Texas Instruments LM2745-19A User Manual
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
All trademarks are the property of their respective owners.
SNVA213A–May 2007–Revised May 2013 AN-1573 LM2745-19A Demonstration Board
Submit Documentation Feedback
1
Copyright © 2007–2013, Texas Instruments Incorporated
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
www.ti.com
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
Copyright © 2007–2013, Texas Instruments Incorporated
Submit Documentation Feedback
www.ti.com
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,
SNVA213A–May 2007–Revised May 2013 AN-1573 LM2745-19A Demonstration Board
Submit Documentation Feedback
3
Copyright © 2007–2013, Texas Instruments Incorporated
Switch Node Voltage and Output Voltage Ripple
www.ti.com
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
AN-1573 LM2745-19A Demonstration Board SNVA213A–May 2007–Revised May 2013
Copyright © 2007–2013, Texas Instruments Incorporated
Submit Documentation Feedback
Loading...