ISL6742_14 Datasheet, PDF (13/18 Pages) Intersil Corporation – Advanced Double-Ended PWM Controller

ISL6742
Synchronous Rectifier Outputs and Control
The ISL6742 provides double-ended PWM outputs, OUTA
and OUTB, and synchronous rectifier (SR) outputs, OUTAN
and OUTBN. The SR outputs are the complements of the
PWM outputs. It should be noted that complemented outputs
are used in conjunction with the opposite PWM output, i.e.
OUTA and OUTBN are paired together and OUTB and
OUTAN are paired together.
Referring to Figure 11, the SRs alternate between being both
on during the free-wheeling portion of the cycle
(OUTA/OUTB off), and one or the other being off when
OUTA or OUTB is on. If OUTA is on, its corresponding SR
must also be on, indicating that OUTBN is the correct SR
control signal. Likewise, if OUTB is on, its corresponding SR
must also be on, indicating that OUTAN is the correct SR
control signal.
CT
OUTA
OUTB
OUTAN
(SR1)
OUTBN
(SR2)
FIGURE 12. WAVEFORM TIMING WITH PWM OUTPUTS
DELAYED, 0V < VADJ < 2.425V
CT
OUTA
OUTB
OUTAN
(SR1)
OUTBN
(SR2)
FIGURE 11. BASIC WAVEFORM TIMING
A useful feature of the ISL6742 is the ability to vary the
phase relationship between the PWM outputs (OUTA,
OUTB) and their complements (OUTAN, OUTBN) by
±300ns. This feature allows the designer to compensate for
differences in the signal propagation delays between the
PWM FETs and the SR FETs. A voltage applied to VADJ
controls the phase relationship. Figures 12 and 13
demonstrate the delay relationships.
CT
OUTA
OUTB
OUTAN
(SR1)
OUTBN
(SR2)
FIGURE 13. WAVEFORM TIMING WITH SR OUTPUTS
DELAYED, 2.575V < VADJ < 5.00V
Setting VADJ to VREF/2 results in no delay on any output.
The no delay voltage has a ±75mV tolerance window.
Control voltages below the VREF/2 zero delay threshold
cause the PWM outputs, OUTA/OUTB, to be delayed.
Control voltages greater than the VREF/2 zero delay
threshold cause the SR outputs, OUTAN/OUTBN, to be
delayed. It should be noted that when the PWM outputs,
OUTA/OUTB, are delayed, the CS to output propagation
delay is increased by the amount of the added delay.
The delay feature is provided to compensate for mismatched
propagation delays between the PWM and SR outputs as
may be experienced when one set of signals crosses the
primary-secondary isolation boundary. If required, individual
output pulses may be stretched or compressed as required
using external resistors, capacitors, and diodes.
13
FN9183.2
October 31, 2008