Datasheet

APPLICATION INFORMATION
Programming the TPS78101 Adjustable LDO
APPLICATION EXAMPLES
V =V 1+´
OUT FB
R
R
1
2
()
(1)
R
1 2
= 1 R- ´
V
V
OUT
FB
()
(2)
TPS781
GND
EN
V
SET
IN OUT
V
IN
V
OUT
1 Fm
1 Fm
4.2Vto5.5V 2.2Vto3.3V
On
Off
V High=V
SET OUT(LOW)
V Low=V
SET OUT(HIGH)
TPS78101
GND
EN
IN OUT
V
IN
V
OUT
1 Fm
1 Fm
FB
R
1
R
2
V =V (1+)´
OUT FB
R
R
1
2
TPS781 Series
www.ti.com ........................................................................................................................................................... SBVS102B – MARCH 2008 – REVISED MAY 2008
Regulator
The TPS781 series of LDOs typically take less than
The output voltage of the TPS78101 adjustable
800 µ s to transition from a lower voltage of 2.2V to a
regulator is programmed using an external resistor
higher voltage of 3.3V under an output load of
divider as shown in Figure 49 . The output voltage
150mA; see Figure 45 . Additionally, the TPS781
operating range is 1.2V to 5.1V, and is calculated
series contain active pull-down circuitry that
using Equation 1 :
automatically pulls charge out of the voltage capacitor
to transition the output voltage from the higher
voltage to the lower voltage, even with no load
connected. Output voltage overshoots and
Where:
undershoots are minimal under this load condition.
V
FB
= 1.216V typ (the internal reference voltage)
The TPS781 series typically take less than 800 µ s to
transition from V
SET
low (3.3V to 2.2V), or V
SET
high
Resistors R
1
and R
2
should be chosen for
(2.2V to 3.3V); see Figure 45 and Figure 46 . Both
approximately 1.2 µ A divider current. Lower value
output states of the TPS781 series are
resistors can be used for improved noise
factory-programmable between 1.5V to 4.2V. Note
performance, but the solution consumes more power.
that during startup or steady-state conditions, it is
Higher resistor values should be avoided as leakage
important that the EN pin and V
SET
pin voltages never
current into/out of FB across R
1
/R
2
creates an offset
exceed V
IN
+ 0.3V.
voltage that artificially increases/decreases the
feedback voltage and thus erroneously
decreases/increases V
OUT
. Table 2 lists several
common output voltages and resistor values. The
recommended design procedure is to choose R
2
=
1M Ω to set the divider current at 1.2 µ A, and then
calculate R
1
using Equation 2 :
Figure 48. Typical Application Circuit
The TPS781 series is also used effectively in
dynamic voltage scaling (DVS) applications. DVS
applications are required to dynamically switch
between a high operational voltage to a low standby
voltage in order to reduce power consumption.
Modern multimillion gate microprocessors fabricated
with the latest sub-micron processes save power by
transitioning to a lower voltage to reduce leakage
currents while maintaining content. This architecture
Figure 49. TPS78101 Adjustable LDO Regulator
enables the microprocessor to transition quickly into
Programming
an operational state (wake up) without requiring a
reload of the states from external memory, or a
reboot.
Table 2. Output Voltage Programming Guide
OUTPUT VOLTAGE R
1
R
2
1.8V 0.499M Ω 1M Ω
2.8V 1.33M Ω 1M Ω
5.0V 3.16M Ω 1M Ω
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