9

LT1572

switch “on” resistance by 2:1, but reduces switch current

switch dissipation under current limit conditions.

than its internal clamp level of 2V. The LT1172 switch

externally clamped between these two levels with a diode.

See AN19 for details.

Diode Characteristics

The catch diode used in the LT1572 is a power Schottky

diode with a very low storage time and low forward

voltage. This gives good efficiency in switching regulator

applications, but some thought must be given to maxi-

mum operating voltage and high temperature reverse

leakage.

Peak repetitive reverse voltage rating on the diode

is 20V. In a boost converter, maximum diode reverse

voltage is equal to regulated output voltage, so this limits

maximum output voltage to 20V. In a negative-to-positive

converter, maximum diode voltage will be equal to the

sum of output voltage

plus input voltage. Use the equa-

tions in Application Note 19 or SwitcherCAD or calculate

maximum diode voltage for other topologies.

Diode reverse leakage increases rapidly with temperature.

This leakage is not high enough to significantly impact

efficiency or diode power dissipation, but it can be of

concern in shutdown mode if the diode is connected in

such a way that the leakage adds to regulator shutdown

current. Use the graphs of diode leakage versus voltage

and temperature to ensure proper high temperature sys-

tem performance.

The LT1572 diode is internally bonded to more than two

package pins to reduce internal bond wire currents.

All

pins must be used to prevent excessive current in the

individual internal bond wires. This is important in low

load current applications because the LT1572 will draw

high surge currents during start-up (to charge the output

capacitor) even with no output load current.

OPERATIO

Average supply current (including driver current) is:

DC = switch duty cycle

Switch power dissipation is given by:

2

Total power dissipation is the sum of supply current times

input voltage plus switch power:

In a typical example, using a boost converter to generate

12V at 0.12A from a 5V input, duty cycle is approximately

60%, and switch current is about 0.65A, yielding:

2

× 1Ω × 0.6 = 0.25W

Approximate diode power dissipation for boost and buck

converters is shown below. For other topologies or more

accurate results, see Application Note 19 or use

SwitcherCAD.

In most applications, full load current is used to calculate

die temperature. However, if overload conditions must

also be accounted for, three approaches are possible.

First, if loss of regulated output is acceptable under

overload conditions, the internal

thermal limit of the

LT1572 will protect the die in most applications by shut-

ting off switch current.

Thermal limit is not a tested

parameter, however, and should be considered only for

noncritical applications with temporary overloads.

The second approach for lower current applications is to

leave the second switch emitter (E2) open. This increases