TPS2145, TPS2147

TPS2155, TPS2157

SLVS333 − AUGUST 2001

16

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APPLICATION INFORMATION

programming the adjustable LDO regulator

The output voltage of the TPS2145 adjustable regulator is programmed using an external resistor divider as

shown in Figure 18. The output voltage is calculated using:

LDO_OUT

+ V

R1

R2

Resistors R1 and R2 should be chosen for approximately 4-

µA (minimum) divider current. Lower value resistors

can be used but offer no inherent advantage and waste more power. Higher values should be avoided as a

minimum load is required to sink the LDO forward leakage and maintain regulation. The recommended design

procedure is to choose R2 = 200 k

Ω to set the divider current at 4-µA and then solve the LDO_OUT equation

for R1.

R1

R2

Cf

0.1

µF

10

µF

LDO_OUT

LDO_ADJ

VIN/SWIN1

LDO_EN

GND

4.7

µF

0.1

µF

TPS2145

Figure 18. External Resistor Divider

OUTPUT VOLTAGE PROGRAMMING GUIDE

OUTPUT VOLTAGE

R1

R2

Cfb

3.3

625 k

Ω

200 k

Ω

NR†

3.0

550 k

Ω

200 k

Ω

NR†

2.5

425 k

Ω

200 k

Ω

2 pf

1.8

250 k

Ω

200 k

Ω

6 pf

1.5

175 k

Ω

200 k

Ω

10.3 pf

1.0

50 k

Ω

200 k

Ω

46 pf

† NR − Not required

overcurrent

A sense FET is used to measure current through the device. Unlike current-sense resistors, sense FETs do not

increase the series resistance of the current path. When an overcurrent condition is detected, the device

maintains a constant output current. Complete shutdown occurs only if the fault is present long enough to

activate thermal limiting.

Three possible overload conditions can occur. In the first condition, the output is shorted before the device is

enabled or before VIN has been applied. The TPS2145 and TPS2147 sense the short and immediately switch

to a constant-current output.

In the second condition, the short occurs while the device is enabled. At the instant the short occurs, very high

currents may flow for a very short time before the current-limit circuit can react. After the current-limit circuit has

tripped (reached the overcurrent trip threshold), the device switches into constant-current mode.

(2)