ELM623FA 20A Fully Integrated Synchronous Boost Converter
http://www.elm-tech.com■General description

ELM623FA is a high efficiency fully integrated synchronous Boost converter with built-in main switch andsynchronous switch. The device has 20A switch peak current capability and provides output voltage up to 18V.
Synchronous rectification increases efficiency, reduces power losses and eases thermal requirements, allowing
ELM623FA to be used in high power step-up applications. The 2.7V to 18V input voltage range supports awide range of battery and AC powered inputs. The 70µA no load quiescent current extends operating run timein battery-powered systems. The operating frequency can be externally set for a 50kHz to 1MHz range, as for
operation mode is automatic PSM/PWM mode. ELM623FA implements a programmable soft-start function, an
adjustable cycle by cycle switching peak current limit function and thermal shutdown protection. ELM623FA isavailable in a small 20 pin 4mm×4mm QFN package.

■Features• Adjustable Input UVLO through EN pin• Resistor or Inductor DCR current sensing• Programmable Soft-start• Cycle-by-Cycle Current Limit• Thermal Shutdown• Integrated 18V rating 9mΩ power switches with 20A peak current capability• Input Range: 2.7V to 18.0V• Output Voltage: 3.0V to 18.0V• Low Quiescent Current: 70μA• Low shutdown supply current : 3.5μA• Adjustable Frequency: 50kHz to 1MHz
• Package: QFN20-4x4■Application• USB Type C-PD and Thunderbolt Port for PCs• Industrial Battery Powered POS Terminals• Quick Charge Power Banks• Electronic Cigarette• Hi Power Bluetooth Speaker■Recommend operating conditions
Parameter
Input Voltage
Output Voltage
Operating temperature

Symbol
Vin
Vout
Top

Limit
+2.7 to +18.0+3.0 to 18.0
-40 to +85

Unit
V
V
°C

* Note: The device is not guaranteed to function outside of the recommended operating conditions.

Rev.1.0
1 / 15

ELM623FA 20A Fully Integrated Synchronous Boost Converter
http://www.elm-tech.com■Maximum absolute ratings
Parameter
VH to GND (Note 1)
ISP to GND (Note 1)
ISN to GND (Note 1)
OUT to GND (Note 1)
ISP to ISN (Note 1)
PGND to GND (Note 1)
SW to GND (Note 1)
Dynamic Vsw in 50ns duration (Note 1)
BST to SW (Note 1)
EN to GND (Note 1)
FB to GND (Note 1)
COMP to GND (Note 1)
VCC to GND (Note 1)
Power dissipation at Ta=+25°C(Note 2, 3)
Storage temperature (Note 1)

Symbol
Vvh
Visp
Visn
Vout
Visp_isnVpgnd
Vsw
Vbst_sw
Ven
Vfb
Vcomp
Vcc
Pd
Tstg

Limit

Unit

-0.3 to +20.0

V-0.3 to +0.3

V

-1 to Vout+1-3 to Vout+3

VV-0.3 to +6.0

V

2.0
-55 to +150

W°C

* : 1. Stress exceeding those listed "Absolute maximum ratings" may damage the device.
2. Measured on JESD51-7, 4-Layer PCB.
3. The maximum allowable power dissipation is a function of the maximum junction temperature TJ_MAX, the junction toambient thermal resistance θja, and the ambient temperature Ta. The maximum allowable continuous power dissipation at any
ambient temperature is calculated by Pd_max= (Tj_max-Ta)/θja. Exceeding the maximum allowable power dissipation will cause
excessive die temperature, and the regulator will go into thermal shutdown. Internal thermal shutdown circuitry protects the device
from permanent damage.

■Selection Guide
ELM623FA-N
Symbolabc

Package
Product version
Taping direction

F: QFN20-4x4
A
N: Refer to PKG file

ELM623 F A - N
↑ ↑ ↑
a b c

* Taping direction is one way.

■Pin configuration

Rev.1.0
2 / 15

ELM623FA 20A Fully Integrated Synchronous Boost Converter
http://www.elm-tech.com
Pin description
Pin No.

Pin name

1, 19, 20

OUT

Output of the Boost converter.

2, 15, 16, 17, 18

SW

Switching node of the Boost converter. Connect this pin to the inductor.

3

BST

Bootstrap capacitor node for Synchronous MOSFET. Connect the bootstrap
capacitor 0.1µF from BST pin to the SW pin.

