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OUT
FB
V
R = R x - 1
1 2
V
L
L
1
VIN
VOUT
FB
C
2
EN
GND
TPS6122x
V
OUT
R
1
R
2
V
IN
C
1
0
20
40
60
80
100
0.7
1.7 2.7
3.7
4.7
V - Input Voltage - V
I
h - Efficiency - %
I = 50 mA
O
I = 100 A
O
m
I = 10 mA
O
I = 1 mA
O
TPS61220, TPS61221, TPS61222
www.ti.com
SLVS776A –JANUARY 2009–REVISED APRIL 2014
Figure 25. TPS61222 Performance
10.2.2 Specific Application, Variable Output Voltage Supply
Figure 26. Application Circuit For Adjustable Output Voltage Option
10.2.2.1 Design Requirements
• Single 4.2 V output at up to 50 mA
• Power source, two AA alkaline cells
• Greater than 80% conversion efficiency
10.2.2.2 Detailed Design Procedure
The design procedure for this application is identical to that for the fixed-output supply except for programming
the output voltage.
10.2.2.2.1 Device Selection
This application example uses the TPS61220 so that the output voltage can be set at 4.2 V.
10.2.2.2.2 Programming The Output Voltage
In the adjustable output versions, an external resistor divider is used to adjust the output voltage. The resistor
divider must be connected between VOUT, FB and GND as shown in Figure 26. When the output voltage is
regulated properly, the typical voltage value at the FB pin is 500 mV for the adjustable devices. The maximum
recommended value for the output voltage is 6.0 V. The current through the resistor divider should be about 100
times greater than the current into the FB pin. The typical current into the FB pin is 0.01 μA, and the voltage
across the resistor between FB and GND, R
2
, is typically 500 mV. Based on those two values, the recommended
value for R
2
should be lower than 500 kΩ, in order to set the divider current to 1 μA or higher. The value of the
resistor connected between VOUT and FB, R
1
, depending on the needed output voltage (V
OUT
), can be
calculated using Equation 5:
(5)
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