6

LT1676

BLOCK DIAGRA

SWDR

SWDR

SWON

SWON

BOOST

1676 BD

BOOST

SWOFF

SWOFF

LOGIC

OSC

BIAS

FB

GND

SYNC

SHDN

FB

AMP

BOOST

COMP

gm

I

I

I

I

R1

I

COMP

Q4

Q3

Q2

Q1

Q5

D1

5

3

2

1

6

4

8

7

OPERATIO

The LT1676 is a current mode switching regulator IC that

has been optimized for high efficiency operation in high

input voltage, low output voltage Buck topologies. The

Block Diagram shows an overall view of the system.

Several of the blocks are straightforward and similar to

those found in traditional designs, including: Internal Bias

Regulator, Oscillator and Feedback Amplifier. The novel

portion includes an elaborate Output Switch section and

Logic Section to provide the control signals required by

the switch section.

The LT1676 operates much the same as traditional

current mode switchers, the major difference being its

specialized output switch section. Due to space con-

straints, this discussion will not reiterate the basics of

current mode switcher/controllers and the “Buck” topol-

ogy. A good source of information on these topics is

Application Note 19.

Output Switch Theory

One of the classic problems in delivering low output

voltage from high input voltage at good efficiency is that

minimizing AC switching losses requires very fast volt-

age (dV/dt) and current (dI/dt) transition at the output

device. This is in spite of the fact that in a bipolar

implementation, slow lateral PNPs must be included in

the switching signal path.

Fast positive-going slew rate action is provided by lateral

PNP Q3 driving the Darlington arrangement of Q1 and Q2.

The extra

β available from Q2 greatly reduces the drive

requirements of Q3.

Although desirable for dynamic reasons, this topology

alone will yield a large DC forward voltage drop. A second

lateral PNP, Q4, acts directly on the base of Q1 to reduce

the voltage drop after the slewing phase has taken place.

To achieve the desired high slew rate, PNPs Q3 and Q4 are

“force-fed” packets of charge via the current sources

controlled by the boost signal.