot/overshoot period
is highly dependent on various external factors, such as theambient temperature, which can lead to a longer durationof overshoot/undershoot. The extended duration of overshoot/undershoot of LED current can cause a flicker in the
LED operation; this flicker is not present in the configuration
with slew-rate control.

Current Overshoot~ 22%

CLBG

LBEAMn

LBS
LBG

GND
EN/PWM
DIMn
LBEAMnBINSEL
COMP
FSET
NTC
HBG

SPI
Communication

SCLK
MOSI
MISO
SSn

Regulators

VCC
VDRV
VREG

Diagnostics

FF1
FF2

BOOT
SW
LSG
SP
HSG
CT
LED-

RG_LBG
Cbt

Figure 25: Transient behavior during the transition from high-beam operation
to low-beam operation, with simple slew control.

R LSG

SW1

R SP

R SEN

Slew Time~ 500 µs

HSG
CT
C

V ANODE
V CATHODEI LED

Current Dip ~ 23%

LBEAMnSLEW

A80803Figure 24: High-voltage buck configuration using A80803 with simple slewrate control.

Figure 26: Transient behavior during transition from low-beam operation tohigh-beam operation, with simple slew control.

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APPLICATION INFORMATIONPROTECTION
The A80803 is a highly configurable device with numerous
protection schemes. However, this section focusses on how
the A80803 handles a single LED short, complete LED string
short, and LED open fault during high-beam and low-beam
operation.

Single-LED Short
A fault for a single-LED short is detected by two undervoltage comparators—one for each mode of operation (highbeam and low-beam). Comparator levels are programmableaccording to the number of LEDs. For this application, theundervoltage comparator settings are:• High-Beam undervoltage (HB_OUT_UV) comparator
between LP and LED– node (set at 16.8 V)

Protection against the effects of a single-LED short duringhigh-beam and low-beam operation is shown in Figure 27
and Figure 28, respectively. When a single LED is shorted:• In high-beam operation, the typical LED voltage
reduces from ~18 V to ~15 V. This trips the high-beam
undervoltage (HB_OUT_UV) comparator, which has athreshold of 16.8 V.
• In low-beam operation, the typical LED voltage reducesfrom ~12 V to ~9 V. This trips the low-beam undervoltage(LB_OUT_UV) comparator, which has a threshold of
10.4 V.
As shown in these figures, reporting of high-beam andlow-beam undervoltage faults is configured on the FF2 line,
which shows a high-to-low transition after a minimum configured fault delay of 50 ms.

• Low-Beam undervoltage (LB_OUT_UV) comparator
between LP and CT (set at 10.4 V)

VLED ~ 15 V

VLED ~ 18 V

VLED ~ 9 V

VLED ~ 12 V

V ANODEV ANODE
V CATHODE

V CATHODE
1-LED Short

50 ms Fault to FF2 Delay

I LED

1-LED Short

FF2

Figure 27: Device protection during a single-LED short during high-beam
operation.

50 ms Fault to FF2 Delay

I LED
FF2

Figure 28: Device protection during a single-LED short during low-beam
operation.

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LED-String Short

LED Open

A short of an LED string is detected if the voltage between
the LP and CT terminals drops below 1 V while the PWM
MOSFET is in the on state. As soon as this fault is detected,
the low-side gate (LSG), high-side gate (HSG), and PWM
MOSFET switch to the off state. The device restarts after a hiccup shutdown period (tHIC) of 10 ms. If the fault is removed,
the device operates in a nominal way; else, the device entersa low-side current limit operation and waits for 64 switchingcycles to elapse before turning off the MOSFET for the hiccup shutdown period.