4

VH

The supply pin to On-Chip LDO Regulator. The operating voltage range on thispin is 2.7V to 18V (20V abs max). Bypass VH to PGND with a 0.1μF ceramic
capacitor.
When the input voltage Vin is < 5.5V, connect VH to the output of Boost
converter to get maximum voltage for gate drivers.
When the input voltage Vin is > 5.5V, connect VH to the input voltage toimprove efficiency.

5

FB

Error amplifier input and feedback pin for output voltage regulation. Connect
FB to the center tap of a resistor divider to set the output voltage.

6

ISP

Positive Current Sense Amplifier Input. The current sense resistor is normallyplaced at the input of the Boost converter in series with the inductor. The common
mode voltage range on the ISP and ISN pins is 2.5V to 20V (20V Abs Max).

7

ISN

Negative Current Sense Amplifier Input.

8

COMP

Output of the internal transconductance error amplifier. The feedback loop
compensation network is connected from COMP pin to GND.

SS

Soft-start programming pin. An external capacitor sets the ramp rate of theinternal error amplifier's reference voltage during soft-start period. Typicallyconnect SS to GND with a 3.3nF ceramic capacitor.

9

Description

FREQ

Oscillator Frequency Set Input. A resistor from FREQ to GND sets the oscillator
frequeny from 50kHz to 1000kHz (Typical Rfreq=270kΩ sets Fosc=250kHz).
Leave this pin float to set 460kHz default frequency.

EN

Enable input. Pull EN above 1.22V to turn on the converter, and pull EN below
1.10V to shutdown the converter. EN pin can be used to implement externallyadjustable input voltage under voltage lockout (UVLO) with two resistors.
Connect EN to the center tap of a resistor divider to set the input UVLO
threshold.

12

VCC

5.4V On-Chip Low Dropout Linear Regulator Output (LDO). This regulator
powers all internal circuitry including the low side and high side N-channel
MOSFET gate drivers. Bypass VCC to GND with a 1μF or greater ceramic capacitor.
When the input voltage Vin is < 5.5V, Connect Vcc to Vin through a Diode.

13

GND

Signal Ground pin of the converter. All small-signal components andcompensation components should be connected to this signal ground.

14

PGND

Power ground of the converter. It is connected to the source of the main
MOSFET. Connect this pin to the (-) terminal(s) of Cin and Cout.

10

11

Rev.1.0
3 / 15

ELM623FA 20A Fully Integrated Synchronous Boost Converter
http://www.elm-tech.com■Standard circuit
VOUT

R4
R4
4.7R
4.7R

C1
1μF

ISP

SS

CSS
3.3nF
VBAT

VBAT
3V to 4.35V

VH

ISN

ELM623FA

D1 RB751(Vin<5.5V, Optional)

C5 4.7μF

R9 270K
Cs 47nF Rs 1K
Cp 220pF

FREQ

BST

CIN
22μFx4

C3
180pF

R1
RFF
300K 133K

C1
1μF

VBAT

ISP

SS
ISN

ELM623FA

D1 RB751(Vin<5.5V, Optional)

CBS 0.1μF

C5 4.7μF

VOUT
12V/2A

R9 270K
Cs 47nF Rs 1K

200K
RR
200K
EN1
EN1

RR
EN2
EN2
150K
150K

GND
PGND

FB

R4
4.7R

VH

CSS
3.3nF

L
2.2μH

OUT

COMP

EN

R2 14.7K

CFF
1nF

VCC

SW

RSENSE
5m

VOUT

CC
FFFF
1nF
1nF

BST

CIN
22μFx4

L
1.5μH

C3
180pF

RLS1
200
CBS 0.1μF
VOUT
12V/2AOUT
EN

R2 14.7K

COUT
22μFx4FREQ

COMP

Cp 220pF

VBAT

VCC

SW

VBAT
3V to 4.35V

CLS RLS2
2.2uF 100

GND
PGND

FB

R1
RR
FFFF
300K 133K
300K

REN1 200K
REN2
150K

VBAT
COUT
22μFx4■Block diagram
OUT

VCC
DB
S
R Q

BST

UVLO

1µA
OSC

FREQ

CONTROL
LOGIC
AND
CHARGE
PUMP

DRH

SW

VCC
DRL

CLK

PGND
ICMP

-+

IZERO

ISN
UVLO

2.5V
0.8V

VH
5.4V
LDO

2.0V

+
-

Slope
Comp

ISP
+
EA -

1.22V

1.203V
SS

FB

COMP

VCC

GND

SS

EN

Rev.1.0
4 / 15

ELM623FA 20A Fully Integrated Synchronous Boost Converter
http://www.elm-tech.com■Electrical characteristics

