LT1533CS datasheet(10/20 Pages) LINER

10 / 20 page

LT1533

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

θJA is 100°C/W.

For example, with fOSC = 40kHz, VIN = 10V, 0.4A average

current and 0.1A of ripple, the maximum duty cycle is

44%. Assume slew resistors are both 17k and VSAT is

0.26V, then:

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

In an S16 package the die junction temperature would be

°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 3, 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:

C PIN RIPPLE

RIPPLE

OUT

= ()(

)( )( )

125

where VRIPPLE = Output ripple (VP-P)

gm = Error amplifier transconductance

RVC = Series resistor on VC pin

VOUT = DC output voltage

Thermal Considerations

Computing power dissipation for this IC requires careful

attention to detail. Reduced output slewing causes the part

to dissipate more power than would occur with fast edges.

However, much improvement in noise can be produced

with modest decrease in supply efficiency.

Power dissipation is a function of topology, input voltage,

switch current and slew rates. It is impractical to come up

with an all-encompassing formula. It is therefore recom-

mended that package temperature be measured in each

application. The part has an internal thermal shutdown to

prevent device destruction, but this should not replace

careful thermal design.

1. Dissipation due to input current:

VIN









where I is the average switch current.

2. Dissipation due to the drivers saturation:

PVSAT = (VSAT)(I)(DCMAX)

where VSAT is the output saturation voltage which is

approximately 0.1 + (0.4)(I), DCMAX is the maximum

duty cycle.

3. Dissipation due to output slew using approximations

for slew rates:

SLEW

CSL

SAT

VSL

OSC

() +













()







()+

() −













()







()

















()

33 10

220 10

∆

Note if VSAT and ∆I are small with respect to VIN and I,

then:

SLEW

CSL

VSL

OSC

= ()(

)

()







+ ()()

()



















()( )()

33 10

220 10

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