1- to 3-Cell, High-Current, Low-Noise,

Step-Up DC-DC Converters with Linear Regulator

______________________________________________________________________________________

17

Implementing Soft-Start

To implement soft-start, set CLK/SEL low on power-up;

this forces PFM operation and reduces the peak switch-

ing current to 435mA. Once the circuit is in regulation,

CLK/SEL can be set high for full-power operation.

Adding a Manual Power Reset

A momentary pushbutton switch can be used to turn

the MAX1705/MAX1706 on and off (Figure 10). ONA is

pulled low and ONB is pulled high to turn the part off.

When the momentary switch is pressed, ONB is pulled

low and the regulator turns on. The switch must be

pressed long enough for the microcontroller (µC) to exit

reset (200ms) and drive ONA high. A small capacitor is

added to help debounce the switch. The µC issues a

logic high to ONA, which holds the part on regardless

of the switch state. To turn the regulator off, press the

switch again, allowing the µC to read the switch status

and pull ONA low. When the switch is released, ONB is

pulled high.

___________________Chip Information

TRANSISTOR COUNT: 1649

SUBSTRATE CONNECTED TO GND

MAX1705

2 NiCd/NiMH

2.4

3.3

730

2 NiCd/NiMH

2.4

5.0

500

3 NiCd/NiMH or 1 Li-Ion

3.6

5.0

850

MAX1706

450

350

550

Table 5. Typical Available Output Current

µC

I/O

MAX1705

MAX1706

ONA

ONB

OUT

I/O

0.1

µF

ON/OFF

270k

270k

Figure 10. Momentary Pushbutton On/Off Switch

MAX1705

MAX1706

PA

RF

LX

POUT

GND

LDO

CONTROL

INPUTS

I/O

Figure 9. Typical Phone Application

1 NiCd/NiMH

1.2

3.3

300

200

NUMBER OF CELLS

INPUT VOLTAGE

(V)

STEP-UP OUTPUT VOLTAGE:

(PA POWER SUPPLY)

(V)

TOTAL OUTPUT CURRENT

(mA)