© 2005 National Semiconductor Corporation
LM2744 Evaluation Board
Introduction
This application notes describes the LM2743 printed circuit
board (PCB) design and provides an example typical appli-
cation circuit. The demo board allows component design
flexibility in order to demonstrate the versatility of the
LM2744 IC.
The demo board contains a voltage-mode, high-speed syn-
chronous buck regulator controller with an external adjust-
able reference voltage between 0.5V and 1.5V. The demo
board design incorporates the LM4140 high precision low
noise reference IC providing 1.0V to the reference pin
(V
to 6V (V
supply rails (V
frequency, adjustable from 50 kHz to 1 MHz with one exter-
nal resistor.
The demo board design regulates to an output voltage of
1.2V at 3.5A with a switching frequency of 1MHz. Note, the
demo board is optimized for a 1MHz, 14V input voltage
compensation design; if another switching frequency and
input voltage is desired, please consult the LM2744 data
sheet for control loop compensation procedures. For addi-
tional design modifications refer to the Design Consideration
section of the LM2744 data sheet.
Additional Footprints
A Schottky diode footprint (D1) is available in parallel to the
low side MOSFET. This component can improve efficiency,
due to the lower forward drop than the low side MOSFET
body diode conducting during the anti-shoot through period.
Select a Schottky diode that maintains a forward drop
around 0.4 to 0.6V at the maximum load current (consult the
I-V curve). In addition select the reverse breakdown voltage
to have sufficient margin above the maximum input voltage.
Footprint C13 is available for a multilayer ceramic capacitor
(MLCC) connected as close as possible to the source of the
low side MOSFET and drain of the high side MOSFET. This
will provide low supply impedance to the high speed switch
currents, thus minimizing the input supply noise. For ex-
ample; a MLCC is used (C13) in combination with aluminum
REF
). Though the control sections of the IC are rated for 3
CC
), the driver sections are designed to accept input
IN
) as high as 14V. It operates at a fixed-
AN201343
National Semiconductor
Application Note 1357
Ricardo Capetillo
October 2005
electrolytic input filter capacitors, placed in designators C12
and C14, because MLCC have lower impedance than elec-
trolytics. If MLCCs are used in designators C12 and C14
component C13 is not necessary.
The PCB is designed on two layers with 1oz. copper on a
62mil FR4 laminate.
Guidelines for Additional Options
When using a DC power supply to set a reference voltage
(V
the DC power supply. A good starting point is 10µF, but may
need to be varied depending on the magnitude of the DC
power supply noise (any make of capacitor will do as long as
the capacitance is maintained within the operating tempera-
ture range). Remove R10 and place a zero ohm jumper in
designator R12 .
Designators R12 and R13 are provided for DDR SDRAM
(double data rate synchronous dynamic random access
memory) active termination design. Set V
supply voltage by using designators R12 and R13 as a
voltage divider. Remove resistors R7, R10 and capacitor
C21, and connect the DDR supply voltage rail to terminal
V
sink or source current in excess of 3A – a load transient
response applied to the output of Figure 2 is provided in
Figure 3.
Do not exceed 5.6V on the VCC pin of the demo board. The
board layout connects together both the input voltage of the
LM4140-1.0 (pin 2) and the control section of the LM2744
(VCC). The maximum DC supply voltage for the control
section of the LM2744 is 6V, while 5.6V is the maximum
rating for any input pin of the LM4140. If the design requires
the control section of the LM2744 to be 6V, a shunt zener
reference may be placed at designator location (D3) to main-
tain the input voltage of the LM4140 between 1.8V and 5.5V.
The cathode of the zener is connected to the input of the
LM4140 and the anode to GND. The resistance of R10 must
be selected to supply the appropriate amount of biasing
current into the zener and the LM4140 (refer to the Electrical
Characteristic table of the LM4140 data sheet).
DCS
REF
, refer to Figure 2. The modified circuit in Figure 1 can
), connect a capacitor (C20) from V
DCS
REF
to half the DDR
to GND to filter
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