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

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:

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

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

=

=

−

















High-Efficiency PWM, Step-Down

P-Channel DC-DC Controller

10

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