LT1534CS datasheet(8/16 Pages) LINER

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LT1534/LT1534-1

The negative feedback amplifier allows for direct regula-

tion 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

RCSL. The two control loops are combined internally to

provide a smooth transition from current slew control to

voltage slew control.

OPERATIO

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 LT1534. 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.

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 LT1534 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.

It is beyond the scope of this data sheet to get into EMI

fundamentals. AN70 contains much information concern-

ing noise in switching regulators and should be consulted.

Oscillator Frequency

The oscillator determines the switching frequency and

therefore the fundamental positioning of all harmonics.

The use of good quality external components is important

to ensure oscillator frequency stability. The oscillator is a

sawtooth design. A current defined by external resistor RT

is used to charge and discharge the capacitor CT. The

discharge rate is approximately ten times the charge rate.

By allowing the user to have control over both compo-

nents, trimming of oscillator frequency can be more easily

achieved.

The external capacitance CT is chosen by:

CT(nF) = 2180/[fOSC(kHz) • RT(kΩ)]

where fOSC is the desired oscillator frequency in kHz.

For RT equal to 16.9k, this simplifies to:

CT(nF) = 129/fOSC(kHz)

(e.g., CT = 1.29nF for fOSC = 100kHz)

A good quality temperature stable capacitor should be

chosen.

Nominally RT should be 16.9k. Since it sets up current, its

temperature coefficient should be selected to compliment

the capacitor. Ideally, both should have low temperature

coefficients.

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