High-Efficiency, PWM, Step-Down,

N-Channel DC-DC Controller

14

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For load currents up to 3A, 100µF (C2) in parallel with

0.1µF (C3) is adequate. Smaller bypass capacitors may

also be acceptable for lighter loads. The input voltage

source impedance determines the size of the capacitor

required at the V+ input. As with the output filter capaci-

tor, a low-ESR capacitor (Sanyo OS-CON, Sprague 595D

or equivalent) is recommended for input bypassing.

Charge-Pump Capacitors

Figure 3a shows the charge-pump doubler circuit con-

figured with a 0.1µF charge-pump capacitor C8 and a

1.0µF reservoir capacitor C9. The ratio of the capaci-

tors, along with the input voltage, determines the

amount of ripple on HIGH. If the input supply range

exceeds 12V, increase C9 to 4.7µF to reduce the

charge-pump ripple. C9 should be 10µF for less.

Figure 3b shows the charge-pump tripler circuit.

Refer to Table 2 to determine the proper charge-pump

configuration (which is based on the minimum expect-

ed supply voltage at V+).

Some interaction occurs between the switch oscillator

and the charge-pump oscillator. This interaction modu-

lates the inductor-current waveform, but has negligible

impact on the output.

Soft-Start and Reference Capacitors

Soft-start provides a ramp to the full current limit. A typi-

cal value for the soft-start capacitor (C5) is 0.1µF,

which provides a 380ms soft-start time. Use values in

the 0.001µF to 1µF range. The nominal time for C5 to

reach its steady-state value is given by:

MAX746 to power-up, although it does affect the start-

up time. The start-up time is also a function of the input

voltage and load current. With a 3A load current, a 10V

input voltage, and a 0.1µF soft-start capacitor, it typi-

cally takes 240ms for the MAX746 to power up. A

0.47µF soft-start capacitor increases the start-up time

to approximately 2.3sec.

Bypass REF with a 1µF capacitor (C6).

Compensation Capacitor

With a fixed 5V output, connect a compensation capac-

itor (C7) between CC and AGND to optimize transient

response. Appropriate compensation is determined by

the size and ESR of the output filter capacitor (C1), and

by the load current.

In the standard 5V application circuit, 2.7nF is appro-

priate for load currents up to 3A; for lighter loads,

C7’s value can be reduced. If 2.7nF does not com-

pensate adequately, use the following equations to

determine C7.

For fixed 5V-output operation:

C7 = _____________

12k

Ω

For adjustable-output operation, FB becomes the

compensation input pin, and CC and OUT are left

unconnected. Connect C7 between FB and GND in

parallel with R4 (Figure 6). C7 is determined by:

C7 = ___________________

R4

 R5

For example, with a fixed 5V output with C1 = 470µF

Ω (at a frequency of 100kHz):

C7 = _____________ = 1560pF

12k

Ω

Table 1. Measured Output Voltage Sag

with 30mA to 3A Load Step*

Table 2. Charge-Pump Configuration

V+

CHARGE-PUMP CONFIGURATION

V+

≤ 6V

6V < V+ < 6.5V

*

V+

Voltage tripler with 1N914 diodes for D2,

D3, D5, and D6

Voltage doubler with 1N5817 Schottky

diodes for D2 and D3

Voltage doubler with 1N914 diodes for

D2 and D3

temperature range, increase the 6.5V limit to 7V.

OUTPUT

FILTER

CAPACITOR

C1 (

mF)

OUTPUT VOLTAGE SAG (mV)

FOR VARIOUS INPUT VOLTAGES

440

400

250

210

140

90

660

260

190

160

70

50

880

200

100

90

40

25