10

LT1336

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

µF boot-

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

µF (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

Ω, 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 de-

creased. The sense resistor should be at least 1.5

Ω to

maintain adequate inductor current limit. The bootstrap

capacitor value should be 1

µF or larger to minimize ripple

voltage. An example of a boost regulator is shown in

Figure 1.

Figure 1. Using the Boost Regulator

SWITCH

2

Ω

1/4W

1

µF

D2

1N4148

S

HV = 40V MAX

–

+

1336 F01

LT1336

200

µH*

D1

1N4148

* SUMIDA RCR-664D-221KC

+

+

SWGND

TGATEDR

TGATEFB

BOOST

TSOURCE

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 peak current is reached, sensed by the 2

Ω

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.

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.

II

V

V

IN V

OUT

BOOST

+









+

=









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,

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