Top=+25°C, Unless otherwise noted. Typical values are at Vh=12V, Visp=Visn=3.6V and Ven=2V.
Parameter
SymbolTest conditions
Min. Typ. Max. Unit
Supply and enable
Input supply voltage
Vh
2.7
18.0
V
Output voltage
Vout
3
18
V
Linear regulator output voltage
Vcc 6V<Vh<18V
5.1
5.4
5.7
V
Vh=Vcc Rising
2.0
2.5
Under-voltage lockout threshold
UVLO
V
Vh=Vcc Falling
1.8
EN Pin on threshold
Ven_on Ven rising, turn on the device
1.120 1.220 1.320
V
1.10
V
EN Pin off threshold
Ven_off Ven falling, turn off the device
EN Pin Input current
Ven_in Ven=2V
100
nA
Operating quiescent current into VH Ivh Vfb=1.25V, Device no switching
70
100
μA
Shutdown current
Is
Ven=0V3.5
10.0
μA
Error amplifier
PWM Mode
1.185 1.203 1.221
Feedback voltage
Vfb
V
Light load PSM mode
1.211
Feedback Current
Ifb Vfb=1.22V
-100
1
+100
nA
Error Amplifier Transconductance Iea/Vtd Vcomp=1.6V
1.3
mA/V
3.0
Hi Clamp voltage, VFB=1.1V
COMP pin clamp voltage
Vcomp
V
0.4
Hi Clamp voltage, VFB=1.3V
Current sense amplifier
Maximum current sense threshold
Vmce ΔV(isP-isN)
90
100
110
mV
Zero current sense threshold
Vzce ΔV(isP-isN)0mV
ISP/ISN current sense input current Iisp/isn VisP=VisN=3.6V
10
20μA
ISP/ISN current sense input range Visp/isn
2.7
30.0
V
Oscillator frequency
Rfreq=220kΩ
240
300
360
Oscillator frequency
Fosc
kHz
Rfreq float
460
Maximum duty cycle
Dmax
93
%
Minimum on time
Tmin
200
ns
Power switches
Low side main switch on resistance Rds(on)_L9mΩ
Rds(on)_H
High side switch on resistance9mΩPower switch leakage current
Ileak Ven=0V, Vsw=18V and 0V
1
20
μA
Thermal shutdown
Thermal shutdown threshold
Tsd
160
°C
Thermal shutdown hysteresis
Tsh
35
°C

Rev.1.0
5 / 15

ELM623FA 20A Fully Integrated Synchronous Boost Converter■Detailed description (Refer to the functional block diagram)

http://www.elm-tech.com

ELM623FA is a fully-integrated synchronous boost converter with a 9mΩ synchronous switch to output. The
device is capable of providing an output voltage up to 18V and delivering up to 25W power from a 2.7 V to 18V
wide input. Voltage regulation is achieved employing constant frequency current mode pulse width modulation(PWM) control. The switching frequency is set either externally from 50kHz to 1MHz by an external timing
resistor from FREQ pin to GND or default 460kHz by floating FREQ pin. The PWM control circuitry turns
on the low side MOSFET at the beginning of each oscillator clock cycle, as the error amplifier compares theoutput voltage feedback signal at the FB pin to the internal 1.203V reference voltage. The low side MOSFET
is turned-off when the inductor current reaches a threshold level set by the error amplifier output. After the low
side MOSFET is turned off, the high side synchronous rectifier MOSFET is turned on until the beginning of thenext oscillator clock cycle or until the inductor current reaches the zero current sense threshold. The input voltage is applied across the inductor and stores the energy as inductor current ramps up during the portion of theswitching cycle when the low side MOSFET is on. Meanwhile the output capacitor supplies load current. Whenthe low side MOSFET is turned off by the PWM comparator, the inductor transfers stored energy via the synchronous rectifier MOSFET to replenish the output capacitor and supply the load current. This operation repeats
every switching cycle. The device skips pulse to improve efficiency at light load. In the light load mode, theconverter only operates when the output voltage trips below a FB set threshold voltage 1.211V. It ramps up theoutput voltage with one or several pulses and skips pulse once the output voltage exceeds the set threshold voltage (PSM:Pulse Skipping Modulation). The devices feature internal slope compensation to avoid sub-harmonic
oscillation that is intrinsic to peak current mode control at duty cycles higher than 50%. They also feature optional lossless inductor DCR current sensing, cycle-by-cycle current limit and over-temperature protection.

