LT1336 Linear Technology, LT1336 Datasheet - Page 13

• Manufacturer Part Number: LT1336
• Description: Half-Bridge N-Channel Power MOSFET Driver with Boost Regulator
• Manufacturer: Linear Technology

APPLICATIONS
Note that for HV > 2V
on, making the diode losses more significant than the
switch. The worst case for the diode is during a short
circuit, when V
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
regulators under 10A, using low R
MOSFETs eliminates the need for heat sinks. R
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
Switch
Switch
On =
Off =
OUT
V
HV – V
HV
U
OUT
approaches zero and the diode con-
OUT
HV
REF PWM
, the switch is off longer than it is
INFORMATION
Total Period
Figure 4. Adding Synchronous Switching to a Step-Down Switching Regulator
OUT
U
OUT A
OUT A
Total Period
W
DS(ON)
INTOP
INBOTTOM
LT1336
GS
N-channel
U
holds the
TSOURCE
BGATEDR
TGATEDR
TGATEFB
BGATEFB
R
negative inductor current is returned to HV when the
switch turns back on. However, I
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
GS
HV
+
R
SENSE
+
1336 F04
2
V
R losses will occur
OUT
LT1336
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