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APPLICATION INFORMATION

DESIGN PROCEDURE

Programming the Output Voltage

R3 + R4

V

O

V

FB

* 1 + 200 kW

V

O

500 mV *

1

(1)

C

parR3 +

10 pF

200 kW

R4

–1

(2)

SW

C1

Power

Supply

L1

R1

R2

VBAT

VOUT

FB

C2

C3

LBO

PGND

LBI

SYNC

EN

GND

TPS6109x

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 k

Ω, 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

k

Ω. From that, the value of resistor R3, depending on the needed output voltage (V

Equation 1:

If as an example, an output voltage of 5.0 V is needed, a 1.8-M

Ω resistor should be chosen for R3. If for any

reason the value for R4 is chosen significantly lower than 200 k

Ω 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

k

Ω. From that, the value of resistor R1, depending on the desired minimum battery voltage V

calculated using Equation 3.

11