10

LT1534/LT1534-1

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:

PV

mA

I

VIN

IN

=+









11

60

where I is the average switch current.

2. Dissipation due to the driver saturation:

duty cycle.

3. Dissipation due to output slew using approximations

for slew rates:

P

VI

I

R

IV

V

Rf

SLEW

IN

CSL

IN

SAT

VSL

OSC

=









































()

































()

2

2

9

2

2

9

4

33 10

4

220 10

∆

then:

P

IR

V

R

fV

I

SLEW

CSL

IN

VSL

OSC

IN

)

































()( )()

33 10

220 10

99

One word of caution. Sometimes a feedback zero is added

to the control loop by placing a capacitor across R1 above.

If the feedback zero capacitively pulls the FB pin above the

internal regulator voltage (2.4V typ), output regulation

may be disrupted. A series resistance with the feedback

pin can eliminate this potential problem.

Negative Output Voltage Setting

Negative output voltage can be sensed using the NFB pin.

In this case regulation will occur when the NFB pin is at

– 2.5V. The input bias current for the NFB pin is –25

µA

resistor values.

Figure 3

NFB PIN

1534 F02

R2

R1

Referring to Figure 3, R1 is chosen such that:

RR

V

RA

OUT

12

25

25

2 25

=

−

+µ

•

.

.•

A suggested value for R2 is 2.5k. The NFB pin is normally

left open if the FB pin is being used.

Dual Polarity Output Voltage Sensing

Certain applications may benefit from sensing both posi-

tive and negative output voltages. When doing this each

output voltage resistor divider is individually set as previ-

ously described. When both FB and NFB pins are used, the

LT1534 will act to prevent either output from going

beyond its set output voltage. The highest output (lightest

load) will dominate control of the regulator. This technique

would prevent either output from going unregulated high

at no load. However, this technique will also compromise

output load regulation.

Shutdown

If the shutdown pin is pulled low, the regulator will turn off.

The supply current will be reduced to less than 20

µA.