Application information

L6591

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Application information

The L6591 is an advanced current-mode PWM controller specific for fixed-frequency, peak-

current-mode-controlled ZVS half-bridge converters. In these converters the switches

(MOSFET’s) are controlled with complementary duty cycle: the high-side MOSFET is driven

ON for a duty cycle D and the low-side MOSFET for a duty cycle 1-D. For a proper operation

the maximum allowed duty cycle must be limited below 50 %.

and the turn-on of the other one ensures soft-switching and enables high-frequency

operation with high efficiency and low EMI emissions. See Section 5.6: Oscillator and dead-

The device is able to operate in different modes (Figure 4), depending on the converter’s

load conditions:

1.

Fixed frequency at heavy load. A relaxation oscillator, externally programmable with a

capacitor and a resistor generates a sawtooth and releases clock pulses during the

falling edges of the sawtooth. In this region the low-side MOSFET is turned off by the

even pulses of the clock signal and the high-side MOSFET is turned on after a delay;

the high-side MOSFET is turned off and, after a delay, the low-side MOSFET is turned

on in response to the control loop.

2.

Burst-mode control with no or very light load. When the load is extremely light or

disconnected, the converter will enter a controlled on/off operation with fixed duty cycle,

where a series of few switching cycles are spaced out by long periods where both

MOSFET

’s are in OFF-state. A load decrease will be then translated into a frequency

reduction, which can go down even to few hundred hertz, thus minimizing all

frequency-related losses and making it easier to comply with energy saving

regulations. Being the peak current very low, no issue of audible noise arises.

Figure 4.

Multi-mode operation