An open LED fault is detected by two overvoltage comparators—one for each mode of operation (high-beam andlow-beam). Comparator levels are programmable according
to the number of LEDs. For this application, the overvoltage
comparator settings are:• High-Beam overvoltage (HB_OUT_OV) comparator
between the LP and LED– nodes (set at 23.6 V)• Low-Beam overvoltage (LB_OUT_OV) comparator
between the LP and CT nodes (set at 15.2 V)

Protection against the effects of an LED-string short in
high-beam and low-beam operation is shown in Figure 29
and Figure 30, respectively. As shown, as soon as the fault
occurs, the device waits for the hiccup shutdown period toelapse before entering the low-side current-limit operation
spanning 64 switching cycles. The fault pin transitions fromhigh to low after a minimum configurable delay of 50 ms.

V ANODEThe LED-open scenario in high-beam operation is shown in
Figure 31. As shown, cathode voltage drops suddenly, whichtrips the high-beam overvoltage comparator (HB_OUT_OV)
present between the LP and LED– nodes. After a minimum
configurable fault delay of 50 ms, LED current drops to zeroand a fault is asserted. In this operation, the device shows an
open LED, output overvoltage, and high-beam fault.
Similarly, the A80803 buck controller during low-beam
operation is shown in Figure 32. The low-beam operation
overvoltage comparator (LB_OUT_OV) trips due to voltage
overshoot between the LP and CT nodes. The LED current
reduces to zero and a fault is asserted on the fault pin after adelay of 50 ms.

64 Switching
Cycle

V ANODE

VLED ~ 18 V

V CATHODE

V CATHODE
I LED

I LED
FF2

10 ms Hiccup Period
50 ms Fault to FF2 Delay

Figure 29: Device protection from the effects of a complete LED-string shortduring high-beam operation.

50 ms Fault to FF2 Delay

FF2

Figure 31: Device protection during LED open (between the anode and the CTnode) during high-beam operation.

V ANODE

VLED ~ 12 V

V ANODE

V CATHODE

64 Switching
CyclesV CATHODE

I LED

I LED
FF2

10 ms Hiccup Period
50 ms Fault to FF2 Delay

Figure 30: Device protection from the effects of a complete LED-string shortduring low-beam operation.

50 ms Fault to FF2 Delay

FF2

Figure 32: Device protection during LED open (between the anode and the CTnode) during low-beam operation.

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APPLICATION INFORMATIONEXPERIMENTAL SETUP DETAILSThe developed PCB for the A80803 buck controller showing
the pads where various components are placed as shown in
Figure 33. This design can be used for automotive headlightapplications where power is sourced directly from a 48 V
power supply and can be extended to similar applications,such as taillights, stopping lights, and daylight runninglights (DRL)/position lights. The circuit is configured in buck
topology to operate at a 48 V input voltage range with input
voltage variation from 40 V to 56 V.

The developed experimental board to drive six serial LEDs inhigh-beam operation and four serial LEDs in low-beam operation using an A80803 buck controller is shown in Figure 33.
Various components of the board—such as the switching
inductor, switching MOSFETs, LBG and PWMOUT PMOS,
differential amplifiers, LDO, programming connector, andvarious input/output terminals—are highlighted in Figure 34.

LBG
PMOS
Inductor

Differential
AmplifierOutputTerminals

A80803
DevicePWMOUT
PMOS
Swicthing
MOSFET’s

Programming
Connector
8V LDO

Input
Terminals

Figure 33: Developed PCB of the A80803 buck controller showing the component pads.

Figure 34: Developed experimental board of the A80803 buck controller
showing various components.

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APPLICATION INFORMATIONSCHEMATIC AND BILL OF MATERIALSThe developed schematic for the high-voltage buck controller driving six LEDs in high-beam and four LEDs in low-beamwith ~1000 mA LED current is shown in Figure 35. The schematic includes a complete circuit of the transition from highbeam operation to low-beam operation with power supply
from a 48 V battery input voltage. For smooth transitionsCo