■Application information
ELM623FA external component selection is driven by the load requirement, and begins with the selection ofswitching frequency, inductor and Rsense. Finally, input and output capacitors are selected.

■Switching frequency selections
The first step is to determine the switching frequency of the Boost converter. There are tradeoffs to consider
when selecting a higher or lower switching frequency. Typically, the designer uses the highest switching frequency possible since this results in the smallest solution size. A higher switching frequency allows for lower
value inductors and smaller output capacitors compared to a Step-up converter that switches at a lower frequency. A lower switching frequency will produce a larger solution size but typically has a better efficiency by
reducing MOSFET switching losses. The switching frequency is also limited by the minimum on-time of theconverter based on the input voltage and the output voltage of the application. Minimum on-time, tONmin (200ns
Typ.), is the smallest time duration that the ELM623FA is capable of turning on the low side MOSFET and correctly sensing inductor current for peak current mode control. To determine the maximum allowable switchingfrequency, first estimate the continuous conduction mode (CCM) duty cycle using Equation 1 with the maximum input voltage.
Vout - Vin(max)Vout
D(min)
fosc(max) =(Equation 1)=
200ns
ton(min)
To determine the timing resistance at FREQ pin for a given switching frequency use the curve in Figure 1.
Rev.1.0
6 / 15

ELM623FA 20A Fully Integrated Synchronous Boost Converter
http://www.elm-tech.com

Figure 1. Switching frequency versus resistor value at the FREQ pin.

■Input capacitor selection
Place a high quality 0.1μF in parallel with at least a 10µF or higher ceramic type X5R or X7R bypass capacitor
at the VIN pin to power ground PGND for proper decoupling. Based on the application requirements additionalbulk capacitance are needed to meet input voltage ripple, transient and EMI requirements. The value of the
Cin is a function of the source impedance, and in general, the higher the source impedance, the larger input
capacitance. The required amount of input capacitance is also greatly affected by the duty cycle. High output
current applications that also experience high duty cycles can place great demands on the input supply, both interms of DC current and ripple current. The input capacitor voltage rating should exceed the maximum input
voltage range.

■Inductor selection
The selection of the inductor affects the steady-state operation as well as transient behavior and loop stability.
These factors make it an important component in a switching power supply design. The three most important
inductor specifications to consider are inductor value, DC resistance (DCR), and saturation current rating. In a
step-up topology the average inductor current is equal to the input current. The highest average current throughthe inductor and the converter depends on the maximum output load, converter efficiency η, the minimum input
voltage (Vinmin), and the output voltage (Vout). The inductor saturation current rating should be greater (bysome margin) than the maximum average inductor current. Estimation of the maximum average inductor current
can be done using Equation 2:
Il(max) = Iout(max) ×

Vout
Vin(min) × ŋ(Equation 2)

For example, for an output current of 2A at 12V with 90% efficiency, at least 8.9A of average current flows
through the inductor at a minimum input voltage of 3V.
The inductor value has a direct effect on ripple current. Let the parameter ∆Il represent the inductor peak-peakripple current. The inductor ripple current contributes to the output current ripple that must be filtered by theoutput capacitor. Therefore, choosing high inductor ripple currents impacts the selection of the output capacitor. Higher values of ∆Il lead to discontinuous mode (DCM) operation at moderate to light loads. The inductor
ripple current ∆Il decreases with higher inductance or frequency and increases with higher Vin. Estimation of
the inductor ripple current can be done using Equation 3:
Rev.1.0
7 / 15

ELM623FA 20A Fully Integrated Synchronous Boost Converter
http://www.elm-tech.com

∆Il =

Vin
fosc・L

(1-

Vin
Vout

)

(Equation 3)

Accepting larger values of ∆Il allows the use of low inductances, but results in higher output voltage ripple andgreater core losses. A reasonable starting point for setting ripple current is ∆Il = 0.3~0.5*Ilmax.
ELM623FA Boost converters have been optimized to operate with an effective inductance in the range of 1μH
to 10μH. Larger or smaller inductor values can be used to optimize the performance of the device for specificoperating conditions.

