Datasheet

LT1336
14
1336fa
Figure 4. Adding Synchronous Switching to a Step-Down Switching Regulator
switch turns back on. However, I
2
R losses will occur 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 di-
rection, 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 most power supplies cannot sink current, this
Switch On=
V
OUT
HV






Total Period
( )
Switch Off=
HV–V
OUT
HV






Total Period
( )
Note that for HV > 2V
OUT
, the switch is off longer than it
is on, making the diode losses more significant than the
switch. The worst case for the diode is during a short cir-
cuit, when V
OUT
approaches zero and the diode conducts
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 ap-
plication, where the top MOSFET is the switch and the
bottom MOSFET replaces the Schottky diode. Since both
conduction paths have low losses, this approach can result
in very high efficiency (90% to 95%) in most applications.
For regulators under 10A, using low R
DS(ON)
N-channel
MOSFETs eliminates the need for heat sinks. R
GS
holds the
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 induc-
tor current 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 significant efficiency loss (as might be expected) since
the negative inductor current is returned to HV when the
applicaTions inForMaTion
+
+
INTOP
INBOTTOM
TGATEDR
TGATEFB
TSOURCE
BGATEDR
BGATEFB
LT1336
HV
V
OUT
R
GS
R
SENSE
1336 F04
REF PWM
OUT A
OUT A