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APPLICATION INFORMATION

DESIGN PROCEDURE

Programming the Output Voltage

R1 + R2

V

O

V

FB

* 1 + 180 kW

V

O

500 mV *

1

(1)

C

parR1 +

3 pF

200 kW

R2

* 1

(2)

SW

C1

Power

Supply

L1

VBAT

VOUT

FB

C2

EN

GND

TPS61070

BoostOutput

R1

R2

Inductor Selection

I

L +

I

O

VOUT

VBAT

0.8

(3)

TPS61070, TPS61071

TPS61072, TPS61073

SLVS510A – JULY 2006 – REVISED JANUARY 2007

The TPS6107x dc/dc converters are intended for systems powered by a single-cell, up to triple-cell alkaline,

NiCd, NiMH battery with a typical terminal voltage between 0.9 V and 5.5 V. They can also be used in systems

powered by one-cell Li-ion or Li-polymer with a typical voltage between 2.5 V and 4.2 V. Additionally, any other

voltage source with a typical output voltage between 0.9 V and 5.5 V can power systems where the TPS6107x

is used. Due to the nature of boost converters, the output voltage regulation is only maintained when the input

voltage applied is lower than the programmed output voltage.

The output voltage of the TPS6107x dc/dc converter can be adjusted with an external resistor divider. The

typical value of the voltage at the FB pin is 500 mV. 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 µA, and the voltage across R2 is typically 500 mV. Based on

those two values, the recommended value for R2 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 R1, depending on the needed output voltage (V

using Equation 1:

If as an example, if an output voltage of 3.3 V is needed, a 1.0 M

Ω resistor should be chosen for R1. If for any

reason the value chosen for R2 is significantly lower than 200 k

Ω, additional capacitance in parallel to R1 is

recommended, in case the device shows instable regulation of the output voltage. The required capacitance

value can be calculated using Equation 2:

Figure 23. Typical Application Circuit for Adjustable Output Voltage Option

A boost converter normally requires two main passive components for storing energy during the conversion. A

boost inductor and a storage capacitor at the output are required. To select the boost inductor, it is

recommended to keep the possible peak inductor current below the current limit threshold of the power switch in

the chosen configuration. For example, the current limit threshold of the TPS6107x's switch is 600 mA. The

For example, for an output current of 75 mA at 3.3 V, at least 340 mA of average current flows through the

inductor at a minimum input voltage of 0.9 V.

14

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