■ISP and ISN pins
ELM623FA can use either a discrete sense resistor (Rsense) or inductor DCR (DC resistance) sensing for current sensing. The choice between the two current sensing schemes is largely a design trade-off between cost,
power consumption and accuracy. DCR sensing is becoming popular because it does not require current sensing
resistors and is more power efficient, especially in high current applications. However, current sensing resistors
provide the most accurate current limits for the converter.
The ISP and ISN pins are the inputs to the current sense amplifier. The common mode input voltage rangeof the current sense amplifier is 2.7V to 18V. The current sense resistor is normally placed at the input of theconverter in series with the inductor. The ISP pin also provides power to the current comparator. ISP draws approximately 10~30µA during normal operation. There is a typical 10µA bias current that flows into the ISN pin.
The sense lines should be Kelvin-sense connection underneath the current sense resistor (shown in Figure 2). If
inductor DCR sensing is used (Figure 3b), sense resistor R1 should be placed close to the switching node, andthe mutual filter capacitor C1 should be placed close to the ELM623FA to prevent noise from coupling into sensitive small-signal nodes.
TO SENSE FILTER,
NEXT TO THE
CONTROLLER

VIN

INDUCTOR OR
RSENSE

Figure 2. Sense lines placement with inductor or sense resistor.

Rev.1.0
8 / 15

ELM623FA 20A Fully Integrated Synchronous Boost Converter
http://www.elm-tech.com
VIN

VH

VIN

VH
ISP

ISP

C1

OPTIONAL
ISN

ISN

VCC

VCC

BST

BST

R2

R1

DCR

INDUCTOR
L

OUT
OUT
SW

SW

DRL

DRL

GND

GND

(3a) Using a current sense resistor.

VOUT(3b) Using the inductor DCR to sense current.

Figure 3. Two current sensing methods.

■Current sensing resistor
A typical sensing circuit using a discrete resistor is shown in Figure 3a. Rsense is chosen based on the required
maximum average inductor current. The current comparator has a maximum threshold Vsensemax of 100mV(typical) and 90mV(minimum). To ensure that the application will deliver full load current over the full operating temperature range, choose the minimum value 92mV for the maximum current sense threshold Vsensemax.
The current comparator threshold sets the peak of the inductor current, yielding a maximum average inductor
current, Ilmax, equal to the peak value less half the peak-to-peak ripple current, ∆Il. To calculate the sense resistor value, use the Equation 4:
Rsense =Vsensemax
Ilmax +

∆Il
2

=

90mV
Ilmax +

∆Il
2

(Equation 4)

Rsense also affects the current mode control loop gain. Choose Rsense in the 3~20mΩ range.

■Inductor DCR Sensing
For applications requiring the highest possible efficiency at high load currents, ELM623FA is capable of sensing the voltage drop across the inductor DCR, as shown in Figure 3b. The DCR sensing reduces conduction loss
through a sense resistor and improve efficiency by a few percent. A flat frequency response is achieved when
the inductor time constant matches that of the RC sense network. If the external R1||R2 • C1 time constant ischosen to be exactly equal to the L/DCR time constant, the voltage drop across the external capacitor is equal tothe drop across the inductor DCR multiplied by R2/(R1+R2). R2 scales the voltage across the sense terminals
for applications where the DCR is greater than the target sense resistor value. The DCR of the inductor can beless than 5mΩ for high current inductors, and the R2 resistor is not used. To properly dimension the externalfilter components, the DCR of the inductor must be known. It can be measured using a good RLC meter, but the
DCR tolerance is not always the same and varies with temperature. Consult the manufacturer’s data sheets forRev.1.0
9 / 15

ELM623FA 20A Fully Integrated Synchronous Boost Converter
http://www.elm-tech.com
detailed information. Using the inductor ripple current value from the inductor section, the equivalent sense resistor value is calculated with Equation 5:
90mV
Vsensemin
R2=
Rsense(equiv) = DCR・
=
R1+R2
∆Il
∆Il
Ilmax +
Ilmax +
2
2

(Equation 5)

Choose R1|| R2 around 200Ω to reduce error due to the ISN pin 10µA(Typical) input bias current. C1 is calculated by Equation 6 and usually selected to be in the range of 100nF to 10µF.
C1 =

L
L
R1+R2
=
・
DCR・R1|| R2 DCR R1・R2(Equation 6)■Setting input under-voltage lockout (UVLO)The EN pin voltage must be greater than 1.22V(typical) to enable ELM623FA. The device enters shutdown
mode when the EN voltage is less than 1.10V. In shutdown mode, the input supply current for the device is lessthan 5μA. When the EN pin voltage is higher than the shutdown threshold but less than 1.22V, the devices arein standby mode. Adjustable input UVLO can be accomplished using the EN pin. As shown in Figure 4, a resistor divider from the VIN pin to GND sets the input UVLO level. Choose the bottom UVLO resistor Ruvlo_bot
in the 10kΩ~200kΩ range to set the divider current at 10µA or higher. Typically select Ruvlo_bot=100kΩ. The
value of top resistor Ruvlo_top, depending on the the desired turn-on voltage Vstart at the VIN pin, can be calculated with Equation 7:
Ruvlo_top = Ruvlo_bot ×(

Vstart
-1
Ven
VIN

)

= 100kΩ ×(

Vstart-1
1.22V)(Equation 7)

RUVLO_TOPENRUVLO_BOT

Figure 4. Input UVLO setting.