A CT C
HSG

6

24.9k

10

SW2

11
12

Dz2
5v

10k
Rf2

10k

P1
SPI Connector

Rd2
10k

13
21

VREG
Re2
10k

10k

Rf1

Rd1

1
2
3
4
5

14
15
16
17
5

GND1

GND2

1

2

3

32
BOOT

SW
HSG

VREG
VCC

LSG

BINSELSP

LBEAMnEN/PWMDIMn

PWMOUT

A80803

LP

FFn1

LN

FFn2

CT

SCLK

SLEW

MOSI

LBG

MISO

LBS

SSn
GND
PAD

8

0

VDRV

U1
A80803

LED-

Rz1
45.3R
Cp1
33pF

Cz1
820nF

R40

30

Rn2
2.21k

HSG

10R

D30
1

Q3
R30

2

R31
29

10R
Rsn3
0R12

787

PWM

28

Rs3

LP

27

Vlow

LN

25

Cs1
47nF

CT

26
23
22

Rsn4
0R12

LBG

Rs2

0E

Rs1
5k

C

U2
TL081PW
26

Rs4 100k3

Rs5 100k

LBS

24

15k

C
A

Cam
100nF, 25V

GND

Rs6
15k

GND
GND

Rn1
Rfs1
100k

D40

31

7

VCC

4

10nF

C20

HBG

2.2uF

VREG

7

Re1 10k
Dz1
5v

4.7uF

LBG

20

Rb2
Rlb 10k

3

2.2uF

VLOW

NT C

Cr6

Rb1
10k

4.7uF, 25V

Cbt
0.1uF

19

Cr5

Cr3

9

ZXTR2008Z

Cr4

VCC

Vlow
Cr2

Q2

LBS

R21
0E

VIN

1

100nF, 100V

D3

3

Vout

VBAT

LN

47nF

FSET

VIN

GND

2
Cr1

Csn

Q4

33uH, 2.0A

4

LP

L1
C

CT
Rln
150R

VR1

VBAT

0R39
PWM

D2

COM P

3

Rsn2

A

18

2

GND

0R39
Q1

1

HIGH

Rsn1

100nF, 25V

X2

4.7uF, 50V

X1

D1

LOW

A list of the complete bill of materials (BOM) for the developed board is provided in Table 1, which details two configurations of the buck controller—one with slew-rate control
and the other with the simple slew option.

Cb1
4.7uF, 100V

Cb2
4.7uF, 100V

Cb3
100nF, 100V

Cb4
100pF, 100V

VBAT

between high-beam and low-beam operation, additional
circuitry for the slew-rate control is provided.

2.21k

VREG

Figure 35: Schematic of the A80803 buck controller

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Table 1: Bill of Materials for A80803 Buck Controller

Input/Output Power
Connection

Regulator(Board and Device)

Power-Switching

Slew Components

A80803Components

Comment

Slew Control(Op-Amp)Simple Slew Option1210

4.7 µF, 100 V

4

4

0805

100 nF, 100 V

1

1

Cb4

0603

100 pF, 100 V

11

D1, D2sma

100 V, 3 A

2

2

X1, X2BLOCK 3x

CON_200P

2

2

VR1

SOT89 LDOZXTR2008Z

1

1

Designator

Footprint

Cb1, Cb2, Co1, Co2
Cb3

Cbt

0603

100 nF, 100 V

1

1

Cr1

0603

100 nF, 100 V

1

1

Cr2

0603

4.7 µF, 25 V

1

1

Cr3

0603

100 nF, 25 V

1

1

Cr4, Cr60603

2.2 µF, 25 V

2

2

Cr5

0603

4.7 µF, 25 V11

Rsn1, Rsn2

1206/0805

390 mΩ

2

2

Csn

0603

47 nF, 25 V

1

1

Rln

0603

150 Ω

1

1

Q1, Q2

SOT89_PMOS

PCP1302, P-Ch

2

2

R21

0603

2.4 kΩ

1

1

C20

0603

4.7 nF, 25 V

1

1

L1

SM_13.8x12.5

33 µH, 2.0 A

1

1

Q3, Q4

LFPAK56_2

BUK9Y65-N-ch22

R30, R40

0603

10 Ω

1

1

D30, D40

SOD123W

100 V, 1 A

2

2

R31

0603

787 Ω

1

1

Rsn3, Rsn4

1206/0805

120 mΩ

2

2

Rs1

0603

5 kΩ/0 Ω

1 (5 kΩ)

