LT1533CS datasheet(7/20 Pages) LINER

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LT1533

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.

The negative voltage feedback amplifier allows for direct

regulation of negative output voltages. The voltage on the

NFB pin gets amplified by a gain of – 0.5 and driven onto

the FB input, i.e., the NFB pin regulates to – 2.5V while the

amplifier output internally drives the FB pin to 1.25V as in

normal operation. The negative feedback amplifier input

impedance is 100k (typ) referred to ground.

Slew Control

Control of output voltage and current slew rates is done via

two feedback loops. One loop controls the output switch

collector voltage dV/dt and the other loop controls the

emitter current dI/dt. Output slew control is achieved by

comparing the currents generated by these two slewing

events to currents created by external resistors RVSL and

OPERATIO

RCSL. The two control loops are combined internally to

provide a smooth transition from current slew control to

voltage slew control.

Internal Regulator

Most of the control circuitry operates from an internal 2.4V

low dropout regulator that is powered from VIN. The

internal low dropout design allows VIN to vary from 2.7V

to 23V with virtually no change in device performance.

When the part is put into shutdown, the internal regulator

is turned off, leaving only a small (12

µA typ) current drain

from VIN.

Protection Features

There are three modes of protection in the LT1533. The

first is overcurrent limit. This is achieved via the clamping

action of the VC pin. The second is thermal shutdown that

disables both output drivers and pulls the VC pin low in the

event of excessive chip temperature. The third is under-

voltage lockout that also disables both outputs

and pulls the VC pin low whenever VIN drops below 2.5V.

50% Duty Cycle Mode

Since the LT1533 has dual out-of-phase outputs, it is ideal

for driving push-pull transformers. For simple DC trans-

former applications, the part can be forced into a 50% duty

cycle mode using the DUTY pin. Grounding the DUTY pin

will override the internal control circuitry and force the

outputs to switch with a 50% duty cycle at one-half the

oscillator frequency. Slew control also applies in the 50%

duty cycle mode.

APPLICATIONS INFORMATION

Reducing EMI from switching power supplies has tradi-

tionally invoked fear in designers. Many switchers are

designed solely on efficiency and as such produce wave-

forms filled with high frequency harmonics that then

propagate through the rest of the power supply.

The LT1533 provides control over two of the more impor-

tant variables for controlling EMI with switching inductive

loads: switch voltage slew rate and switch current slew

rate. The use of this part will reduce noise and EMI over

conventional switch mode controllers. Because these

variables are under control, a supply built with this part will

exhibit far less tendency to create EMI and less chance of

wandering into problems during production.

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