LTC1522CS8 datasheet(4/8 Pages) LINER

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LTC1522

BLOCK DIAGRAM

Operation

The LTC1522 uses a switched capacitor charge pump to

boost VIN to a regulated 5V ±4% output voltage. Regula-

tion is achieved by sensing the output voltage through an

internal resistor divider and enabling the charge pump

when the output voltage droops below the lower trip point

of COMP1. When the charge pump is enabled, a 2-phase,

nonoverlapping clock controls the charge pump switches.

Clock 1 closes the S1 switches which enables the flying

capacitor to charge up to the VIN voltage. Clock 2 closes

the S2 switches that stack CFLY in series with VIN and

connect the top plate of CFLY to the output capacitor at

VOUT. This sequence of charging and discharging contin-

ues at a free-running frequency of 700kHz (typ) until the

output has risen to the upper trip point of COMP1 and the

charge pump is disabled. When the charge pump is

disabled, the LTC1522 draws only 4

µA (typ) from VIN

which provides high efficiency at low load conditions.

In shutdown mode, all circuitry is turned off and the part

draws only leakage current from the VIN supply. VOUT is

also disconnected from VIN. The SHDN pin is a CMOS

input with a threshold of approximately VIN/2; however,

the SHDN pin can be driven by logic levels that exceed the

VIN voltage. The part enters shutdown mode when a logic

high is applied to the SHDN pin. The SHDN pin should not

be floated; it must be driven with a logic high or low.

Short-Circuit/Thermal Protection

During short-circuit conditions, the LTC1522 will draw

between 100mA and 200mA from VIN causing a rise in

the junction temperature. On-chip thermal shutdown

circuitry disables the charge pump once the junction

temperature exceeds

≈160°C, and reenables the charge

pump once the junction temperature falls back to

≈145°C.

The LTC1522 will cycle in and out of thermal shutdown

indefinitely without latchup or damage until the VOUT

short is removed.

Capacitor Selection

For best performance, it is recommended that low ESR

(< 0.5

Ω) capacitors be used for both CIN and COUT to

reduce noise and ripple. The CIN and COUT capacitors

should be either ceramic or tantalum and should be 3.3

µF

or greater (aluminum capacitors are not recommended

because of their high ESR). If the input source impedance

is very low, CIN may not be needed. Increasing the size of

COUT to 10µF or greater will reduce output voltage ripple.

APPLICATIONS INFORMATION

VREF

CLOCK 1

CLOCK 2

CONTROL

LOGIC

S2A

S2B

S1A

S1B

CFLY

0.22

µF

CIN

µF

C+

C–

COUT

µF

µA

VOUT

SHDN

LTC1522 BD

CHARGE PUMP SHOWN IN DISCHARGE CYCLE

VIN

CHARGE PUMP

–

COMP1

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