Datasheet

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APPLICATION INFORMATION
DESIGN PROCEDURE
Programming the Output Voltage
R3  R4 

V
O
V
FB
 1

 200 k 

V
O
500 mV
 1

(1)
C
parR3
 10 pF 

200 k
R4
–1

(2)
SW
C1Power
Supply
L1
R1
R2
VBAT
VOUT
FB
C2 C3
LBO
PGND
LBI
SYNC
EN
GND
TPS6109x
V
OUT
Boost Output
Low Battery
Output
R3
R4
R5
Programming the LBI/LBO Threshold Voltage
TPS61090
TPS61091, TPS61092
SLVS484A – JUNE 2003 – REVISED APRIL 2004
The TPS6109x dc/dc converters are intended for systems powered by a dual or triple cell NiCd or NiMH battery
with a typical terminal voltage between 1.8 V and 5.5 V. They can also be used in systems powered by one-cell
Li-Ion with a typical stack voltage between 2.5 V and 4.2 V. Additionally, two or three primary and secondary
alkaline battery cells can be the power source in systems where the TPS6109x is used.
The output voltage of the TPS61090 dc/dc converter section can be adjusted with an external resistor divider.
The typical value of the voltage on the FB pin is 500 mV. The maximum allowed 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 µA, and the voltage across R4 is typically 500 mV. Based on those
two values, the recommended value for R4 should be lower than 500 kΩ, in order to set the divider current at 1
µA or higher. Because of internal compensation circuitry the value for this resistor should be in the range of 200
kΩ. From that, the value of resistor R3, depending on the needed output voltage (V
O
), can be calculated using
Equation 1 :
If as an example, an output voltage of 5.0 V is needed, a 1.8-MΩ resistor should be chosen for R3. If for any
reason the value for R4 is chosen significantly lower than 200 kΩ additional capacitance in parallel to R3 is
recommended. The required capacitance value can be easily calculated using Equation 2
Figure 18. Typical Application Circuit for Adjustable Output Voltage Option
The current through the resistive divider should be about 100 times greater than the current into the LBI pin. The
typical current into the LBI pin is 0.01 µA, and the voltage across R2 is equal to the LBI voltage threshold that is
generated on-chip, which has a value of 500 mV. The recommended value for R2is therefore in the range of 500
kΩ. From that, the value of resistor R1, depending on the desired minimum battery voltage V
BAT,
can be
calculated using Equation 3 .
11