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SETTING THE OUTPUT VOLTAGE

V

OUT +

1.233 V

1 )

R1

R2

(5)

C

FF +

1

2

p

fS

20

R1

(6)

TPS61040

TPS61041

SLVS413C – OCTOBER 2002 – REVISED AUGUST 2007

The maximum load current of the conveter is the current at the operation point where the coverter starts to enter

the continuous conduction mode. Usually the converter should always operate in discontinuous conduction

mode.

Last, the selected inductor should have a saturation current that meets the maximum peak current of the

Another important inductor parameter is the dc resistance. The lower the dc resistance, the higher the efficiency

of the converter. See Table 3 and the typical applications for the inductor selection.

Table 3. Recommended Inductor for Typical LCD Bias Supply (see Figure 15)

DEVICE

INDUCTOR VALUE

COMPONENT SUPPLIER

COMMENTS

10

μH

Sumida CR32-100

High efficiency

10

μH

Sumida CDRH3D16-100

High efficiency

TPS61040

10

μH

Murata LQH4C100K04

High efficiency

4.7

μH

Sumida CDRH3D16-4R7

Small solution size

4.7

μH

Murata LQH3C4R7M24

Small solution size

High efficiency

TPS61041

10

μH

Murata LQH3C100K24

Small solution size

The output voltage is calculated as:

For battery-powered applications, a high-impedance voltage divider should be used with a typical value for R2 of

≤200 kΩ and a maximum value for R1 of 2.2 MΩ. Smaller values might be used to reduce the noise sensitivity of

the feedback pin.

A feedforward capacitor across the upper feedback resistor R1 is required to provide sufficient overdrive for the

error comparator. Without a feedforward capacitor, or one whose value is too small, the TPS61040/41 shows

double pulses or a pulse burst instead of single pulses at the switch node (SW), causing higher output voltage

ripple. If this higher output voltage ripple is acceptable, the feedforward capacitor can be left out.

The lower the switching frequency of the converter, the larger the feedforward capacitor value required. A good

starting point is to use a 10-pF feedforward capacitor. As a first estimation, the required value for the feedforward

capacitor at the operation point can also be calculated using the following formula:

Where:

R1 = Upper resistor of voltage divider

fS = Switching frequency of the converter at the nominal load current (See the INDUCTOR SELECTION,

MAXIMUM LOAD CURRENT section for calculating the switching frequency)

Copyright © 2002–2007, Texas Instruments Incorporated

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