Datasheet

L6591 Application information
Doc ID 14821 Rev 6 21/41
6 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) 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%.
An externally programmable deadtime T
D
inserted between the turn-off of one MOSFET
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 6.6: Oscillator and
deadtime programming on page 28
section for more information on how to program T
D
.
The device is able to operate in different modes (
Figure 39 ), 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 enters 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 is 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. The peak current is very low and so no issue of audible noise arises.
Figure 39. Multi-mode operation
AM13255v1