dv

31.9 V /ms

dt

=

dv

150 mA

4.7

F

dt

=

m

´

dv

Output slew rate

dt

=

dv

Inrush Current

C

dt

=

´

TPS22932B

SLVS802C – AUGUST 2009 – REVISED MAY 2015

www.ti.com

10.2.2 Detailed Design Procedure

10.2.2.1 VIN to VOUT Voltage Drop

The VIN to VOUT voltage drop in the device is determined by the rON of the device and the load current. The rON

of the device depends upon the VIN condition of the device. Refer to the rON specification of the device in the

Electrical Characteristics table of this data sheet. When the rON of the device is determined based upon the VIN

conditions, use Equation 1 to calculate the VIN to VOUT voltage drop:

ΔV = ILOAD × rON

where

•

ΔV = Voltage drop from VIN to VOUT

•

ILOAD = Load current

•

rON = ON-resistance of the device for a specific VIN

•

An appropriate ILOAD must be chosen such that the IMAX specification of the device is not violated.

(1)

10.2.2.2 Managing Inrush Current

When the switch is enabled, the output capacitors must be charged up from 0-V to VIN. This charge arrives in the

form of inrush current. Inrush current can be calculated using the following equation:

where

•

C = Output capacitance

•

(2)

The TPS22932B offers a very slow controlled rise time for minimizing inrush current. This device can be selected

based upon the maximum acceptable slew rate which can be calculated using the design requirements and the

inrush current equation. An output capacitance of 4.7

μF will be used because the amount of inrush increases

with output capacitance:

(3)

(4)

To ensure an inrush current of less than 150 mA, a device with a slew rate less than 31.9 V/ms must be used.

The TPS22932B has a typical rise time of 145

μs at 3.3 V. This results in a slew rate of 22.8 V/ms which meets

the requirement.

18

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