Datasheet

 
 
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
ref
ǒ
1)
R1
R2
Ǔ
where V
ref
= 0.8 V typical (internal reference voltage).
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 kΩ to set the divider current at 4-µA and then solve the LDO_OUT equation
for R1.
R1
R2
C
f
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 kΩ 200 kΩ NR
†
3.0 550 kΩ 200 kΩ NR
†
2.5 425 kΩ 200 kΩ 2 pf
1.8 250 kΩ 200 kΩ 6 pf
1.5 175 kΩ 200 kΩ 10.3 pf
1.0 50 kΩ 200 kΩ 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)