Rainbow Electronics MAX747 User Manual
Page 10
gate capacitance, because that energy is dissipated
by the gate-drive circuitry, not the P-FET.
The
Standard Application Circuit (Figure 1a, 1b) uses
an 8-pin Si9405DY surface-mount P-FET that has 0.1
Ω
on resistance with a 10V V
GS
. Optimum efficiency is
obtained when the voltage at the drain swings between
the supply rails (within a few hundred mV).
Diode Selection
The MAX747’s high switching frequency demands a
high-speed rectifier. Schottky diodes are recommended.
Ensure that the Schottky diode average current rating
exceeds the load current level.
Capacitor Selection
Output Filter Capacitor
The output filter capacitor C1 should have a low
effective series resistance (ESR), and its capacitance
should remain fairly constant over temperature. This is
especially true when in CCM, since the output filter
capacitor and the load form the dominant pole that
stabilizes the loop. 430µF is adequate for load currents
up to 2.3A in Figure 1a. At low input/output
differentials, it may be necessary to use much larger
output filter capacitors to maintain adequate load-
transient response. See the
AC Stability with Low
Input/Output Differentials section.
Sprague 595D surface-mount solid tantalum capacitors
and Sanyo OS-CON through-hole capacitors are
recommended due to their extremely low ESR. OS-CON
capacitors are particularly useful at low temperatures.
For best results when using other capacitors, increase
the output filter capacitor’s size or use capacitors in
parallel to reduce ESR.
Input Bypass Capacitor
The input bypass capacitor C2 reduces peak currents
drawn from the voltage source, and also reduces noise
at the voltage source caused by the MAX747’s fast
switching action (this is especially important when other
circuitry is operated from the same source). The input
capacitor ripple current rating must exceed the RMS
input current.
For load currents up to 2.5A, 100µF (C2) in parallel with
a 0.1µF (C3) is adequate. Smaller bypass capacitors
may be acceptable for lighter loads. The input voltage
source impedance determines the capacitor size
required at the V+ input. As with the output filter
capacitor, a low-ESR capacitor (Sanyo OS-CON,
Sprague 595D, or equivalent) is recommended for
input bypassing.
Soft-Start and Reference Capacitors
A typical value for the soft-start capacitor C4 is 0.1µF,
which provides a 380ms ramp to full current limit. Use
values in the 0.001µF and 1µF range. The nominal time
for C4 to reach its steady-state value is given by:
Note that t
SS
does
not
equal the time it takes for the
MAX747 to power up, although it does affect start-up
time. Start-up time is also a function of the input voltage
and load current. With a 2.5A load current, a 7V input
voltage, and a 0.1µF soft-start capacitor, power-up
takes typically 360ms.
Bypass REF with a 0.22µF capacitor (C5).
Compensation Capacitor
With a fixed +5V output, connect the compensation
capacitor (C6) between CC and GND to optimize
transient response. Appropriate compensation is
determined by the ESR of the output filter capacitor
(C1) and the feedback voltage-sense resistor network.
270pF is adequate for applications where V+
≤
9V.
Over the full input voltage range, increase C6 to 470pF.
C6 also depends on the load current, so for light loads,
C6’s value can be reduced. If appropriate
compensation is not obtained using 470pF, use the
following equations to determine C6:
For fixed 5V output operation,
For adjustable-output operation, FB becomes the
compensation input pin and CC is left unconnected.
Connect C6 between FB and GND in parallel with R4
(Figure 5). C6 is determined by:
For example, with a fixed 5V output, C1 = 330µF and
an ESR
C1
of 0.04
Ω
(at a 100kHz frequency),
C6
(C1) (ESR
)
24k
783pF
C1
=
Ω
=
C6
(C1) (ESR
)
R4 II R5
C1
=
C6
(C1) (ESR
)
24k
C1
=
Ω
t
(sec)
(C4) (3.8
10 )
SS
6
=
×
I
RMS AC input current
I
V
(V
V
)
V
RMS
LOAD
OUT
IN
OUT
IN
=
=
−
MAX747
High-Efficiency PWM, Step-Down
P-Channel DC-DC Controller
10
______________________________________________________________________________________