1 (0 Ω)

Cs1

0603

47 nF, 25 V

1

DNP

Rs2

0603

0Ω

1

DNP

Cam

0603

100 nF, 25 V

1

DNP

Rs3, Rs6

0603

15 kΩ

2

DNP

Rs4, Rs50603

100 kΩ

2

DNP

U2

PKG_SOIC8

TL081PW

1

DNP

U1

MLPQ32_5x5

A808031

1

Rb1, Rlb, Rd1, Rd2,
Rf1, Rf2, Re1, Re2

0603

10 kΩ

8

8

Rb2

0603

24.9 kΩ1

1

D3

SOD123W

100 V, 3 A

1

1

Dz1, Dz2

SOD123/X.85

5 V Zener

2

2

SW_En

SW DIP 1POS

SW-SPST

1

1

Cp1

0603

33 pF, 25 V

1

1

Cz1

0603

820 nF, 25 V

1

1

Rz1

0603

45.3 Ω

1

1

Rfs1

0603

100 kΩ

1

1

Rn1, Rn20603

2.21 kΩ2

2

P1

HDR1X5Header 5

1

1

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APPLICATION INFORMATIONCONCLUSIONREFERENCES

This application note has presented an implementation ofa high-input-voltage (48 V) buck controller for automotive
applications such as headlights, tail-stop lights, and position lights/DRLs. The slew-rate control feature of the device
has been explored in detail. The highly integrated A80803
device has numerous protection features, and this report has
detailed:[1] A80803, Multi-Topology LED Driver with PWM Dimmingand Seamless High/Low Intensity Modes, Datasheet,
Allegro MicroSystems, June 2021. https://www.allegromicro.com/en/products/regulate/led-drivers/leddrivers-for-lighting/a80803• Implementation of a buck controller with slew-rate
control.
• Device operation and design configuration of the slewrate components.
• Differences in transient behaviors during high-beam to
low-beam and low-beam to high-beam transitions, withand without slew-rate control.
• Device protection during various fault scenarios.
• Evaluation board schematics and bill of material (BOM).

ACKNOWLEDGEMENT
The authors would like to thank Nai-Chi Lee for continuous
evaluation and provision of insight that greatly improved thisapplication note.

[2] A80803 Evaluation Board and Programming GUI, User
Manual, Allegro MicroSystems, February 2022. https://www.allegromicro.com/-/media/files/applicationnotes/an296281-auto-led-headlamp-applications.
pdf?sc_lang=en[3] LED Headlamps for Mainstream Vehicles: High/Low
Beam LED Controller IC Solves Common Design Challenges, Allegro MicroSystems, October 2021. https://www.allegromicro.com/-/media/files/brochures/
am035-led-headlamp-controller-ics.pdf?sc_lang=en&ha
sh=9D73B0DCFBF2AAA0EFE13A1EF7F9EC7B[4] Nai-Chi Lee and Vijay Mangtani, Apparatus and Methods for LED Control, US Patent 9,781,789 B1, issued
October 3, 2017. https://patents.google.com/patent/
US9781789B1/en

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Revision History
NumberDate

Description

Responsibility–

September 19, 2023

Initial release

V. Bist, S. Wekhande

Copyright 2023, Allegro MicroSystems.

The information contained in this document does not constitute any representation, warranty, assurance, guaranty, or inducement by Allegro to the customer with respect to the subject matter of this document. The information being provided does notguarantee that a process based on this information will be reliable, or that Allegro has explored all of the possible failure modes.
It is the customer’s responsibility to do sufficient qualification testing of the final product to ensure that it is reliable and meetsall design requirements.
Copies of this document are considered uncontrolled documents.

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