Datasheet
APPLICATION INFORMATION
DESIGN PROCEDURE
Programming the Output Voltage
L
L
1
VIN
VOUT
FB
C
2
EN
GND
TPS6122x
fixedoutputvoltage
V
OUT
V
IN
C
1
æ ö
ç ÷
è ø
OUT
FB
V
R = R x - 1
1 2
V
(1)
L
L
1
VIN
VOUT
FB
C
2
EN
GND
TPS6122x
V
OUT
R
1
R
2
V
IN
C
1
TPS61220
TPS61221
TPS61222
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............................................................................................................................................................................................... SLVS776 – JANUARY 2009
The TPS6122x DC/DC converters are intended for systems powered by a single cell battery to up to three
Alkaline, NiCd or NiMH cells with a typical terminal voltage between 0.7 V and 5.5 V. They can also be used in
systems powered by one-cell Li-Ion or Li-Polymer batteries with a typical voltage between 2.5 V and 4.2 V.
Additionally, any other voltage source with a typical output voltage between 0.7 V and 5.5 V can be used with the
TPS6122x family.
Fixed output voltage versions
At fixed voltage versions, the output voltage is set by a resistor divider internally. The FB pin is used to sense the
output voltage. To configure the fixed output devices properly, the FB pin needs to be connected directly to
VOUT as shown in Figure 24 .
Figure 24. Typical Application Circuit for Fixed Output Voltage Option
Adjustable output voltage version
In the adjustable output versions, an external resistor divider is used to adjust the output voltage. The resistor
divider needs to be connected between VOUT, FB and GND as shown in Figure 25 . 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 5.5 V. The current through the resistive divider should be about 100
times greater than the current into the FB pin. The typical current into the FB pin is 0.01 µ A, 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 k Ω , in order to set the divider current to 1 µ A 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 1 :
As an example, if an output voltage of 3.3 V is needed, a 1-M Ω resistor is calculated for R
1
when for R
2
a 180-k Ω
has been selected.
Figure 25. Typical Application Circuit for Adjustable Output Voltage Option
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