8

LT1572

OPERATIO

to the comparator input. The LT1572 will then regulate the

value of the flyback pulse with respect to the supply

output voltage in the traditional transformer coupled flyback

topology regulator. By regulating the amplitude of the

flyback pulse, the output voltage can be regulated with no

direct connection between input and output. The output is

fully floating up to the breakdown voltage of the trans-

former windings. Multiple floating outputs are easily ob-

tained with additional windings. A special delay network

inside the LT1572 ignores the leakage inductance spike at

the leading edge of the flyback pulse to improve output

regulation.

The error signal developed at the comparator input is

functions. It is used for frequency compensation, current

limit adjustment, soft starting, and total regulator shut-

down. During normal regulator operation this pin sits at a

voltage between 0.9V (low output current) and 2.0V (high

output current). The error amplifiers are current output

adjusting current limit. Likewise, a capacitor coupled

external clamp will provide soft start. Switch duty cycle

pin below 0.15V causes total regulator shutdown, with

only 50

µA supply current for shutdown circuitry biasing.

See AN19 for full application details.

E1 and E2 Pins

The LT1572 has the emitters of the power transistor

brought out separately from the ground pin. This elimi-

nates errors due to ground pin voltage drops and allows

the user to reduce switch current limit 2:1 by leaving the

second emitter (E2) disconnected. The first emitter (E1)

should always be connected to the ground pin. Note that

switch “on” resistance doubles when E2 is left open, so

efficiency will suffer somewhat when switch currents

exceed 300mA. Also, note that chip dissipation will actu-

ally

increase with E2 open during normal load operation,

even though dissipation in current limit mode will

de-

crease.

Other Application Help

More circuits and application help for the LT1572 can be

found in the LT1172 data sheet, both in loose form and in

the

1994 Linear Databook Volume III. Extensive additional

help is contained in Application Note 19. All application

circuits using the LT1172 can also use the LT1572 as long

as the 20V maximum reverse voltage of the diode is not

exceeded. A CAD program called SwitcherCAD is also

available. This program can be used with the LT1572 by

simply treating the LT1572 as an LT1172 and ignoring the

predicted die temperature results obtained from

SwitcherCAD itself.

Thermal Management

Thermal management is particularly important with the

LT1572 because both switch and diode power dissipation

increase rapidly at low input voltage when using the

popular boost topology. Regulator and diode die tempera-

ture must be calculated

separately because they are not

connected to an isothermal plane inside the package.

Diode

plus regulator thermal resistance is approximately

70

°C/W when the LT1572 is soldered to 1oz copper traces

over an internal or backside copper plane using FR4 board

material. However, individual calculation of die tempera-

ture must take thermal coupling into account. To accom-

plish this, thermal resistance is broken into two sections,

a common (coupled) section and a second uncoupled

section. Die temperatures are calculated from:

The following formulas can be used as a rough guide to

calculate LT1572 power dissipation. For more details, the

reader is referred to Application Note 19 (AN19), “Effi-

ciency Calculations” section.