6

LT1370

The LT1370 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 voltage mode 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 frequency compensation under widely vary-

ing 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 pro-

vides a 2.3V supply for all internal circuitry. This low

dropout design allows input voltage to vary from 2.7V to

25V with virtually no change in device performance. A

500kHz 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 1.245V bandgap reference biases the positive input of

the error amplifier. The negative input of the amplifier is

brought out for positive output voltage sensing. The error

amplifier has nonlinear transconductance to reduce out-

put overshoot on start-up or overload recovery. When

the feedback voltage exceeds the reference by 40mV,

error amplifier transconductance increases 10 times,

which reduces output overshoot. The feedback input also

invokes oscillator frequency shifting, which helps pro-

tect components during overload conditions. When the

feedback voltage drops below 0.6V, the oscillator fre-

quency is reduced 5:1. Lower switching frequency allows

full control of switch current limit by reducing minimum

switch duty cycle.

–

+

NFBA

NFB

S/S

FB

100k

50k

0.005

Ω

EA

GND

LT1370 • BD

GND SENSE

1.245V

REF

5:1 FREQUENCY

SHIFT

OSC

SYNC

SHUTDOWN

DELAY AND RESET

LOW DROPOUT

2.3V REG

ANTI-SAT

LOGIC

DRIVER

SW

SWITCH

–

+

IA

COMP