Datasheet

+
-
+
V
RAMP
+
-
V
REF
+
-
R
FB2
R
FB1
C
C1
C
C2
R
C1
R
C2
C
C3
L
R
L
C
O
R
O
V
IN
R
C
+
-
LM2745, LM2748
SNOSAL2E –APRIL 2005–REVISED APRIL 2013 www.ti.com
Support Components
C
IN
2 - A small (0.1 to 1 µF) ceramic capacitor should be placed as close as possible to the drain of the high-side
MOSFET and source of the low-side MOSFET (dual MOSFETs make this easy). This capacitor should be X5R
type dielectric or better.
R
CC
, C
CC
- These are standard filter components designed to ensure smooth DC voltage for the chip supply. R
CC
should be 1 to 10Ω. C
CC
should 1 µF, X5R type or better.
C
BOOT
- Bootstrap capacitor, typically 100 nF.
R
PULL-UP
– This is a standard pull-up resistor for the open-drain power good signal (PWGD). The recommended
value is 100 kΩ connected to V
CC
. If this feature is not necessary, the resistor can be omitted.
D
1
- A small Schottky diode should be used for the bootstrap. It allows for a minimum drop for both high and low-
side drivers. The MBR0520 or BAT54 work well in most designs.
R
CS
- Resistor used to set the current limit. Since the design calls for a peak current magnitude (I
OUT
+ (0.5 x
ΔI
OUT
)) of 4.8A, a safe setting would be 6A. (This is below the saturation current of the output inductor, which is
7A.) Following the equation from the Current Limit section, a 1.3 kΩ resistor should be used.
R
FADJ
- This resistor is used to set the switching frequency of the chip. The resistor value is approximated from
the Frequency vs Frequency Adjust Resistor curve in the Typical Performance Characteristics section. For 300
kHz operation, a 100 kΩ resistor should be used.
C
SS
- The soft-start capacitor depends on the user requirements and is calculated based on the equation given in
the section titled START UP/SOFT-START. Therefore, for a 7 ms delay, a 12 nF capacitor is suitable.
Control Loop Compensation
The LM2745/8 uses voltage-mode (‘VM’) PWM control to correct changes in output voltage due to line and load
transients. VM requires careful small signal compensation of the control loop for achieving high bandwidth and
good phase margin.
The control loop is comprised of two parts. The first is the power stage, which consists of the duty cycle
modulator, output inductor, output capacitor, and load. The second part is the error amplifier, which for the
LM2745/8 is a 9 MHz op-amp used in the classic inverting configuration. Figure 29 shows the regulator and
control loop components.
Figure 29. Power Stage and Error Amp
One popular method for selecting the compensation components is to create Bode plots of gain and phase for
the power stage and error amplifier. Combined, they make the overall bandwidth and phase margin of the
regulator easy to see. Software tools such as Excel, MathCAD, and Matlab are useful for showing how changes
in compensation or the power stage affect system gain and phase.
20 Submit Documentation Feedback Copyright © 2005–2013, Texas Instruments Incorporated
Product Folder Links: LM2745 LM2748