■Bootstrap capacitor selection
Place a 10nF~0.1μF X5R or X7R ceramic capacitor between BST and SW pins for the proper operation. This
capacitor provides gate drive voltage to turn on the high-side MOSFET.

■VCC Low-dropout linear regulator
ELM623FA features an internal P-channel low dropout linear regulator (LDO) that supplies power to the VCC
pin from the VH supply pin. VCC powers the gate drivers and ELM623FA’s internal circuitry. The LDO output
VCC is regulated to 5.4V. It can supply at least 20mA and must be bypassed to ground with 1µF~4.7μF X5R
or better grade ceramic capacitor. The capacitor should have a 10V or higher voltage rating. Good bypassing isneeded to supply the high transient currents required by the MOSFET gate drivers. A VCC under-voltage detection
Rev.1.0
10 / 15

ELM623FA 20A Fully Integrated Synchronous Boost Converter
http://www.elm-tech.com
circuit prevents the internal PWM control circuitry and power switches from operation when VCC voltage isbelow 2V (typical).

■Output capacitor selection
In a step-up converter, the output has a discontinuous current, so output capacitor Cout must be capable of reducing the output voltage ripple and filtering the high di/dt path of the supply. It is recommended to use X5R or
X7R ceramic capacitors placed as close as possible to the VOUT pin and power ground PGND pin. The effectsof ESR (equivalent series resistance) and the bulk capacitance must be considered when choosing the right capacitor for a given output ripple voltage. The steady ripple voltage due to charging and discharging the bulk output capacitance in a single phase step-up converter is given by Equation 8. This value does not take into accountthe ESR of the output capacitor.
Ioutmax × Dmax
∆Vout ==
Cout × fosc

Vout - Vin(min)Vout
Cout × fosc

Iout(max) ×(Equation 8)

Where Cout is the output filter capacitor.
For example: Build 5V nominal output voltage from the minimum 3V input supply voltage. Select switchingfrequency 600kHz. Choose output capacitor to get less than 50mV ripple (1% of Vout) at maximum 4Amp output current. The minimum output capacitor is 53μF required to limit the output voltage ripple.
5V - 3V
4A ×
5V
Ioutmax × Dmax
Cout ≥= 5V × 1% × 600kHz = 53μF
∆Vout × fosc

(Equation 9)

Multiple capacitors placed in parallel may be needed to meet the ESR and RMS current handling requirements.
Ceramic capacitors have excellent low ESR characteristics but can have a DC Bias effect, which will have astrong influence on the final effective capacitance. Capacitance deratings for aging, temperature and dc bias
increase the minimum value required. The voltage rating must be greater than the output voltage with some tolerance for output voltage ripple and overshoot in transient conditions. For this example 4×22μF, 25V ceramic
capacitors with 5mΩ of ESR are used. The 40% derated capacitance is 52.8μF, approximately equal to the calculated minimum.

■Setting output voltage
ELM623FA output voltage is set by an external feedback resistor divider carefully placed across the output,
as shown in Figure 5. Great care should be taken to route the VFB line away from noise sources, such as theinductor or the SW line. Also, keep the FB trace as short as possible to avoid noise pickup. The typical valueof the voltage on the FB pin is 1.203V. The maximum allowed value for the output voltage is 18V. Choose thebottom resistor Rfb_bot in the 10kΩ~200kΩ range to set the divider current at 6µA or higher. Typically selectRfb_bot=100kΩ. The value of top resistor Rfb_top, depending on the needed output voltage Vout, can be calculated using Equation 10:
Rfb_top = Rfb_bot ×

(

Vout
-1
Vfb

)

= 100kΩ ×

(

Vout
-1
1.203V)(Equation 10)

Rev.1.0
11 / 15

ELM623FA 20A Fully Integrated Synchronous Boost Converter
http://www.elm-tech.com
VOUT

RFB_TOPFB
RFB_BOT

Figure 5. Output voltage setting.

