MAX5051
diode is a high-voltage, small-signal Schottky type. It
may be helpful to connect a resistor in series with this
diode to minimize noise as well as reduce the peak
charging currents. As in any other switching power-
supply circuit, the gate-drive loops must be kept to a
minimum. Plan PC board layout with the critical current
carrying loops of the circuit as a starting point.
Secondary-Side Synchronization
The MAX5051 has additional (LXH and LXL) outputs to
make the driving of secondary-side synchronous recti-
fiers possible with a signal from the primary. These sig-
nals lead in time, the actual gate drive applied to the
main power FETs, and allow the secondary-side syn-
chronous FETs to be commutated in advance of the
power pulse. The synchronizing pulse is generated
approximately 90ns ahead of the main pulse that drives
the two power FETs.
Synchronization is accomplished by connecting a small
pulse transformer between LXH and LXL, along with
some clamp diodes (D1 and D2 in Figure 4). This is a
small integrated two-switch driver configuration that
allows for full recovery of the stored energy in the mag-
netizing inductance of the pulse transformer, thereby
significantly reducing the running bias current of the
controller. It also allows for correct transfer of DC levels
without requiring series capacitors with large time con-
stants, assuring correct drive levels for the secondary
circuit.
Select a pulse transformer, T1, so the current buildup in
its magnetizing inductance is low enough not to create
a significant voltage droop across the internal driver
FETs. Use the following formula to calculate the
approximate value of the primary magnetizing induc-
tance of T1:
where R
dsLXH
and R
dsLXL
are the internal high- and low-
side pulse transformer driver on-resistances, f
s
is the
switching frequency, L
M
is the pulse transformer primary
magnetizing inductance, t
s
is the transition time at the
drains of these FETs (typically < 40ns), and C
ds
is the
total drain-source capacitance (approximately 10pF).
Alternatively, a high-speed optocoupler (Figure 5) can
be used instead of the pulse transformer. The look-
ahead pulse accommodates the propagation delays of
the high-speed optocoupler as well as the delays
through the gate drivers of the secondary-side FETs.
Choose optocouplers with propagation delays of less
than 50ns.
Error Amplifier And Reference Soft-Start
The error amplifier in the MAX5051 has an uncommitted
inverting input (FB) and output (COMP). Use this ampli-
fier when secondary isolation is not required. COMP
can then be directly connected to CON (the input of the
PWM comparator). The noninverting input of the error
amplifier is connected to the soft-start generator and is
also available externally at CSS. A capacitor connected
to CSS is slewed linearly during initial startup with the
70µA internal current source (see Figure 2). This pro-
vides a linearly increasing reference to the noninverting
input of the error amplifier forcing the output voltage
also to slew proportionally. This method of soft-start is
superior to other methods because the loop is always
T1: PULSE ENGINEERING, PE-68386.
D1, D2: CENTRAL SEMICONDUCTOR, CMOSH-3.
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