Datasheet

C
1
+
(3 10
*7
) (R
1
) R
2
)
(R
1
R
2
)
V
OUT
+ V
REF
ǒ
1 )
R
1
R
2
Ǔ
R
1
+
ǒ
V
OUT
V
REF
* 1
Ǔ
R
2
TPS79501
GND
FB
IN OUT
EN
V
IN
V
OUT
R
1
C
1
R
2
1 Fm
1 Fm
OUTPUTVOLTAGE
PROGRAMMINGGUIDE
1.8V
3.6V
14.0kW
57.9kW 30.1kW
30.1kW
33pF
15pF
OUTPUT
VOLTAGE
R
1
R
2
C
1
TPS795xx
SLVS350H –OCTOBER 2002–REVISED AUGUST 2010 www.ti.com
PROGRAMMING THE TPS79501 The approximate value of this capacitor can be
ADJUSTABLE LDO REGULATOR calculated as Equation 3:
The output voltage of the TPS79501 adjustable
regulator is programmed using an external resistor
(3)
divider as shown in Figure 24. The output voltage is
The suggested value of this capacitor for several
calculated using Equation 1:
resistor ratios is shown in the table within Figure 24.
If this capacitor is not used (such as in a unity-gain
configuration), then the minimum recommended
(1)
output capacitor is 2.2 mF instead of 1 mF.
where:
REGULATOR PROTECTION
• V
REF
= 1.2246 V typ (the internal reference
voltage)
The TPS795xx PMOS-pass transistor has a built-in
back diode that conducts reverse current when the
Resistors R
1
and R
2
should be chosen for
input voltage drops below the output voltage (for
approximately 40-mA divider current. Lower value
example, during power down). Current is conducted
resistors can be used for improved noise
from the output to the input and is not internally
performance, but the device wastes more power.
limited. If extended reverse voltage operation is
Higher values should be avoided, as leakage current
anticipated, external limiting might be appropriate.
at FB increases the output voltage error.
The TPS795xx features internal current limiting and
The recommended design procedure is to choose
thermal protection. During normal operation, the
R
2
= 30.1 kΩ to set the divider current at 40 mA,
TPS795xx limits output current to approximately 2.8
C
1
= 15 pF for stability, and then calculate R
1
using
A. When current limiting engages, the output voltage
Equation 2:
scales back linearly until the overcurrent condition
ends. While current limiting is designed to prevent
gross device failure, care should be taken not to
(2)
exceed the power dissipation ratings of the package.
In order to improve the stability of the adjustable
If the temperature of the device exceeds
version, it is suggested that a small compensation
approximately +165°C, thermal protection circuitry
capacitor be placed between OUT and FB.
shuts it down. Once the device has cooled down to
below approximately +140°C, regulator operation
resumes.
Figure 24. TPS79501 Adjustable LDO Regulator Programming
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