LTC1503CS8-2 datasheet(7/12 Pages) LINER

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LTC1503-1.8/LTC1503-2

maintain regulation. This will only occur as VIN/VOUT nears

a 3-to-2 or 1-to-1 transition point. For example, under light

load conditions, the LTC1503-X can operate in 2-to-1

mode when VIN equals 4.1V with greater than 90% effi-

ciency. However, when the load is increased, the part can

no longer supply enough output current in 2-to-1 mode to

maintain regulation. This causes VOUT to droop below the

regulation point until COMP2 trips and forces the part to

skip from 2-to-1 mode to 3-to-2 mode. The COMP2

threshold is about 17mV (VOUT referred) below the main

comparator regulation point. Hysteresis in COMP2 will

force the part to transition in and out of mode skipping.

This will result in a slight VOUT decrease of approximately

20mV under mode skipping conditions.

Shutdown/Soft-Start Operation

The SHDN/SS pin is used to implement both low current

shutdown and soft-start. The soft-start feature limits

inrush currents when the regulator is initially powered up

or taken out of shutdown. Forcing a voltage lower than

0.35V (typ) will put the part into shutdown mode. Shut-

down mode disables all control circuitry and forces the

charge pump VOUT into a high impedance state. A 2µA pull-

up current on the SHDN/SS pin will force the part into

active mode if the pin is left floating or is driven with an

open-drain output that is in a high impedance state. If the

pin is not driven with an open-drain device, it must be

forced to a logic high voltage of 2.2V (min) to ensure

proper VOUT regulation. The SHDN/SS pin should not be

driven to a voltage higher than VIN.

To implement soft-start, the SHDN/SS pin must be driven

with an open-drain device and a capacitor must be

connected from the SHDN/SS pin to GND. Once the open-

drain device is turned off, a 2

µA pull-up current will begin

charging the external SS capacitor and force the voltage

on the pin to ramp towards VIN. As soon as the SHDN

threshold is reached (0.35V typ), the internal reference

voltage which controls the VOUT regulation point will

follow the ramp voltage on the SHDN/SS pin (minus a

0.35V offset to account for the SHDN threshold) until the

reference reaches its final band gap voltage. This occurs

when the voltage on the SHDN/SS pin reaches

Figure 1b. Step-Down Charge Pump in 3-to-2 Mode

In 1-to-1 mode, switch S1 and S2 are connected in series

between VIN and VOUT as needed to boost VOUT back into

regulation (see Figure 1c). The REG ENABLE signal from

the main comparator (COMP1) controls switches S1 and

S2 directly. Since all of the VOUT current is sourced from

VIN, the efficiency in 1-to-1 mode is approximately equal

to that of a linear regulator.

Figure 1c. Step-Down Charge Pump in 1-to-1 Mode

Mode Selection and Mode Skipping

The optimal step-down conversion mode is chosen based

on VIN to VOUT differential voltage and output load condi-

tions. Two internal comparators are used to select the

default step-down mode based on the VIN and VOUT

voltage. A separate comparator (COMP2) is used to sense

a droop on VOUT due to a heavy output load and force the

charge pump to skip to a higher output current mode to

φ1

φ2

φ1

(EXTERNAL)

C1+

C1–

C2–

GND

C2 +

1503-1.8/2 F01b

VIN

VOUT

φ1

φ2

(EXTERNAL)

C1+

C1–

1503-1.8/2 F01c

VIN

VOUT

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