AN296288
MCO-0001416

APPLICATION INFORMATIONAN EFFICIENT VOLTAGE-OPTIMIZED LED DRIVER (A80804)
WITH BUCK-BOOST CONTROLLER (A4450)
By Shashank Wekhande, Vashist Bist, and Sarang Gadgil, Application Engineers
Allegro MicroSystems

INTRODUCTION

A80804 [2]). The LED driver accomplishes two distinct functions
simultaneously: regulating current in each LED string andtrimming the output voltage of the preregulator to the minimum
required to maintain regulation across all LED strings. This
active trim scheme is implemented through the MINOUT
feature of the A80804. The MINOUT voltage is combined withthe preregulator feedback network to dynamically control thevoltage to the LED strings at the lowest operational level, as
determined by the configuration of the LED strings.

A linear LED driver dissipates substantial power to dynamicallymaintain a voltage drop equal to the difference between its
input and output voltages. To limit the impact on system thermal
management, linear devices are typically implemented only inlower-current applications. A more-efficient approach consists ofadding a switching preregulator to efficiently reduce the voltage
to levels closer to the requirements of LED strings. However,variation in LED forward voltage drop and active switching ofseries-connected LEDs require the output voltage of such a
preregulator to be designed for the worst-case scenario, which
limits the efficiency at lower LED voltage.

At higher temperature levels, when the IC is in thermal foldback,current through the LEDs is reduced. This results in lower forward
voltage (VF) of the LEDs and increased voltage drop across the
MOSFETs. With MINOUT feedback, the IC corrects the voltage
drop across the MOSFETs to the optimal level by adjusting thevoltage of the preregulator. This leads to an extended operationaltemperature range. The MINOUT pin provides feedback to thepreregulator, which allows for optimal voltage control of theA80804. The connection of the MINOUT pin is shown in Figure 1.

This application note presents the implementation of an
adaptive preregulator whose output voltage is dynamicallyadjusted to provide optimal voltage to a linear LED driver. The
proposed buck-boost regulator module (Allegro MicroSystems
A4450 [1]) supplies the linear LED driver (Allegro MicroSystems
PREREGULATOR
VOUT

C1

RFB1

C2

FB
RFB2

VB

RFB3

Q3
R3

Q4

D1 25

3 G3

G1 24

2 S3

S1 23
A80804

4 D4

D2 22

6 G4

G2 21
GND

5 S4

MINOUT

R4

26

VIN

1 D3

28

16

S2 20

Figure 1: Connection of MINOUT pin to the preregulator.
[1] https://www.allegromicro.com/en/products/regulate/regulators/single-output-regulators/a4450[2] https://www.allegromicro.com/en/products/regulate/led-drivers/led-drivers-for-lighting/a80804

Q1
R1

Q2
R2

AN296288
MCO-0001416

APPLICATION INFORMATION

DEVICE INTRODUCTIONSYSTEM SPECIFICATION

A80804 LED Driver

The proposed system is shown in Figure 1. To understandthe principles of operation, it is important to notice that thevoltage on the DX and DY pins is the difference between
the preregulator output voltage and the LED string forward
voltage drop. The A80804 requires a minimum of 0.5 V oneach of these pins to maintain regulation. Any additional
voltage on these pins translates to additional power losses.

The A80804 is a linear, programmable current controller
capable of accurately regulating current in four LED strings
using external MOSFETs; each of the four LED strings offers adedicated enable/pulse-width modulation (PWM) input andcurrent-sense resistor. Each channel can be independently
dimmed with an external PWM signal, or all channels canbe dimmed through internal PWM and automatic phaseshifting. The string current can be switched between low andfull intensity for applications including stop/tail or daytime
running lighting (DRL)/position lighting. Multiple analog
dimming options are available to support applications
including LED binning, negative temperature coefficient(NTC) foldback, or a hybrid approach. A selectable LED
current slew-rate control is available to tune PWM edge
times and improve electromagnetic interference (EMI)performance.
The A80804 offers several fault detection and protection
capabilities required for automotive applications. The
A80804 can be configured for either one-out-all-out or N-1
operation.