■The control loop compensation
The series Rc-Cc filter at COMP pin sets the dominant pole-zero loop compensation. The resistor Rc in series
with a capacitor Cc creates a compensating zero. A capacitor Cc1 in parallel to these two components can be
added to form a compensating pole. In a step-up topology, the maximum crossover frequency is typically limited by the right-half plane zero (RHPZ). The compensation design should be done at the minimum input voltage
and full load when the RHPZ is at the lowest frequency. The crossover frequency should also be limited to lessthan 1/4 of the RHPZ frequency.
Table 1. Gives Rc, Cc and Cc1 values for certain inductors, input and output voltages providing a very stablesystem. For a faster response time, a higher Rc value can be used to enlarge the bandwidth, as well as a slightly
lower value of Cc to keep enough phase margin. These adjustments should be performed in parallel with theload transient response monitoring of ELM623FA.
Table 1. Recommended compensation network values.
Application
Iout_max Fsw
Inductor Rsense
Cin
Cout
1-Cell step-up to 5V
1.5µH
4*22µF 4*22µF
5A
250kHz5mΩ
16V
16V
Vin range: 3V~4.35V
Isat=18A
1-Cell step-up to 9V
1.5µH
4*22µF 4*22µF
3A
250kHz5mΩ
Vin range: 3V~4.35V
Isat=18A
25V
25V
1-Cell step-up to 12V
1.5µH
4*22µF 4*22µF
2A
250kHz5mΩ
Vin range: 3V~4.35V
Isat=18A
25V
25V
2-Cell step-up to 12V
1.5µH
4*22µF 4*22µF
2A
250kHz5mΩ
Vin range: 6V~8.40V
Isat=18A
25V
25V

Rc
1kΩ
1kΩ
1kΩ
1kΩ

Cc and Cc1
Cc=47nF
Cc1=220pF
Cc=47nF
Cc1=220pF
Cc=47nF
Cc1=220pF
Cc=47nF
Cc1=220pF■Thermal information
Implementation of integrated circuits in low-profile and fine-pitch surface-mount packages typically requires
special attention to power dissipation. Many system-dependent issues such as thermal coupling, airflow, added
heat sinks and convection surfaces, and the presence of other heat-generating components affect the powerdissipation limits of a given component.
Three basic approaches for enhancing thermal performance are listed below:• Improve the power dissipation capability of the PCB design• Improve the thermal coupling of the component to the PCB• Introducing airflow in the system

Rev.1.0
12 / 15

ELM623FA 20A Fully Integrated Synchronous Boost Converter
http://www.elm-tech.com
The maximum junction temperature (Tj) of the ELM623FA device is 160°C and worse case won’t excess 145°C,
The thermal resistance of the 20-pin QFN package is θja = 50°C/W, if the Exposed PAD is soldered. Specified
regulator operation is assured to a maximum ambient temperature Ta of +85°C. Therefore, the maximum power
dissipation for the 20-pin QFN package it is about 1W. More power can be dissipated if the maximum ambient
temperature of the application is lower.
Pd(max) =Tj(max) - Ta 145°C - 85°C=
= 1.2Wθja
50°C/W(Equation 11)■Layout consideration
For designing ELM623FA a boost power supply, especially those operating at high output voltage and current
application, PCB layout is a very important in design step. To prevent radiation of high frequency noise (for example, EMI), proper layout of the high-frequency switching path is essential. Minimize the length and area of
all traces connected to the SW pin to reducing the high frequency noise of coupling to GND plane to cause EMI
or power system unstable.
Check the following layout rules:
1. Put the input capacitors GND, output capacitors GND and the PGND of ELM623FA in the same of power
plane to reduce impedance to avoid EMI and increase efficiency of power system.
2. The GND and PGND kept separate and used dot short skill to avoid noise coupling to GND to cause powersystem unstable as ELM623FA EV board PCB design.
3. ISP and ISN trace routing like a differential pair to shielding each other to filter the common mode noise. And
far away the SW trace to avoid to be coupled that will cause the current limit protection circuit fault. Ensure
accurate current sensing with Kelvin connections at the sense resistor or the DCR of inductor.
4. Keep the switching node (SW and PGND) and boost node (BST) away from sensitive small-signal nodes.

