Typical Waveforms

Application Information

Minimum Pulse Width

In order for the shoot-through prevention circuitry in the

LM2722 to work properly, the pulses into the PWM_IN pin

must be longer than 55ns. The internal logic waits until the

first FET is off plus 20ns before turning on the opposite FET.

If, after a falling edge, a rising edge occurs sooner than the

specified time, t

the top gate when the bottom gate is off. As the rising edge

occurs sooner and sooner, the driver may start to ignore the

pulse and produce no output. This condition results in the

PWM_IN pin in a high state and neither FET turned on. To

get out of this state, the PWM_IN pin must see a low signal

for greater than 55ns, before the rising edge.

This will also assure that the gate drive bias voltage has

been restored by forcing the top FET source and C

ground first. Then the internal circuitry is reset and normal

operation will resume.

Conversely, if, after a rising edge, a falling edge occurs

sooner than the specified miniumum pulse width, t

IC may intermittently fail to turn on the bottom FET. As the

falling edge occurs sooner and sooner, the driver will start to

ignore the pulse and produce no output. This will result in the

t

for non-synchronous operation. The circuit will resume syn-

chronous operation when the rising PWM pulses exceed

55ns in duration.

High Input Voltages or High Output Currents

At input voltages above twice the output voltage and at

higher power levels, the designer may find snubber networks

and gate drive limiting useful in reducing EMI and preventing

injurious transients. A small resistor, 1

Ω to 5Ω, between the

driver outputs and the MOSFET gates will slightly increase

the rise time and fall time of the output stage and reduce

switching noise. The trade-off is 1% to 2% in efficiency.

A series R-C snubber across in parallel with the bottom FET

can also be used to reduce ringing. Values of 10nF and 10

Ω

to 100

Ω are a good starting point.

20028913

FIGURE 1. Switching Waveforms of Test Circuit

20028914

FIGURE 2. When Input Goes High

20028915

FIGURE 3. When Input Goes Low

20028916

FIGURE 4. Minimum Positive Pulse

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