Datasheet

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PROGRAMMING THE TPS79401
C
1
+
(3 10
*7
) (R
1
) R
2
)
(R
1
R
2
)
(3)
V
OUT
+ V
REF
ǒ
1 )
R
1
R
2
Ǔ
(1)
REGULATOR PROTECTION
R
1
+
ǒ
V
OUT
V
REF
* 1
Ǔ
R
2
(2)
OUTPUT VOLTAGE
PROGRAMMING GUIDE
OUTPUT
VOLTAGE
R
1
R
2
C
1
1.8 V
3.6 V
14.0 kΩ
61.9 kΩ
30.1 kΩ
30.1 kΩ
22 pF
15 pF
TPS79401
GND
FB
IN OUT
EN
V
IN
V
OUT
1
µ
F
2.2
µ
F
R
1
R
2
C
1
TPS794xx
SLVS349E – NOVEMBER 2001 – REVISED DECEMBER 2005
In order to improve the stability of the adjustable
ADJUSTABLE LDO REGULATOR version, it is suggested that a small compensation
capacitor be placed between OUT and FB.
The output voltage of the TPS79401 adjustable
regulator is programmed using an external resistor The approximate value of this capacitor can be
divider as shown in Figure 23 . The output voltage is calculated as Equation 3 :
calculated using Equation 1 :
The suggested value of this capacitor for several
resistor ratios is shown in the table within Figure 23 .
where:
If this capacitor is not used (such as in a unity-gain
• V
REF
= 1.2246 V typ (the internal reference
configuration), then the minimum recommended
voltage)
output capacitor is 2.2 µ F instead of 1 µ F.
Resistors R
1
and R
2
should be chosen for
approximately 40- µ A divider current. Lower value
resistors can be used for improved noise
The TPS794xx PMOS-pass transistor has a built-in
performance, but the device wastes more power.
back diode that conducts reverse current when the
Higher values should be avoided, as leakage current
input voltage drops below the output voltage (for
at FB increases the output voltage error.
example, during power down). Current is conducted
from the output to the input and is not internally
The recommended design procedure is to choose
limited. If extended reverse voltage operation is
R
2
= 30.1 k Ω to set the divider current at 40 µ A,
anticipated, external limiting might be appropriate.
C
1
= 15 pF for stability, and then calculate R
1
using
Equation 2 :
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.
Figure 23. TPS79401 Adjustable LDO Regulator Programming
9
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