6

LTC1503-1.8/LTC1503-2

General Operation

The two most common methods for providing regulated

step-down DC/DC conversion are linear DC/DC conversion

(used by LDOs) and inductor-based DC/DC conversion.

Linear regulation provides low cost and low complexity, but

the conversion efficiency is poor since all of the load cur-

down conversion provides the highest efficiency, but the

solution cost and circuit complexity are much higher. The

LTC1503-X provides the efficiency advantages associated

with inductor-based circuits as well as the cost and sim-

plicity advantages of an inductorless converter.

The LTC1503-X is a switched capacitor step-down DC/DC

converter. The part uses an internal switch network and

fractional conversion ratios to achieve high efficiency over

control circuitry selects the appropriate step-down con-

optimize efficiency. The part has three possible step-down

modes: 2-to-1, 3-to-2 or 1-to-1 (gated switch) step-down

mode. Only two external flying caps are needed to operate

switch the step-down ratio as needed to maintain output

regulation under heavy load conditions.

Regulation is achieved by sensing the divided down output

voltage and enabling the charge pump as needed to boost

the output back into regulation. This method of regulation

allows the LTC1503-X to achieve high efficiency at very

light loads. The part has shutdown capability as well as

user controlled inrush current limiting. In addition, the

part can withstand an indefinite short-circuit condition on

Step-Down Charge Pump Operation

Figure 1a shows the charge pump switch configuration

that is used for 2-to-1 step down. When the charge pump

is enabled in this mode, a two phase nonoverlapping clock

generates the switch control signals. On phase one of the

clock, flying capacitor C1 is connected through switches

Figure 1a. Step-Down Charge Pump in 2-to-1 Mode

tion. Since charge current is sourced from ground on

phase one of the clock, current multiplication is realized

relative to a linear regulator.

The 3-to-2 conversion mode also uses a nonoverlapping

clock for switch control but requires two flying capacitors

and a total of seven switches (see Figure 1b). On phase

thereby reducing the average voltage on the flying caps

and raising the voltage on the output capacitor. On phase

two, the two flying capacitors are connected on parallel

manner, charge is again transferred from the flying caps

to the output on both phases of the clock, and the voltage

regulation. As in 2-to-1 mode, charge current is sourced

from ground on phase one of the clock which results in

S4

φ2

S1

φ1

S3

φ2

S2

φ1

C1

(EXTERNAL)

1503-1.8/2 F01a

APPLICATIO S I FOR ATIO