Datasheet

FB
0.8V
Vo
+
-
TPS65250
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 )
)
TPS65250
SLVSAA3C –JUNE 2010–REVISED OCTOBER 2012 www.ti.com
Out-of-Phase Operation
Buck 1 has a low conduction resistance compared to Buck 2 and 3. Normally buck 1 is used to drive higher
system loads. Buck 2 and 3 are used to drive some peripheral loads like I/O and line drivers . The combination of
buck 2 and 3’s loads may be on par with Buck 1’s. In order to reduce input ripple current, buck 2 operates in
phase with buck 3; buck 1 and buck 2 operate 180 degrees out-of-phase. This enables the system, having less
input ripple, to lower component cost, save board space and reduce EMI.
Delayed Start-Up
If a delayed start-up is required on any of the buck converters fit a ceramic capacitor to the ENx pins. The delay
added is ~1.67 ms per nF connected to the pin. Note that the EN pins have a weak 1-MΩ pull-up to the 3V3 rail.
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 31. Voltage Divider Circuit
Error Amplifier
The device has a transconductance error amplifier. The transconductance of the error amplifier is 130 µA/V
during normal operation. The frequency compensation network is connected between the COMP pin and ground.
Loop Compensation
TPS65250 is a current mode control dc/dc converter. The error amplifier is a 130-µA/V transconductance
amplifier. A type-II compensation circuit is adequate for the converter to have a phase margin between 60 and 90
degrees.
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