Datasheet

P
1
F =
2 L1x C1x´p
Ox
R1x
V = 0.765 V 1+
R2x
æ ö
´
ç ÷
è ø
VINx
12V ± 10%
PGND
1uF
PGND
C
4
SW1
VIN1
VBST1
EN1
VFB2
VFB1
GND
VREG5
PGND1
6
VIN2
VBST2
EN2
SW2
SS1
5
1
3
SS2
9
10
11
PGND2
2
4
7
13
12
TPS542951
HTSSOP16
14
8
15
16
PGND
SGND
SGND
C11
10 Fm
VO1
3.3 V
C21
22 F
x2
m
R11
73.2 kW
R21
22.1 kW
L11
H2.2 m
C31
0.1 Fm
C32
0.1 Fm
L12
1.5 Hm
C12
10 Fm
VO2
1.5 V
C22
22 F
x2
m
R12
21.5 kW
R22
22.1 kW
SGND
SGND
C42
0.01 Fm
C41
0.01 Fm
TPS542951
www.ti.com
SLVSBI4B JULY 2012REVISED OCTOBER 2013
DESIGN GUIDE
Step By Step Design Procedure
To begin the design process, you must know a few application parameters:
Input voltage range
Output voltage
Output current
In all formulas x is used to indicate that they are valid for both converters. For the calculations the estimated
switching frequency of 700 kHz is used.
Figure 24. Schematic Diagram for the Design Example
Output Voltage Resistors Selection
The output voltage is set with a resistor divider from the output node to the VFBx pin. It is recommended to use
1% tolerance or better divider resistors. Start by using Equation 3 to calculate V
Ox
.
To improve the efficiency at very light loads consider using larger value resistors, but too high resistance values
will be more susceptible to noise and voltage errors due to the VFBx input current will be more noticeable.
(3)
Output Filter Selection
The output filter used with the TPS542951 is an LC circuit. This LC filter has double pole at:
(4)
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