High-Frequency, High-Power, Low-Noise,

Step-Up DC-DC Converter

12

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Diode Selection (D1)

The MAX1708’s high switching frequency demands a

high-speed rectifier. Use Schottky diodes (Table 3).

The diode’s current rating must exceed the maximum

load current, and its breakdown voltage must exceed

LX switching node and the output filter capacitor. The

diode also must be able to dissipate the power calcu-

lated by the following equation:

diode forward voltage at the peak switch current.

Capacitor Selection

Input Bypass Capacitor (C1)

A 150μF, low-ESR input capacitor will reduce peak cur-

rents and reflected noise due to inductor current ripple.

Lower ESR allows for lower input ripple current, but

combined ESR values up to 100m

Ω are acceptable.

Smaller ceramic capacitors may also be used for light

loads or in applications that can tolerate higher input

current ripple.

Output Filter Capacitor (C2)

The output filter capacitor ESR must be kept under

30m

Ω for stable operation. Polymer capacitors of

150μF (Panasonic EEFUE0J151R) typically exhibit

10m

Ω of ESR. This translates to approximately 35mV of

output ripple at 3.5A switch current. Bypass the

MAX1708 IC supply input (OUT) with a 0.1μF ceramic

capacitor to GND and a 2

Ω series resistor (R2, as

shown in Figure 1).

MAX1708 IC Power Dissipation

The major components of MAX1708 dissipated power

examples provided in brackets ({ }) correspond to the

following condition:

An important parameter to compute the power dissipat-

ed in the MAX1708 is the approximate peak switch cur-

where:

Ω}

f = switching frequency {600kHz}

Applications Information

Using a Momentary On/Off Switch

A momentary pushbutton switch can be used to turn

the MAX1708 on and off. As shown in Figure 5, when

ONA is pulled low and ONB is pulled high, the device

is off. When the momentary switch is pressed, ONB is

pulled low and the regulator turns on. The switch

should be on long enough for the microcontroller to exit

reset. The controller issues a logic high to ONA, which

guarantees that the device will stay on regardless of

the subsequent switch state. To turn the regulator off,

depress the switch long enough for the controller to

read the switch status and pull ONA low. When the

switch is released, ONB pulls high and the regulator

turns off.

I

I

D

A

D

V

VV

SW

OUT

IN

OUT

D

'

{.

}

'

{.}

=

=

+

333

06

μC

270k

Ω

ONB

ONA

0.1

μF

270k

Ω

ON/OFF

MAX1708

I/O

I/O

Figure 5. Momentary Pushbutton On-Off Switch