The input voltage specifications of the system are:• Nominal input voltage (Vin_nom) = 12 to 15 V
• Minimum input voltage (Vin_min) = 8 V• Maximum input voltage (Vin_max) = 18 V
Key buck-boost regulator specifications are:• Operating frequency (fSW) = 400 kHz• Output capacitor (Cout) = 33 µF• Regulator feedback voltage (Vfb) = 800 mV
Key LED specifications are:
• LED typical forward voltage (Vf_typ) = 3.1 V• LED maximum VF (Vf_max) = 3.3 V

A4450 Buck-Boost Controller• LED minimum VF (Vf_min) = 2.8 V

The A4450 is a power management IC that can control
either a buck or a buck-boost topology to efficiently convert
automotive battery voltages into a tightly regulated voltage.
It offers many useful features for automotive applications,
including:

The MINOUT pin provides an output voltage proportional tothe minimum voltage of the Dx pins for all active channels, as
shown in Figure 2.
A80804800 mV• An enable input compatible with battery voltage levels.
• A diagnostic power-on-reset output (NPOR) signal.
• Several protection features that include pulse-by-pulse
current limit, hiccup mode short-circuit protection, switch
node short-circuit protection, missing freewheeling diodeprotection, and thermal shutdown.
The A4450 is most suitable for applications where theinput voltage can vary below and above the regulated output
voltage.

MINOUT+

Sampleand Hold

Vref

Min
+-

DY
DX
GX
SX

Figure 2: Block diagram of MINOUT block.

The voltage from MINOUT is combined with the voltage
regulator FB pin to create a closed-loop system and trim its
regulated output. The voltage on MINOUT is calculated as:
VMINOUT = 10 × (VDX_MIN – 0.8) + 3,

and therefore ranges between 0.3 V and 3.5 V.

If the regulating string voltage changes for any reason,
MINOUT voltage changes as well and provides proper
feedback signal to the preregulator to adjust the output
voltage to the optimal level.
The calculation for the selection of the bias resistors needed
for the implementation of the MINOUT feature is discussed
next.

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AN296288
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APPLICATION INFORMATIONMINOUT Resistor Calculation

TEST RESULTS

System specifications pertaining to the margins required in
the calculation of the minimum and maximum output voltage
for the implementation shown in Figure 1 are:This section presents the steady-state, transient, and stability
performance of the overall system.

• Regulation voltage drop on sense resistor(Vsense) = 500 mV

Steady-State Performance
The operation of the LED driver is shown in Figure 3 andFigure 4, respectively without and with the MINOUT feature, at
supply voltages of 8 V, 12 V, and 18 V.

• MOSFET voltage drop (Vmos) = 200 mV

8V• Maximum margin for transient and regulator tolerance(Vmargin,max) = 0.7 V• Minimum margin for transient and regulator tolerance(Vmargin,min) = 0.3 V
Based on the above specifications, the maximum andminimum output voltage can be calculated as:
Equation 1—Maximum output voltage (Vout_max):
𝑉𝑜𝑢𝑡_𝑚𝑎𝑥 = 4 × 𝑉𝑓𝑚𝑎𝑥 + 𝑉𝑠𝑒𝑛𝑠𝑒 + 𝑉𝑚𝑜𝑠 + 𝑉𝑚𝑎𝑟𝑔𝑖𝑛,𝑚𝑎𝑥
𝑉𝑜𝑢𝑡_𝑚𝑎𝑥 = 4 × 3.3 + 0.5 + 0.2 + 0.7 = 14.6 𝑉
Equation 2—Minimum output voltage (Vout_min):
𝑉𝑜𝑢𝑡_𝑚𝑖𝑛 = 4 × 𝑉𝑓𝑚𝑖𝑛 + 𝑉𝑠𝑒𝑛𝑠𝑒 + 𝑉𝑚𝑜𝑠 + 𝑉𝑚𝑎𝑟𝑔𝑖𝑛,𝑚𝑖𝑛
𝑉𝑜𝑢𝑡_𝑚𝑎𝑥 = 4 × 2.8 + 0.5 + 0.2 + 0.3 = 12.6 𝑉

