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

Low-Noise, Step-Up DC-DC Converters

14

______________________________________________________________________________________

Power-On Reset Delay

Adding a timing capacitor from POK to GND generates

a power-on reset delay. The reset time constant is

determined by the pull-up resistor and timing capacitor

(Figure 11). When power is turned on, POK is low and

the capacitor is shorted. When the output voltage

reaches regulation, POK goes high and the capacitor

slowly charges to the output voltage.

The timing resistor value depends on the controller’s

RESET input leakage current. The voltage drop across

the timing resistor should not exceed the difference

between the output voltage and the µC reset threshold

voltage. This resistor should be large enough to mini-

mize the shutdown current.

µC-Controlled Shutdown

The MAX848/MAX849 turn on when ON1 = 1 or ON2 = 0.

The µC monitors the battery voltage and turns off the

device (forces ON1 low and ON2 high) when the bat-

tery is weak.

Layout Considerations

Due to high inductor current levels and fast switching

waveforms, which radiate noise, proper PC board lay-

out is essential. Protect sensitive analog grounds by

using a star ground configuration. Minimize ground

noise by connecting PGND, the input bypass capacitor

ground lead, and the output filter capacitor ground lead

to a single point (star ground configuration). Also, mini-

mize lead lengths to reduce stray capacitance and

trace resistance.

If an external resistor divider is used to set the output

voltage (Figure 5), the trace from FB to the resistors

must be extremely short and must be shielded from

switching signals, such as CLK, DATA, or LX.

MAX848

MAX849

OUT

POK

RESET

µC

R

C

Figure 11. Power-On Reset Delay

Table 4. Component Selection Guide

PRODUCTION

INDUCTORS

CAPACITORS

DIODES

Surface Mount

Sumida CDR63B, CD73, CDR73B, CD74B series

Coilcraft DO1608, DO3308, DT3316 series

Matsuo 267 series

Sprague 595D series

AVX TPS series

Motorola MBR0520L

Through Hole

Sumida RCH654 series

Sanyo OS-CON series

Nichicon PL series

Motorola 1N5817

Table 5. Component Suppliers

SUPPLIER

PHONE

FAX

AVX

USA: 803-946-0690

800-282-4975

803-626-3123

Coilcraft

USA: 847-639-6400

847-639-1469

Matsuo

USA: 714-969-2491

714-960-6492

Motorola

USA: 602-303-5454

602-994-6430

Sanyo

USA: 619-661-6835

Japan: 81-7-2070-6306

619-661-1055

81-7-2070-1174

Sumida

USA: 847-956-0666

Japan: 81-3-3607-5111

847-956-0702

81-3-3607-5144

Chip Information

TRANSISTOR COUNT: 2059