1-Cell to 3-Cell, High-Power,

Low-Noise, Step-Up DC-DC Converters

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13

Capacitor Selection

Input Bypass Capacitors

A 22µF, low-ESR input capacitor will reduce peak cur-

rents and reflected noise due to inductor current ripple.

Smaller ceramic capacitors may also be used for light

loads or in applications that can tolerate higher input

ripple.

Output Filter Capacitors

Two 100µF (single 100µF for the MAX848), 10V, low-

ESR, output filter capacitors typically exhibit 30mV rip-

ple when stepping up from 1.2V to 3.3V at 200mA

(100mA for the MAX848). Bypass the MAX848/MAX849

supply input, OUT, with a 0.1µF ceramic capacitor

to GND. Also bypass POUT to PGND with a 0.1µF

ceramic capacitor.

The filter capacitors’ equivalent series resistance (ESR)

affects efficiency and output ripple. The output voltage

ripple is the product of the peak inductor current and

the output capacitor’s ESR. Low-ESR, surface-mount

tantalum capacitors are currently available from

Sprague (595D series) and AVX (TPS series). Sanyo

OS-CON organic-semiconductor, through-hole capaci-

tors also exhibit very low ESR, and are especially useful

for operation at cold temperatures. See Table 5 for a list

of suggested capacitor suppliers.

Applications Information

Using a Momentary On/Off Switch

A momentary pushbutton switch can be used to turn

the MAX848/MAX849 on and off.

As shown in Figure 9, ON1 is pulled low and ON2 is

pulled high when the part is off. When the momentary

switch is pressed, ON2 is pulled low and the regulator

turns on. The switch should be on long enough for the

µC to exit reset. The controller issues a logic high to

ON1, which guarantees that the part will stay on,

regardless of the switch state.

To turn off the regulator, the switch is pressed and held.

The controller reads the switch status and pulls ON1

low. The switch is released and ON2 is pulled high.

Power Amplifier (PA) and Radio Supply

in a Typical Phone Application

The MAX849 is an ideal power supply for the power

amplifier (PA) and the radio used in digital cordless

and PCS phones (Figure 10). The PA is directly pow-

ered by the MAX849 for maximum output swing. Post-

linear regulators power the controller and the radio. In

addition, they reduce switching noise and ripple. Table

3 lists the output power available when operating with

one or more NiCd/NiMH cells or one Li-Ion cell.

MAX848

MAX849

ON1

ON2

I/O

I/O

OUT

µC

1M

Ω

1M

Ω

Figure 9. Momentary Pushbutton On/Off Switch

MAX848

MAX849

µC

MAX8865/MAX8866 DUALS

MAX8863/MAX8864 SINGLES

RADIO

PA

Figure 10. Typical Phone Application

NUMBER OF CELLS

INPUT VOLTAGE

(V)

OUTPUT VOLTAGE:

PA POWER SUPPLY

(V)

OUTPUT POWER

(W)

1 NiCd/NiMH

1.2

3.3

0.9

2 NiCd/NiMH

2.4

3.3

2.4

2 NiCd/NiMH

2.4

5.0

2.6

3 NiCd/NiMH or 1 Li-Ion

3.6

5.0

4.3

Table 3. Available Output Power