12 V

Now, the resistors connected on the MINOUT pin, as shownin Figure 1, can be calculated by:• Selecting Rfb2 = 10 kΩ• Calculating Rfb1 and Rfb3 using:
3 =(
1 =(

(max )

)

)

)
)×(

)

2

)

2

)
3

3

− 1)

2

)

18 V

• Maximum MINOUT voltage (Vminout_max) = 3.5 V• Using above formulas, the value of Rfb3 and Rfb1 arecalculated as:□ Rfb3 = 291.8 kΩ (selected value as 294 kΩ)□ Rfb1 = 166.75 kΩ (selected value as 165 kΩ)

Figure 3: Steady-state waveforms without MINOUT feature at an input
voltage of 8 V (top), 12 V (center), and 18 V (bottom). Ch-1: SW node (yellow);
Ch-2: Vout (pink); Ch-3: MINOUT (blue); and Ch-4: Total LED current (green).

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AN296288
MCO-0001416

APPLICATION INFORMATIONAs shown in Figure 3, the typical output voltage of thepreregulator is 13.94 V with an LED current of 1.237 A
without enabling the MINOUT feature. However, when
MINOUT is enabled, the output voltage of the preregulator
drops to a typical value of 13.2 V supplying the same LED
current, as shown in Figure 4.

8V

Power Efficiency
The steady-state performance of the A80804 driver fed
with the A4450 preregulator without and with the MINOUT
feature is shown in Table 1.
The efficiency vs. input voltage of the system without andwith MINOUT is shown in Figure 5. As shown in the figure,the system implemented with the MINOUT feature is more
efficient than the system implemented without the MINOUT
feature.
The LED current variation vs. input voltage of the system
without and with the MINOUT feature is shown in Figure 6.
As shown in the figure, the LED current is almost constant forthe operation of the driver in both modes of operation.
Table 1: Steady-State Performance Without MINOUT
Vin (V)Iin (A)Pin (W)

Vout (V)

Iout (A)Pout (W) Efficiency (%)

8

3.12

24.96

13.96

1.236

17.255

69.13

12

2.01

24.17

13.94

1.236

17.23

71.29

18

1.33

23.86

13.94

1.239

17.272

72.39

Without MINOUT

12 V

With MINOUT
8

2.61

20.88

13.16

1.238

16.292

78.03

12

1.66

19.9

13.22

1.235

16.327

82.05

18

1.09

19.54

13.3

1.235

16.42684.06

85

Efficiency (%)

81

18 V

Without MINOUTWith MINOUT

77

Efficiency

73
69

65

6

8

10

12

14

16

Input Voltage (V)1820

Figure 5: Efficiency vs. supply voltage without and with MINOUT.
1.25
Without MINOUTWith MINOUTILED (A)

1.24

1.23

LED Curent
Variation
Figure 4: Steady-state waveforms with MINOUT feature at an input voltage of
8 V (top), 12 V (center), and 18 V (bottom). Ch-1: SW node (yellow); Ch-2: Vout(pink); Ch-3: MINOUT (blue); and Ch-4: Total LED current (green).

1.22

6

8

10

12

14

Input Voltage (V)

16

18

20

Figure 6: LED current vs. supply voltage without and with MINOUT.

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AN296288
MCO-0001416

APPLICATION INFORMATION

Thermals

Load Voltage Transients (Tail/Stop or DRL/POS)

Thermal images of the A80804 board are shown in
Figure 7 without (left) and with (right) implementation of the
MINOUT feature. As shown in these figures, better thermal
performance is achieved with the MINOUT feature.

In tail/stop or DRL/POS applications, the number of LEDs in astring are intentionally switched, resulting in different optimal
output voltages. The MINOUT function automatically adjusts
the output voltage of the preregulator to the new optimallevel.
Performance is shown in Figure 9 when the number of LEDs
in a string is switched from three LEDs to two LEDs, then backto three. LED current is set to 300 mA per channel. Whenthe string transitions from three LEDs to two LEDs, the output
voltage of the preregulator drops slowly without causing any
disturbance in LED current.

Figure 7: Thermal images (maximum temperature) of A80804 without (left)
and with (right) MINOUT.

