9

LT1336

floating supply. This allows the output to smoothly transi-

tion to 100% duty cycle.

An undervoltage detection circuit disables both channels

undervoltage detect block disables the high side channel

The top and bottom gate drivers in the LT1336 each utilize

two gate connections: 1) a Gate Drive pin, which provides

the turn-on and turn-off currents through an optional

series gate resistor, and 2) a Gate Feedback pin which

connects directly to the gate to monitor the gate-to-source

voltage.

Whenever there is an input transition to command the

outputs to change states, the LT1336 follows a logical

sequence to turn off one MOSFET and turn on the other.

decreased below the turn-off threshold, and finally the

other gate is turned on.

The LT1336 incorporates two independent driver chan-

nels with separate inputs and outputs. The inputs are

TTL/CMOS compatible; they can withstand input voltages

has 300mV of hysteresis. Both channels are noninverting

drivers. The internal logic prevents both outputs from

simultaneously turning on under any input conditions.

When both inputs are high both outputs are actively

held low.

An internal switching regulator permits smooth transition

from PWM to DC operation. In PWM operation the boot-

strap capacitor is recharged each time Top Source pin

goes low. As the duty cycle approaches 100% the output

pulse width becomes narrower and the time available to

produce an elevated upper MOSFET gate supply becomes

shorter than required. As the voltage across the bootstrap

capacitor drops below 10.6V, an inductor-based switch-

ing regulator kicks in and takes over the charging of the

pin is still needed to allow conventional bootstrapping of

the bootstrap capacitor when duty cycles are below 90%.

The LT1336’s internal switching regulator can provide

enough charge to the bootstrap capacitor to allow the top

driver to drive several power MOSFETs in parallel at its

maximum operating frequency. The regulated voltage

exceeded due to normal bootstrap action, the regulator

automatically shuts down.

The switching regulator uses a hysteretic current mode

control. This method of control is simple, inherently stable

and provides peak inductor current limit in every cycle. It

is designed to run at a nominal frequency of around

700kHz which is 7

× the maximum PWM operating fre-

quency of the LT1336. Since the hysteretic current mode

control has no internal oscillator, the frequency is deter-

mined by external conditions such as supply voltage and

load currents and external components such as inductor

value and current sense resistor value.

Deriving the Floating Supply

In a typical half-bridge driver like the LT1158 or the

LT1160, the floating supply for the topside driver is

provided by a bootstrap capacitor. This capacitor is re-

charged each time its negative plate goes low in PWM

operation. As the duty cycle approaches 100% the output

pulse width becomes narrower and the time available to

recharge the bootstrap capacitor becomes shorter than

required (1

µs to 2µs). For instance, at 100kHz and at 95%

duty cycle the output pulse width is only 0.5

µs; clearly this

is insufficient time to recharge the capacitor by

bootstrapping. To get around this problem, the LT1336

incorporates a switching regulator to help recharge the

bootstrap capacitor under such extreme conditions.

The LT1336 provides all the necessary circuitry to con-

struct a boost or flyback switching regulator. This regula-

tor can charge the bootstrap capacitor when it cannot

recharge by bootstrapping. This happens when nearing

100% duty cycle in PWM applications. This is a worst-

case condition because the bootstrap capacitor must still

provide for the gate charging current of the high side