13

LT1336

Figure 4. Adding Synchronous Switching to a Step-Down Switching Regulator

negative inductor current is returned to HV when the

under these conditions due to the recirculating currents.

The LT1336 performs the synchronous MOSFET drive in

a step-down switching regulator. A reference and PWM

are required to complete the regulator. Any voltage mode

or current mode PWM controller may be used but the

LT3526 is particularly well-suited to high power, high

efficiency applications such as the 10A circuit shown in

Figure 6. In higher current regulators a small Schottky

diode across the bottom MOSFET helps to reduce reverse-

recovery switching losses.

Motor Drive Applications

In applications where rotation is always in the same

direction, a single LT1336 controlling a half-bridge can be

used to drive a DC motor. One end of the motor may be

connected either to supply or to ground. A motor in this

configuration is controlled by its inputs which give three

alternatives: run, free running stop (coasting) and fast

stop (“plugging” braking with the motor shorted by one of

the MOSFETs).

Whenever possible, returning one end of the motor to

ground is preferable. When the motor is returned to supply

and the boost topology is used to charge the bootstrap

capacitor, the return current from the top driver will find its

way to the high voltage rail through the top MOSFET. Since

+

+

INTOP

INBOTTOM

TGATEDR

TGATEFB

TSOURCE

BGATEDR

BGATEFB

LT1336

HV

1336 F04

REF PWM

OUT A

OUT A

Switch

Total Period

Switch

Total Period

On =

V

HV

Off =

HV – V

HV

OUT

OUT













on, making the diode losses more significant than the

switch. The worst case for the diode is during a short

ducts the short-circuit current almost continuously.

Figure 4 shows the LT1336 used to synchronously drive a

pair of power MOSFETs in a step-down regulator applica-

tion, where the top MOSFET is the switch and the bottom

MOSFET replaces the Schottky diode. Since both conduc-

tion paths have low losses, this approach can result in very

high efficiency (90% to 95%) in most applications. For

top MOSFET off when HV is applied before the 12V supply.

One fundamental difference in the operation of a step-

down regulator with synchronous switching is that it never

becomes discontinuous at light loads. The inductor cur-

rent doesn’t stop ramping down when it reaches zero, but

actually reverses polarity, resulting in a constant ripple

current independent of load. This does not cause a signifi-

cant efficiency loss (as might be expected) since the