Stability

When a third LED is added to the string of two, the MINOUT
feature gradually increases the output voltage. During thetransition from two LEDs to three LEDs, the slow response of
the MINOUT feature—which gradually increases the output
voltage of the A4450 buck-boost converter—causes acurrent dip.

Bode plots depicting sufficient gain and phase margins areshown in Figure 8 for implementation without (top) and with(bottom) the MINOUT feature. The stability of the overallsystem is primarily set by preregulator compensation loop
and is not affected by the LED driver. The MINOUT function
is quite slow and does not impact the overall frequency
response.

Transitions from Three LEDs to Two LEDs, Then Back to Three

Without MINOUT Implemented

Zoomed Waveform for the Current Dip During Transition from
Two- to Three-LED Operation

With MINOUT Implemented

Figure 8: Bode plot of overall system without (top) and with (bottom)
implementation of the MINOUT feature.

Figure 9: Switching the number of LEDs in series from three to two, then backto three (top); and zoomed waveform for the current dip during two- to threeLED operation (bottom). Ch-1: Vout (yellow); Ch-4: Total LED current (green).

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AN296288
MCO-0001416

APPLICATION INFORMATIONAPPLICATION SIMPLIFIED SCHEMATICSThe application schematic of the LED driver with the front-end A4450 and the A80804 LED controller is shown in Figure 10.
The developed two-layer board without components is shown in Figure 11. Overall board dimensions are 147 mm × 53 mm.
D1

VOUT
VOUT

S4

RPU

RFB3

28
PWM1
29
PWM2
30
PWM3
31
PWM4
32
nFAULT 8

BIAS

FULL

MINOUTPWMIN1PWMIN2PWMIN3PWMIN4nFAULT

BIAS

Q2

S2 20
RS2
PWMOUT

9

GND
PAD

PAD

U1

G2 21

RRNG

NPOR

22 DR20

17

3,7,12PGND

4 ENBAT

D2

FDIS
M ODE
UVL O
VDR

10

NPOR

26

27

CO4

CO3

CO2

CO5

5

10
18
14
13
16

CZ

REN2

G4

RS4

ADIM 2VBIAS
VINDERAT
FSET

RNG 9

MIN
FB

RS1

A80804SL EW
ADIM 1

RZ

SWEN

Q4

6

Q1

S1 23

U2
D4

P1
1
2
3
4
5

25 DR1

G1 24

DR4 4

RP4

15

RP2

FB

COMP

D1

RS3

RFB2

RP1

14

S3

DR1
DR2
DR3
DR4

16

11 SS

CSS

G3

2

CDR4 CDR3 CDR2 CDR1

7
19
11
17

LG

3

15
12

8 FSET/SYNC

RFS

CP

RFB1

RFLT

CIN5

CO6

0

1

LX

6 AVIN

RIN

REN1

Q3

20
19

CFL

DR3 1 D3

R_LG

Cbt
LX
LX

QSW

18

RP3

CB1

CO1

5

5,13
NC

BOOT

1 VIN
2 VIN

VIN
VIN

GND

Dbat

CIN1
CIN2
CIN3
CIN4

VBAT

4

VIN

LSW
DSW

CBIAS

Figure 10: Application schematic of the A4450- and A80804-integrated LED driver.

Figure 11: Developed board of the A4450- and A80804-integrated LED driver.

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AN296288
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APPLICATION INFORMATIONLAYOUT
The layout (top layer and bottom layer) of the LED driver developed with the front-end A4450 and the A80804 LED controller
is shown in Figure 12.
Top Layer

Bottom Layer

Figure 12: Layout of the A4450- and A80804-integrated LED driver showing the top layer (top) and bottom layer (bottom) layouts.

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AN296288
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APPLICATION INFORMATIONBILL OF MATERIALSPIN CONFIGURATIONThe bill of materials for the LED driver developed with thefront-end A4450 and the A80804 LED controller is shown in
Table 2.

A4450
• ENBAT: Connected to VIN to enable device.

Table 2: Bill of Materials for LED Driver Using A4450 and A80804
Devices• FSET: Connected to GND via resistor for 400 kHz
switching frequency.

Designator• RNG: Connected to GND via resistor for preferred 0.65
buck duty cycle at the instant the boost switch starts toswitch.

