LT1336 Linear Technology, LT1336 Datasheet - Page 10

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

LT1336
APPLICATIONS
In applications where switching is always above 10kHz
and the duty cycle never exceeds 90%, Pins 1, 15 and 16
can be left open. The bootstrap capacitor is then charged
by conventional bootstrapping. Only a diode needs to be
connected between V
strap capacitor is usually adequate using this technique
for driving a single MOSFET under 10,000pF. When driv-
ing multiple MOSFETs in parallel, if the total gate capaci-
tance exceeds 10,000pF, the bootstrap capacitor should
be increased proportionally above 0.1 F (see Paralleling
MOSFETs).
Deriving the Floating Supply with the Boost Topology
The advantage of using the boost topology is its simplicity.
Only a resistor, a small inductor, a diode and a capacitor
are needed. However, the high voltage rail may not exceed
40V to avoid reaching the collector-base breakdown volt-
age of the internal NPN switch.
The recommended values for the current sense resistor,
inductor and bootstrap capacitor are 2 , 200 H and 1 F
respectively. Using the recommended component values
the boost regulator will run at around 700kHz. To lower the
frequency the inductor value can be increased and to
increase the frequency the inductor value can be de-
creased. The sense resistor should be at least 1.5 to
maintain adequate inductor current limit. The bootstrap
capacitor value should be 1 F or larger to minimize ripple
voltage. An example of a boost regulator is shown in
Figure 1.
10
* SUMIDA RCR-664D-221KC
R
2
1/4W
SENSE
SV
PV
Figure 1. Using the Boost Regulator
+
+
I
SENSE
LT1336
U
1N4148
TSOURCE
200 H*
TGATEDR
D1
TGATEFB
SWITCH
+
SWGND
BOOST
and the Boost pin. A 0.1 F boot-
S
INFORMATION
U
+
V
–
BOOST
+
W
D2
1N4148
C
1 F
BOOST
HV = 40V MAX
U
1336 F01
+
The boost regulator works as follows: when switch S is on,
the inductor current ramps up as the magnetic field builds
up. During this interval energy is being stored in the
inductor and no power is transferred to V
inductor peak current is reached, sensed by the 2
resistor, the switch is turned off. Energy is no longer
transferred to the inductor causing the magnetic field to
collapse. The collapsing magnetic field induces a change
in voltage across the inductor. The Switch pin voltage rises
until diode D2 starts conducting. As the inductor current
ramps down, the lower inductor current threshold is
reached and switch S is turned off, thus completing the
cycle.
Current drawn from V
this current (~ 1.5mA) flows through the topside driver to
the Top Source pin. This current is typically returned to
ground via the bottom MOSFET or the output load. If the
bottom MOSFET were off and the output load were re-
turned to HV, then the Top Source pin will return the
current to HV through the top MOSFET or the output load.
If the HV supply cannot sink current and no load drawing
greater than 1.5mA is connected to the supply, then a
resistor from HV to ground may be needed to prevent
voltage buildup on the HV supply.
Note that the current drawn from V
V
V
Deriving the Floating Supply with the Flyback
Topology
For applications where the high voltage rail is greater than
40V, the flyback topology must be used. To configure a
flyback regulator, a resistor, a diode, a small 1:1 turns ratio
transformer and a capacitor are needed. The maximum
voltage across the switch, assuming an ideal transformer,
will be about V
transformers will induce an overvoltage spike at the switch
at the instant when it opens. These spikes can be clamped
using a snubbing network or a Zener. Unlike the boost
topology, the current drawn from V
is equal to the current drawn from V
BOOST
BOOST
I
IN V
is significantly higher than the current drawn from
as given by:
I
OUT
+
V
+ 11.3V. Leakage inductance in nonideal
BOOST
V
+
is delivered to V
+
BOOST
(assuming no loss)
+
and delivered to
BOOST
BOOST
.
. When the
. Some of