TPS79425_15 Datasheet, PDF (9/23 Pages) Texas Instruments – ULTRALOW-NOISE, HIGH-PSRR, FAST, RF, 250-mA LOW-DROPOUT LINEAR REGULATORS

TPS794xx
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PROGRAMMING THE TPS79401
ADJUSTABLE LDO REGULATOR
The output voltage of the TPS79401 adjustable
regulator is programmed using an external resistor
divider as shown in Figure 23. The output voltage is
calculated using Equation 1:
ǒ Ǔ V_OUT + V_REF
1
)
R₁
R₂
(1)
where:
• V_REF = 1.2246 V typ (the internal reference
voltage)
Resistors R₁ and R₂ should be chosen for
approximately 40-µA divider current. Lower value
resistors can be used for improved noise
performance, but the device wastes more power.
Higher values should be avoided, as leakage current
at FB increases the output voltage error.
The recommended design procedure is to choose
R₂ = 30.1 kΩ to set the divider current at 40 µA,
C₁ = 15 pF for stability, and then calculate R₁ using
Equation 2:
ǒ Ǔ R₁ +
V_OUT
V_REF
*
1
R₂
(2)
SLVS349E – NOVEMBER 2001 – REVISED DECEMBER 2005
In order to improve the stability of the adjustable
version, it is suggested that a small compensation
capacitor be placed between OUT and FB.
The approximate value of this capacitor can be
calculated as Equation 3:
(3
C₁ +
10^*7) (R₁ ) R₂)
(R₁ R₂)
(3)
The suggested value of this capacitor for several
resistor ratios is shown in the table within Figure 23.
If this capacitor is not used (such as in a unity-gain
configuration), then the minimum recommended
output capacitor is 2.2 µF instead of 1 µF.
REGULATOR PROTECTION
The TPS794xx PMOS-pass transistor has a built-in
back diode that conducts reverse current when the
input voltage drops below the output voltage (for
example, during power down). Current is conducted
from the output to the input and is not internally
limited. If extended reverse voltage operation is
anticipated, external limiting might be appropriate.
The TPS794xx features internal current limiting and
thermal protection. During normal operation, the
TPS794xx limits output current to approximately
2.8 A. When current limiting engages, the output
voltage scales back linearly until the overcurrent
condition ends. While current limiting is designed to
prevent gross device failure, care should be taken
not to exceed the power dissipation ratings of the
package. If the temperature of the device exceeds
approximately 165°C, thermal-protection circuitry
shuts it down. Once the device has cooled down to
below approximately 140°C, regulator operation
resumes.
V_IN
IN
OUT
V_OUT
1 µF
TPS79401
EN
GND FB
R₁
C₁
2.2 µF
R₂
OUTPUT VOLTAGE
PROGRAMMING GUIDE
OUTPUT
VOLTAGE
1.8 V
R₁
14.0 kΩ
R₂
30.1 kΩ
3.6 V
61.9 kΩ 30.1 kΩ
C₁
22 pF
15 pF
Figure 23. TPS79401 Adjustable LDO Regulator Programming
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