5

LT1512

be connected directly to the GND S pin or to the ground

plane close to the point where the GND S pin is connected.

carrying up to 1.5A of current with fast rise and fall times.

Keep the traces on this pin as short as possible to minimize

radiation and voltage spikes. In particular, the path in

Figure 1 which includes SW to C2, D1, C1 and around to

the LT1512 ground pin should be as short as possible to

minimize voltage spikes at switch turn-off.

–

+

S/S

FB

5k

62k

0.08

Ω

EA

GND

1512 F02

GND S

1.245V

REF

500kHz

OSC

SYNC

SHUTDOWN

DELAY AND RESET

LOW DROPOUT

2.3V REG

ANTI-SAT

LOGIC

DRIVER

SW

SWITCH

–

+

IA

COMP

Figure 2

The LT1512 is a current mode switcher. This means that

switch duty cycle is directly controlled by switch current

rather than by output voltage or current. 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

and current is obtained by using the output of a dual

feedback voltage sensing error amplifier to set switch

current trip level. This technique has the advantage of

simplified loop frequency compensation. A low dropout

internal regulator provides a 2.3V supply for all internal

circuitry on the LT1512. This low dropout design allows

input voltage to vary from 2.7V to 25V. 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 unique error amplifier design has two inverting inputs

which allow for sensing both output voltage and current. A

1.245V bandgap reference biases the noninverting input.

The first inverting input of the error amplifier is brought out

for positive output voltage sensing. The second inverting

input is driven by a “current” amplifier which is sensing

output current via an external current sense resistor. The