SJ1 SJ1
Connects SoftStart Capacitor
Connects Tracking
Resistors
AN-1573 LM2745-19A Demonstration Board
1 Introduction
This application notes describes the LM2745 printed circuit board (PCB) design and provides an example
typical application circuit. The LM2745 is a voltage mode PWM buck controller which implements
synchronous rectification. It provides a low cost, high power density, and efficient point of load solution. In
steady state operation the LM2745 is always synchronous, even at no load, thus simplifying the
compensation design. The LM2745 ensures a smooth and controlled start-up when the output is prebiased. The current limit protection does not require a current limit resistor in the power path, but is
achieved by sensing the voltage VDSacross the low side MOSFET. Though the control sections of the IC
are rated for 3 to 6V (VCC), the driver sections are designed to accept input supply rails (VIN) as high as
14V.
2 Specifics Of The Board
This demo board targets the fixed and mobile telecommunications, industrial electronics, and distributed
power markets. The demo board has a VINrange of 8V to 14V and a LDO regulator, the LP2937, powers
VCCby regulating a 5V output voltage. The LM2745 regulates to an output range of 1.2V to 3.3V at 19A
with a switching frequency of 300 kHz. Note, the demo board is optimized for the above parameters, thus
for additional design modifications refer to the Design Consideration section of the LM2745 data sheet.
The PCB is designed on four layers, the top and bottom layers are 2oz. copper and the two inner layers
are 1oz. copper. The board measures 2.19 in. x 1.03 in. x 0.41 in. (56 mm x 26.2 mm x 10.3 mm) (l, w, h)
on a FR4 laminate.
User's Guide
SNVA213A–May 2007–Revised May 2013
3 Feature Options
When the tracking feature of the LM2745 is required for use, remove the jumper that connects the softstart capacitor C10 and connect the resistor divider, on designators R13 and R14 (see Figure 1). The
Track terminal has been provided for your connecting convenience. The demo board is synchronize ready,
just connect an external clock to the SYNC terminal. Note: increasing the switching frequency results in a
lower inductor current ripple and input and output voltage ripple (if the component values are kept the
same). Monitor the MOSFET junction temperature since switching losses will increase, and do not exceed
the maximum junction temperature of the MOSFET. Refer to the MOSFET manufacturer datasheet for
maximum junction temperature specification and heat sinking guidelines.
Figure 1. Soft-Start and Tracking Jumper
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Specification Summary
4 Specification Summary
• Space saving footprint
• Wide ambient temperature range: -40 ºC to 65 ºC
• Input voltage range: 8V to 14V
• Adjustable output voltage: 1.2V to 3.3V
• No minimum load requirement
• Remote ON/OFF
• Power good signal
• Fixed switching frequency: 300 kHz
• Switching frequency synchronize range 250 kHz to 1 MHz
• Current Limit Protection
• Master power supply start-up tracking function
• Start-up with a pre-biased output load
• Adjustable soft-start
• Small size 2.19 in. x 1.03 in. x 0.41 in. (56 mm x 26.2 mm x 10.3 mm)
5 Performance Characteristics
Efficiency
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2
AN-1573 LM2745-19A Demonstration Board SNVA213A–May 2007–Revised May 2013
Figure 2. Efficiency vs. Load Current
V
= 3.3V, fSW= 300 kHz
OUT
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6 Switch Node Voltage and Output Voltage Ripple
Switch Node Voltage and Output Voltage Ripple
Figure 3. VIN= 8V, V
I
= 100 mA, fSW= 300 kHz
LOAD
OUT
20 MHz Bandwidth Limit
Figure 4. VIN= 8V, V
I
= 19A, fSW= 300 kHz
LOAD
OUT
20 MHz Bandwidth Limit
= 3.3V,
= 3.3V,
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Switch Node Voltage and Output Voltage Ripple