■Application schematic• 1-Cell Application:
VOUT

R4
4.7R

C1
1μF

CSS
3.3nF

VBAT

VBAT
3V to 4.35V

VH

ISP

SS
ISN

D1 RB751

R9 270K
Cs 47nF Rs 1K

Cp 220pF
R2 14.7K
CFF
1nF

VCC

SW

FREQ

BST

COMP

C3
180pF

L
2.2μH

CBS 0.1μF

VOUT
12V/2A

OUT
EN

FB
R1
RFF
133K
300K

CIN
22μFx4

ELM623FA(Vin<5.5V, Optional)

C5 4.7μF

RSENSE
5m

REN1 200K

GND
PGND

REN2
150K

VBAT

COUT
22μFx4
CFF
1nF

RFF
300K

Current sensing resistor
Rev.1.0
13 / 15

ELM623FA 20A Fully Integrated Synchronous Boost Converter
http://www.elm-tech.com
VOUT

R4
4.7R

C1
1μF

ISN

ELM623FA

D1 RB751(Vin<5.5V, Optional)

C5 4.7μF
R9 270K
Cs 47nF Rs 1K
REN1 200K

FREQ

BST

RLS1
200

CBS 0.1μF

VOUT
12V/2AOUT
REN1 200K
REN2
150K

GND
PGND

FB

R1
RFF
133K
300K

CIN
22μFx4

L
1.5μH

C3
180pF

EN

R2 14.7K
CFF
1nF

SW

COMP

Cp 220pF

REN2
150K

VCC

VBAT
3V to 4.35V

CLS RLS2
2.2uF 100

ISP

SS

CSS
3.3nF

VBAT

VH

VBAT

COUT
22μFx4Current sensing DCR of inductor• 2-Cell Application:
VBAT

R4
4.7R

C1
1μF

VH

ISP

SS

CSS
3.3nF

ISN

ELM623FAC5 4.7μF

R9 270K
Cs 47nF Rs 1K

SW

FREQ

BST

COMP

Cp 220pF

FB

R1
RFF
133K
300K

CIN
22μFx4

L
1.5μH

C3
180pF

RLS1
200
CBS 0.1μF
VOUT
12V/2AOUT
EN

R2 14.7K

CFF
1nF

VCC

VBAT
6V to 8.4V

CLS RLS2
2.2uF 100

REN1 200K

GND
PGND

REN2
150K

VBAT
COUT
22μFx4Current sensing DCR of inductor■Marking
Mark

Content

MT5087a to eProduct code
Lot No.

Rev.1.0
14 / 15

ELM623FA 20A Fully Integrated Synchronous Boost Converter
http://www.elm-tech.com

EVB BOM List
Qty
4

Ref
Cin

Value
22µF

Description
Ceramic capacitor, 16V, X5R

Package
0805

4

Cout

22µF

Ceramic capacitor, 30V, X5R

0805

1

L

1.5µH for L sense
2.2µH for R sense

MHC106030-1R5M-R8, 15mΩ, 18A(SAT), 9.5A(RMS)
MHC106030-2R2M-R8, 20mΩ, 14A(SAT), 8.5A(RMS)

SMD

1

R1

133kΩ

Resistor, ±1%0603

Resistor, ±1%0603

Resistor, ±5%
Resistor, ±5%

0603
0603

1

R2

Vout=5V
Vout=9V
Vout=12V
Vout=20V

42.2kΩ
20.5kΩ
14.7kΩ

1
1

Rff
R4

8.45kΩ
300kΩ
4.7Ω

1

Rs

1kΩ

Resistor, ±1%

0603

1
1

Ren1
Ren1

156kΩ
200kΩ

Resistor, ±5%
Resistor, ±5%

0603
0603

1
1
1

Ren2
R9
Rsense

150kΩ
270kΩ
5mΩ

Resistor, ±5%
Resistor, ±5%
Resistor, ±1%

0603
0603
1206

1
1
1

Rls1
Rls2
C1

200Ω
100Ω
1µF

Resistor, ±1%
Resistor, ±1%
Ceramic capacitor, 10V, X5R

0603
0603
0603

1
1

C3
C5

180pF
4.7µF

Ceramic capacitor, 10V, X5R
Ceramic capacitor, 10V, X5R

0603
0603

1
1
1

Cff
Css
Cbs

1nF
3.3nF
0.1µF

Ceramic capacitor, 10V, X5R
Ceramic capacitor, 10V, X5R
Ceramic capacitor, 10V, X5R

0603
0603
0603

1
1
1

Cls
CS
CP

2.2µF
47nF
220pF

Ceramic capacitor, 10V, X5R
Ceramic capacitor, 10V, X5R
Ceramic capacitor, 10V, X5R

0603
0603
0603

1
1

D1
Power IC

RB751
ELM623FA

Diode, RB751
Boost DC/DC converter

QFN20-4x4

Rev.1.0
15 / 15