SC1462 View Datasheet(PDF) - Semtech Corporation

Part Name

Description

MFG CO.

SC1462

High Output Current Charge Pump Doubler

Semtech Corporation 

SC1462

POWER MANAGEMENT

Applications Information (Cont.)

Capacitor Selection

Capacitor selection is based on both the maximum ac-

ceptable ripple voltage and the maximum tolerable drop

at the output.

This equation can be used to calculate CBUCKET for a spe-

cific VOUT when VIN and IOUT are known. The maximum value

for the RSW from the Electrical Characteristics section on

Page 2 should be used in order to find a worst-case num-

ber.

Ripple

Layout Consideration

The peak-to-peak output ripple voltage is dependent on

the following factors:

1) Oscillator frequency

2) ESR of COUT

3) Capacitance value of COUT

Standard power board layout should be followed to ensure

proper operation. Any stray inductance should be mini-

mized, this can be achieved by using large power planes

or traces, and reducing the distances between input, out-

put and the load. All components should be placed as

close as possible to the SC1462.

Item 1 is fixed and depend on the charge pump device

used. Item 2, and 3 can be optimized for best perfor-

mance and reduction of the output ripple.

The following formula could be used for an approximation

for V . OUT_RIPPLE

V = OUT_RIPPLE

IOUT

2 × fOSC × COUT + 2 × I × ESR_ COUT

VOUT Drop

The following table shows typical capacitor characteris-

tics.

Component

ESR(Ω) ESL(nH)

0.1μF Ceramic 0603 Package

0.050

1.60

1.0μF Ceramic 1206 Package

0.040

0.47

10μF Ceramic 0603 Package

0.075

0.50

47μF Ceramic, 16V Tantalum D case 0.100

0.60

330μF, 6.3V “OSCON”

0.025

2.50

330μF, 16V AL. Electrolytic

0.143

2.37

820μF, 4V “OSCON”

0.012

2.50

1000μF, 10V Al. Electrolytic

0.053

5.00

The voltage drop at the output is specified with the follow-

ing equation:

VOUT_DROP = IOUT × ROUT

R is dependent on the following factors:

OUT

1) Oscillator frequency

2) ESR of COUT & CBUCKET

3) Capacitance value of COUT and CBUCKET

4) Internal switch resistance (RSW = Total of all 4 switch

resistances).

A good approximation for calculating the output

resistance is shown below.

ROUT =

1

fOSC × CBUCKET

+ 2 × RSW + 4 × ESR_ CBUCKET + ESR_ COUT

© 2005 Semtech Corp.

6

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