CB1Value
47 µF, 35 V

CIN1, CO1
4.7 µF, 35 V
CIN2
0.22 µF, 35 V
CIN3
100 nF, 35 V
CIN4
100 pF, 35 V
CIN5
1.0 µF, 35 V
CDR1, CDR2,
0.1 µF, 35 V
CDR3, CDR4
CBIAS
2.2 µF, 16 V
Cbt
47 nF, 35 V
Cfl
33 µF, 35 V
CO2
10 µF, 35 V
CO3
0.22 µF, 35 V
CO4
10 nF, 35 V
CO5
0.1 µF, 35 V
CO6
0.1 µF, 35 VCP
33 pF, 35 V
CSS
22 nF, 35 V
CZ
2200 pF, 35 V
D1, DSW
SS3P4
Dbat
4 A, 35 V
LSW
4.7 µH, 4 Asat
P1
MHDR1X5
Q1, Q2, Q3, Q4

3 A, 35 V

QSW

PQFN

Description

Footprint

Quantity

Cap-TH 6.3mm

1

1210
0805
0603
0603
0603

2
1
1
1
1

Capacitor

0603

4

Capacitor
Capacitor
Capacitor
Capacitor
Capacitor
Capacitor
Capacitor
Capacitor
Capacitor
Capacitor
Capacitor
Diode
Diode
Inductor
Header, 5-Pin
N-Channel
MOSFETeSOIC8
Package FET

0603
0603
0603
0805
0805
0603
0805
1210
0603
0603
0603
SMP
SMA
Inductor 4040
MHDR1X5

1
1
1
1
1
1
1
1
1
1
1
2
1
1
1

DPAK123

4

eSOIC8

1

Electrolytic
Capacitor
Capacitor
Capacitor
Capacitor
Capacitor
Capacitor

REN1, RFLT,
RLG, RP1,
RP2, RP3,
RP4, RPU
REN2
RFB1
RFB2
RFB3
RFS
Rin
RRNG
RS1, RS2,
RS3, RS4
RZ
SWEN

10 kΩ

Resistor

0603

8

4.99 MΩ
24.9 kΩ4.75 kΩ
165 kΩ
23.7 kΩ5E
22.1 kΩResistor
Resistor
Resistor
Resistor
Resistor
Resistor
Resistor

0603
0603
0603
0603
0603
0603
0603

1
1
1
1
1
1
1

1.68E

Resistor

1210

4

10.7 kΩOFF0603
SW DIP 1POS

1
1

U1

A4450

4×4 QFN

1

U2

A80804

Resistor
SW, SPST
4×4 QFN-20(ES)
5×5 QFN-32(ET)

5×5 QFN

1

A80804• SLEW: Connected to VBIAS for 80 µs slew time.
• ADIM1, ADIM2: Connected to VBIAS to disable analog
dimming.
• VINDERATE: Connected to VBIAS to disable input voltage
derating. When the VIN pin is connected to the input
battery supply and the LEDs are connected to a different
voltage supply, VINDERATE will not work.
• FSET: Driver works in full mode, so PWM mode isdisregarded.
• FDIS: Disable FDIS by connecting to VBIAS.
• MODE: Connect to low for N-1 fault operation.
• UVLO: Disable UVLO by connecting to GND.
• VDR: Sets PWM duty in internal PWM mode. Because thedriver is configured in full mode, the VDR pin voltage is
not relevant.

CONCLUSION
The MINOUT feature of the Allegro A80804 linear LED
driver allows control of a switching DC-DC converter toprovide optimal output voltage to drive a string or multiple
strings of LEDs. This minimizes power dissipation in thelinear current sinks of the LED driver, resulting in a substantial
increase in system efficiency. This efficiency has multiple
benefits, including reduced heating of the application
board and ICs, and is a welcome improvement in many
applications, particularly those that involve high-temperatureenvironments and those that rely on limited power sources,
such as batteries.

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AN296288
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APPLICATION INFORMATION

Revision History
Number–Date
June 27, 2023

Description

Responsibility

Initial release

S. Wekhande, V. Bist,
and S. Gadgi

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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