SC600C Просмотр технического описания (PDF) - Semtech Corporation

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SC600C

mAhXLife™ LED Driver with 5.0V, 4.5V, or 4.0V Output

Semtech Corporation 

SC600

POWER MANAGEMENT

Applications Information

Regulated Fractional Charge Pump Operation

Component Versions

A fractional charge pump is a voltage converter which

implements switched capacitor techniques to produce an

output voltage that is one of several multiples of the input

voltage. Regulated fractional charge pumps (also called

charge pump regulators) use a linear regulator with vari-

ous charge pump configurations to deliver a regulated out-

put over a wide input voltage range. Regulated fractional

charge pumps have improved efficiency over ordinary lin-

ear regulator and charge pump circuit combinations. The

improved efficiency is achieved by implementing multiple

charge pump configurations on one integrated circuit.

The correct charge pump configuration is automatically

selected to meet the regulation requirements at the best

possible efficiency. The SC600 has three charge pump

configurations (modes), which multiply the input voltage

by 1x, 1.5x and 2x.

There are four versions of the SC600. The component

selection table on page 4 highlights the differences be-

tween the component types. The three basic differences

between the component versions are in the output volt-

age, maximum output current capability, and the mode

transition point.

The mode transition point is the value of input voltage at

which the component will transition between 1.5x and 2x

modes. 5.0V, 4.5V, and 4.0V versions are available. The

SC600A (5.0V) is most efficient for applications that use

up to 60mA. The SC600B (4.5V) and SC600C (4.5V) have

different maximum output currents and mode transition

points. The lower mode transition point of the SC600C

allows it to remain in 1.5x mode longer for greater power

savings.

The charge pump configurations are implemented with

two switched or 'bucket' capacitors plus the input and out-

put capacitor. The bucket capacitors are configured for 1x

mode at start-up to source current to the output capacitor

and bring the output up quickly. The charge pump will

begin switching in 1.5x mode. During normal operation,

starting with a fully charged Li-Ion cell, the battery voltage

will begin at about 4.1V. As the battery discharges and

the voltage decays, the SC600 will eventually transition to

2x mode when the battery voltage is approximately 3.50V.

Hysteresis is provided to prevent mode toggling. The out-

put is prevented from exceeding 6.0V. This feature allows

the use of 6.3V ceramic capacitors.

mAhXLiFETM Advantage

The plot on page 1 shows the efficiency of the SC600A.

An example of a 5.0V regulated charge pump doubler is

plotted to demonstrate how effective the SC600’s 1.5x

mode is at improving efficiency when the input voltage is

above 3.5V. Most of the Li-Ion battery life is above 3.5V

where the SC600 achieves more than 20% higher effi-

ciency compared to the 5.0V regulated charge pump dou-

bler with only a 2x mode. Following the efficiency curve

from left to right as the battery discharges, the SC600A

5.0V remains in 1.5x mode until 3.5V and then transitions

to 2x mode.

The SC600B has higher output capability, up to 120mA,

and S600C is rated for 60mA. The SC600D (4.0V) is for

applications using up to 120mA.

Start-Up Conditions

Typical start-up time is less than 50μs. Caution: The SC600

must be enabled while 650kHz or 262kHz is selected to pre-

vent over-voltage during start-up.

LED Bias and Backlighting Applications

When using the SC600 for LED bias, note that the SC600C

and SC600D require the same input power per unit of

output current even though the SC600C is more efficient.

Also, the SC600A will have the same input power as the

SC600B though the SC600A is more efficient.

The following plots of Input Power vs. Input Voltage high-

light the differences in application of the four component

versions. To achieve the lowest possible input power, it

is desirable for the charge pump to remain in 1.5x mode

until the input voltage is as low as possible. The transi-

tion points from 1.5x to 2x are identified in the plot where

the input power steps upward as the input voltage moves

lower.

© 2006 Semtech Corp.

7

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