7

LT1072

1072fc

The LT1072 is a current mode switcher. This means that

switch duty cycle is directly controlled by switch current

rather than by output voltage. Referring to the block

diagram, the switch is turned “on” at the start of each

oscillator cycle. It is turned “off” when switch current

reaches a predetermined level. Control of output voltage is

obtained by using the output of a voltage sensing error

amplifier to set current trip level. This technique has

several advantages. First, it has immediate response to

input voltage variations, unlike ordinary switchers which

have notoriously poor line transient response. Second, it

reduces the 90 phase shift at midfrequencies in the energy

storage inductor. This greatly simplifies closed loop fre-

quency compensation under widely varying input voltage

or output load conditions. Finally, it allows simple pulse-

by-pulse current limiting to provide maximum switch

protection under output overload or short conditions. A

low-dropout internal regulator provides a 2.3V supply for

all internal circuitry on the LT1072. This low-dropout

design allows input voltage to vary from 3V to 60V with

virtually no change in device performance. A 40kHz

oscillator is the basic clock for all internal timing. It turns

“on” the output switch via the logic and driver circuitry.

Special adaptive antisat circuitry detects onset of

saturation in the power switch and adjusts driver current

instantaneously to limit switch saturation. This minimizes

driver dissipation and provides very rapid turn-off of

the switch.

A 1.2V bandgap reference biases the positive input of the

error amplifier. The negative input is brought out for

output voltage sensing. This feedback pin has a second

function; when pulled low with an external resistor, it

programs the LT1072 to disconnect the main error

amplifier output and connects the output of the flyback

amplifier to the comparator input. The LT1072 will then

regulate the value of the flyback pulse with respect to the

supply voltage. This flyback pulse is directly proportional

to 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

transformer windings. Multiple floating outputs are easily

obtained with additional windings. A special delay network

inside the LT1072 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

shutdown. 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 (gm) types, so this voltage can be externally

clamped for adjusting current limit. Likewise, a capacitor

coupled external clamp will provide soft start. Switch duty

pin below 0.15V causes total regulator shutdown, with

only 50µA supply current for shutdown circuitry biasing.

See AN-19 for full application details.

Extra Pins on the MiniDIP and Surface Mount Packages

The 8 and 16-pin versions of the LT1072 have the emitters

of the power transistor brought out separately from the

ground pin. This eliminates 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 100mA. Also, note that chip

dissipation will actually

increase with E2 open during

normal load operation, even though dissipation in current

limit mode will

decrease. See “Thermal Considerations.”

Thermal Considerations When Using Small Packages

The low supply current and high switch efficiency of the

LT1072 allow it to be used without a heat sink in most

applications when the TO-220 or TO-3 package is selected.

These packages are rated at 50°C/W and 35°C/W

respectively. The small packages, however, are rated at

greater than 100°C/W. Care should be taken with these

packages to ensure that the worse case input voltage and

load current conditions do not cause excessive die

temperatures. The following formulas can be used as a

LT1072 OPERATIO