Datasheet

FB
0.8V
Vo
+
-
TPS65251
R1
R2
0.8
2 1
0.8
O
V
R R
V V
æ ö
= ×
ç ÷
-
è ø
T ms V V
ss REF
( ) = ( ) ·
(
C nF
ss
( )
-
I A
ss
(µ )
)
TPS65251
SLVSAA4D –JUNE 2010–REVISED DECEMBER 2012 www.ti.com
Soft Start Time
The device has an internal pull-up current source of 5 µA that charges an external slow start capacitor to
implement a slow start time. Equation 2 shows how to select a slow start capacitor based on an expected slow
start time. The voltage reference (V
REF
) is 0.8 V and the slow start charge current (I
ss
) is 5 µA. The soft start
circuit requires 1 nF per 200 µS to be connected at the SS pin. A 1-ms soft-start time is implemented for all
converters fitting 4.7 nF to the relevant pins.
(2)
Adjusting the Output Voltage
The output voltage is set with a resistor divider from the output node to the FB pin. It is recommended to use 1%
tolerance or better divider resistors. In order to improve efficiency at light load, start with 40.2 kΩ for the R1
resistor and use the Equation 3 to calculate R2.
(3)
Figure 22. Voltage Divider Circuit
Input Capacitor
Use 10-µF X7R/X5R ceramic capacitors at the input of the converter inputs. These capacitors should be
connected as close as physically possible to the input pins of the converters.
Bootstrap Capacitor
The device has three integrated boot regulators and requires a small ceramic capacitor between the BST and LX
pin to provide the gate drive voltage for the high side MOSFET. The value of the ceramic capacitor should be
0.047 µF. A ceramic capacitor with an X7R or X5R grade dielectric is recommended because of the stable
characteristics over temperature and voltage.
Error Amplifier
The device has a transconductance error amplifier. The frequency compensation network is connected between
the COMP pin and ground.
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