LT1534CS datasheet(11/16 Pages) LINER

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LT1534/LT1534-1

APPLICATIONS INFORMATION

where

∆I is the ripple current in the switch, RCSL and

RVSL are the slew resistors and fOSC is the oscillator

frequency.

Power dissipation PD is the sum of these three terms. Die

junction temperature is then computed as:

TJ = TAMB + (PD)(θJA)

where TAMB is ambient temperature and θJA is the package

thermal resistance. For the 16-pin SO with fused leads the

θJA is 50°C/W.

For example, with fOSC = 40kHz, 0.4A average current and

0.1A of ripple, the maximum duty cycle is 88%. Assume

slew resistors are both 17k and VSAT is 0.26V, then:

PD = 0.176W + 0.094W + 0.158W = 0.429W

In an S16 fused lead package the die junction temperature

would be 21

°C above ambient.

Frequency Compensation

Loop frequency compensation is accomplished by way of

a series RC network on the output of the error amplifier (VC

pin). Referring to Figure 4, the main pole is formed by

capacitor CVC and the output impedance of the error

amplifier (approximately 400k

Ω). The series resistor RVC

creates a “zero” which improves loop stability and tran-

sient response. A second capacitor CVC2, typically one-

tenth the size of the main compensation capacitor, is

sometimes used to reduce the switching frequency ripple

on the VC pin. VC pin ripple is caused by output voltage

ripple attenuated by the output divider and multiplied by

the error amplifier. Without the second capacitor, VC pin

ripple is:

CPIN RIPPLE

RIPPLE

OUT

= ()(

)( )( )

125

where VRIPPLE = Output ripple (VP-P)

gm = Error amplifier transconductance

RVC = Series resistor on VC pin

VOUT = DC output voltage

To prevent irregular switching, VC pin ripple should be

kept below 50mVP-P. Worst-case VC pin ripple occurs at

maximum output load current and will also be increased if

poor quality (high ESR) output capacitors are used. The

addition of a 0.0047

µF capacitor on the VC pin reduces

switching frequency ripple to only a few millivolts. A low

value for RVC will also reduce VC pin ripple, but loop phase

margin may be inadequate.

Capacitors

While the IC reduces the source of switcher noise, it is

essential for the lowest noise, that the filter capacitors

should have low parasitic impedance. Sanyo OS-CON,

Panasonic Specialty Polymer and tantalum capacitors are

the preferred types. Aluminum electrolytics are not suit-

able for this application. In general, ESR is more critical

than capacitance. At higher frequencies, ESL can also be

important. Paralleling capacitors can reduce both ESR and

ESL.

Design Note 95 offers more information about capacitor

selection. The following is a brief summary:

Solid tantalum capacitors have small size and low

impedance. Typically they are available for voltages

below 50V. They may have a problem with surge

currents (AVX TPS line addresses this issue).

OS-CON capacitors have very low impedance but are

only available for 25V or less. Form factor may be a

problem. Sometimes their very low ESR can cause loop

stability problems.

Ceramic capacitors are generally used for high fre-

quency and high voltage bypass. They too can have

such a low ESR as to cause loop stability problems.

Often they can resonate with their ESL before ESR

becomes effective.

Specialty Polymer Aluminum: Panasonic has come out

with their series CD capacitors. While they are only

available for voltages below 16V, they have very low

ESR and good surge capability.

VC PIN

1534 F03

RVC

CVC

0.01

µF

CVC2

4.7nF

Figure 4

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