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LT1572

100kHz, 1.25A Switching Regulator with Catch Diode

Linear Technology 

LT1572

OPERATIO

Average supply current (including driver current) is:

IIN ≈ 6mA + ISW(0.004 + DC/40)

ISW = switch current

DC = switch duty cycle

Switch power dissipation is given by:

PSW = (ISW)2 × RSW × DC

RSW = LT1572 switch “on” resistance (1Ω maximum)

Total power dissipation is the sum of supply current times

input voltage plus switch power:

PREG = IIN × VIN + PSW

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:

IIN = 6mA + 0.65(0.004 + DC/40) = 18mA

PSW = (0.65)2 × 1Ω × 0.6 = 0.25W

PREG = 5V × 0.018A + 0.25 = 0.34W

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.

Boost: PDIODE = IOUT × Vf

Buck: PDIODE = IOUT × Vf × (VIN – VOUT)/VIN

Vf = diode forward voltage at a current equal to IOUT for a

buck converter and IOUT × VOUT/VIN for a boost converter.

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

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

limit by 2:1 also, resulting in a net 2:1 reduction in I2R

switch dissipation under current limit conditions.

The third approach is to clamp the VC pin to a voltage less

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

current limit is zero at approximately 1V on the VC pin and

2A at 2V on the VC pin. Peak switch current can be

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.

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