TPS54394_15 Datasheet, PDF (13/30 Pages) Texas Instruments – 3A Dual Channel Synchronous Step-Down Switcher with Integrated FET

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DESIGN GUIDE
TPS54394
SLVSBE6B – JUNE 2012 – REVISED AUGUST 2013
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.
VINx
12V ± 10%
VO1
3.3 V
C11
10 mF
L11
2.2 mH
C21
22 mF
x2
PGND
1 VIN1
2 VBST1
C31
0.1 mF
3 SW1
4 PGND1
5 EN1
TPS54394
HTSSOP16
VIN2 16
VBST2 15
C32
0.1 mF
SW2 ¹⁴
PGND2 ¹³
EN2 12
L12
1.5 mH
C12
10 mF
VO2
1.5 V
C22
22 mF
x2
PGND
R11
72.3 kW
R21
22.1 kW
SGND
6 PG1
7 ^VFB1
8 GND
PG2 11
VFB2 10
C4 1uF
VREG5 ⁹
PGND
Figure 24. Schematic Diagram for the Design Example
R12
21.5 kW
R22
22.1 kW
SGND
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 2 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.
V_Ox
= 0.765 V
´
æ R1x ö
çè¹⁺ _R2x ÷ø
(2)
Output Filter Selection
The output filter used with the TPS54394 is an LC circuit. This LC filter has double pole at:
1
F_P =
2p L_OUT ´ C_OUT
(3)
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