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Figure 5. VIN= 14V, V
I
= 100 mA, fSW= 300 kHz
LOAD
20 MHz Bandwidth Limit
Figure 6. VIN= 14V, V
I
= 19A, fSW= 300 kHz
LOAD
20 MHz Bandwidth Limit
OUT
OUT
= 3.3V,
= 3.3V,
4
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100 µs/DIV
50 mV/DIV
5A/DIV
100 µs/DIV
50 mV/DIV
2A/DIV
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7 Load Transient Response
Load Transient Response
Figure 7. VIN= 14V, V
I
= 2A to 10A
LOAD
CH1: V
, CH2: I
OUT
Figure 8. VIN= 14V, V
I
= 11A to 19A
LOAD
CH1: V
, CH2: I
OUT
OUT
LOAD
OUT
LOAD
= 3.3V
= 3.3V
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100 µs/DIV
100 mV/DIV
5A/DIV
100 µs/DIV
100 mV/DIV
2A/DIV
Load Transient Response
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Figure 9. VIN= 8V, V
I
= 2A to 10A
LOAD
CH1: V
, CH2: I
OUT
Figure 10. VIN= 8V, V
I
= 11A to 19A
LOAD
CH1: V
, CH2: I
OUT
OUT
OUT
= 3.3V
LOAD
= 3.3V
LOAD
6
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8 Bill of Materials
Table 1. Bill of Materials for LM2745 POL EVB (Vin: 8V to 14V, Vout:3.3V, 19A)
Locatio Part Number Type Size Parameters Qty Vendo
n r
U1 LM2745 Syn. Buck Controller TSSOP-14 Vin: 4.5V - 5.5V 1 NSC
U2 LM27937IMP-5.0 Linear Regulator 5V,500mA SOT-223 5V, 500mA 1 NSC
L1 SER2010-202ML Inductor 2uH, 27A, 0.852mohm 1 Coilcraf
Q1-2 IRF6633 N-MOSFET DirectFET-MP 20V, 16A, 4.1mohm, 2 Vishay
Q3 IRF6609 N-MOSFET DirectFET-MT 20V, 150A, 2mohm, 1 Vishay
D1 SL22-E3/2C Schottky Diode SMB 20V, 2A 1 Vishay
D2 MBRS0520 Schottky Diode SOD123 20V, 0.5A 1 Vishay
C1, 2 GRM32ER61C226KE20L Ceramic Capacitor 1210 22µF, 25V, X7R, 5% 2 Murata
C3, 11 GRM319R71H104KA01B Ceramic Capacitor 1206 100nF, 25V, X7R, 10% 2 Murata
C14 GRM319R71H474KA01B Ceramic Capacitor 1206 470nF, 25V, X7R, 10% 1 Murata
C9 GRM1885C1H1210JA01 Ceramic Capacitor 0603 120pF, 50V, C0G, 5% 1 Murata
C6 GRM188R71H222KA01 Ceramic Capacitor 0603 2.2nF, 50V, X7R, 10% 1 Murata
C7 GRM1885C1H101JA01 Ceramic Capacitor 0603 100pF, 50V, C0G, 5% 1 Murata
C8 GRM188R71H332KA01 Ceramic Capacitor 0603 3.3nF, 50V, X7R, 10% 1 Murata
C10 GRM188R71H153KA01 Ceramic Capacitor 0603 15nF, 50V, X7R, 10% 1 Murata
C12 GRM40X7R472K25 Ceramic Capacitor 0805 470nF, 25V, X7R, 10% 1 Murata
C13 12066D226MAT Ceramic Capacitor 1206 22µF, 6.3V, X5R, 20% 1 AVX
C4, 5 6SVPC220M OS-CON C6 220µF, 6.3V, 20% 2 Sanyo
R1,18 CRCW06030R00F Chip Resistor 0603 0ohm 2 Vishay
R2 CRCW06032151F Chip Resistor 0603 2.15k, 1% 1 Vishay
R3, R17 CRCW06031002F Chip Resistor 0603 10k, 1% 2 Vishay
R4 CRCW06032211F Chip Resistor 0603 2.21k, 1% 1 Vishay
R10, 11 CRCW06031003F Chip Resistor 0603 100k, 1% 2 Vishay
R5, 6, 8 CRCW06032R21F Chip Resistor 0603 2.21ohm, 1% 3 Vishay
R7, 9 CRCW06034702F Chip Resistor 0603 47k, 1% 2 Vishay
R12 CRCW06038451F Chip Resistor 0603 8.45k, 1% 1 Vishay
R15 CRCW06031821F Chip Resistor 0603 1.82k, 1% 1 Vishay
R16 CRCW060310R0F Chip Resistor 0603 10ohm, 1% 1 Vishay
- - F-Pin 8
- - Pin 90 deg., SIP, 6 way Pitch: 2.54mm 1
Bill of Materials
t
11nC
46nC
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R3
C8
R12
R15
C10
R10
C11
D2
C14
C2
L1
+
C9
R4
C5
R2
C6
R17
+
C4
C1
R11
Q1
R16
+
C3
VIN
Q3
HG
BOO
T
LG
PGND
FB
PWGD
FREQ/SYNC
SS/TRACK
SGND
EAO
PGND
LM2745
V
OUT
LM2937
V
IN
C13
V
OUT
GND
C12
D1
Q2
SD
R6
R5
R7
R8
R9
R18
U1
U2
C7
SYNC
R14
R13
TRACK
SJ2
PWGD
SD
TRIM
SENSE
SJ1
V
CC
I
SEN
PCB Layout
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9 PCB Layout
Figure 11. 300 kHz Demo Board Schematic
Figure 12. Top Silkscreen
8
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PCB Layout
Figure 13. Top Copper Layer
Figure 14. Bottom Silkscreen
Figure 15. Bottom Copper Layer
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PCB Layout
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Figure 16. Internal Layer-1 (GND Copper)
Figure 17. Internal Layer-2 (GND Copper)
10
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