LT1534CS datasheet(7/16 Pages) LINER

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LT1534/LT1534-1

BLOCK DIAGRA

OPERATIO

In noise sensitive applications, switching regulators tend

to be ruled out as a power supply option due to their

propensity for generating unwanted noise. When switch-

ing supplies are required due to efficiency or input/output

voltage constraints, great pains must be taken to work

around the noise generated by a typical supply. These

steps may include precise synchronization of the power

supply oscillator to an external clock, synchronizing the

rest of the circuit to the power supply oscillator, or halting

power supply switching during noise sensitive operations.

The LT1534 greatly simplifies the task of eliminating

supply noise by enabling the design of an inherently low

noise switching regulator power supply.

The LT1534 is a fixed frequency, current mode switching

regulator with unique circuitry to control the voltage and

current slew rates of the output switch. Slew control

capability provides much greater control over power sup-

ply components that can create conducted and radiated

electromagnetic interference. The current mode control

provides excellent AC and DC line regulation and simplifies

loop compensation.

Current Mode Control

A switching cycle begins with an oscillator discharge pulse

which resets the RS flip-flop, turning on the output driver

(refer to Block Diagram). The switch current is sensed

across an internal resistor and the resulting voltage is

amplified and compared to the output of the error amplifier

(VC pin). The driver is turned off once the output of the

current sense amplifier exceeds the voltage on the VC pin.

Internal slope compensation is provided to ensure stabil-

ity under high duty cycle conditions.

Output regulation is obtained using the error amp to set

the switch current trip point. The error amp is a transcon-

ductance amplifier that integrates the difference between

the feedback output voltage and an internal 1.25V refer-

ence. The output of the error amp adjusts the switch

current trip point to provide the required load current at

the desired regulated output voltage. This method of

controlling current rather than voltage provides faster

input transient response, cycle by cycle current limiting

for better output switch protection and greater ease in

compensating the feedback loop.

The VC pin serves three different purposes. It is used for

loop compensation, current limit adjustment and soft

starting. During normal operation the VC voltage will be

between 0.2V and 1.33V. An external clamp may be used

for lowering the current limit. A capacitor coupled to an

external clamp can be used for soft starting.

NFB

100k

50k

SYNC

GND

1534 BD

RVSL

RCSL

OSCILLATOR

INTERNAL VCC

SHDN

VIN

PGND

COL

OUTPUT

DRIVER

–

NEGATIVE

FEEDBACK

AMP

–

ERROR

AMP

1.25V

COMP

LDO REGULATOR

SLEW CONTROL

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