LT1336CN-TRPBF View Datasheet(PDF) - Linear Technology
Part Name
Description
MFG CO.
LT1336CN-TRPBF Datasheet PDF : 20 Pages
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LT1336
Applications Information
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+ and the Boost pin. A 0.1µF bootstrap
capacitor is usually adequate using this technique for driv-
ing a single MOSFET under 10,000pF. When driving multiple
MOSFETs in parallel, if the total gate capacitance exceeds
10,000pF, the bootstrap capacitor should be increased
proportionally above 0.1µF (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 2Ω, 200µH and
1µF 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
decreased. The sense resistor should be at least 1.5Ω to
maintain adequate inductor current limit. The bootstrap
capacitor value should be 1µF or larger to minimize ripple
voltage. An example of a boost regulator is shown in
Figure 1.
D1
1N4148
200µH*
RSENSE
2Ω
1/4W
ISENSE
SV+
PV+
SWITCH
S
SWGND
BOOST
LT1336
TSOURCE
+
–VBOOST
TGATEDR
D2
1N4148
+ CBOOST
1µF
HV = 40V MAX
+
TGATEFB
* SUMIDA RCR-664D-221KC
1336 F01
Figure 1. Using the Boost Regulator
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 VBOOST. When the
inductor peak current is reached, sensed by the 2Ω 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 + is delivered to VBOOST. Some of
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
returned 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 draw-
ing 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 + and delivered to
VBOOST is significantly higher than the current drawn from
VBOOST as given by:
IIN(V +
)
= IOUT
VBOOST
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 + + 11.3V. Leakage inductance in nonideal
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+ (assuming no loss)
is equal to the current drawn from VBOOST.
1336fa
11
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