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

Step-Up DC-DC Converters with Linear Regulator

16

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from higher-impedance batteries, consider using one

or two 100µF, 100m

Ω, low-ESR tantalum capacitors.

Low-ESR capacitors are recommended. Capacitor ESR

is a major contributor to output ripple—often more than

70%.

Ceramic, Sanyo OS-CON, and Panasonic SP/CB-series

capacitors offer the lowest ESR. Low-ESR tantalum

capacitors are second best and generally offer a good

trade-off between price and performance. Do not

exceed the ripple-current ratings of tantalum capaci-

tors. Avoid aluminum-electrolytic capacitors, since their

ESR is too high.

Adding Bypass Capacitors

Several ceramic bypass capacitors are required for

proper operation of the MAX1705/MAX1706. Bypass

REF with a 0.33µF capacitor to GND. Connect a 0.1µF

ceramic capacitor from OUT to GND and a 0.33µF

ceramic capacitor from POUT to PGND. Place a 22µF,

low-ESR capacitor and an optional 0.33µF ceramic

capacitor from the linear-regulator output LDO to GND.

An optional 22pF ceramic capacitor can be added to

the linear-regulator feedback network to reduce noise

(C2, Figure 2). Place each of these as close to their

respective pins as possible, within 0.2in. (5mm) of the

DC-DC converter IC. High-value, low-voltage, surface-

mount ceramic capacitors are now readily available in

small packages; see Table 4 for suggested suppliers.

Designing a PC Board

High switching frequencies and large peak currents

make PC board layout an important part of design.

Poor design can cause excessive EMI and ground-

bounce, both of which can cause instability or

regulation errors by corrupting voltage- and current-

feedback signals. It is highly recommended that the PC

board example of the MAX1705 evaluation kit (EV kit)

be followed.

Power components—such as the inductor, converter

IC, filter capacitors, and output diode—should be

placed as close together as possible, and their traces

should be kept short, direct, and wide. Place the LDO

output capacitor as close to the LDO pin as possible.

Make the connection between POUT and OUT very

short. Keep the extra copper on the board, and inte-

grate it into ground as a pseudo-ground plane.

On multilayer boards, do not connect the ground pins

of the power components using vias through an internal

ground plane. Instead, place them close together and

route them in a star-ground configuration using compo-

nent-side copper. Then connect the star ground to the

internal ground plane using vias.

Keep the voltage-feedback networks very close to the

MAX1705/MAX1706—within 0.2in. (5mm) of the FB and

FBLDO pins. Keep noisy traces, such as from the LX

pin, away from the reference and voltage-feedback net-

works, especially the LDO feedback, and separated

from them using grounded copper. Consult the

MAX1705/MAX1706 EV kit for a full PC board example.

__________ Applications Information

Use in a Typical

Wireless Phone Application

The MAX1705/MAX1706 are ideal for use in digital cord-

less and PCS phones. The power amplifier (PA) is con-

nected directly to the step-up converter output for

maximum voltage swing (Figure 9). The internal linear

regulator is used for post-regulation to generate low-

noise power for DSP, control, and RF circuitry. Typically,

RF phones spend most of their life in standby mode and

short periods in transmit/receive mode. During standby,

maximize battery life by setting CLK/SEL = GND and

TRACK = OUT; this places the IC in PFM and track

modes (for lowest quiescent power consumption). In

transmit/receive mode, set TRACK = GND and CLK/SEL

= OUT to increase the PA supply voltage and initiate

high-power, low-noise PWM operation. Table 5 lists the

typical available output current when operating with

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

Table 4. Component Suppliers

(847) 956-0702

81-3-3607-5144

USA: (847) 956-0666

Japan: 81-3-3607-5111

Sumida

(619) 661-1055

81-7-2070-1174

USA: (619) 661-6835

Japan: 81-7-2070-6306

Sanyo

(602) 994-6430

USA: (602) 303-5454

Motorola

(714) 960-6492

USA: (714) 969-2491

Matsuo

(847) 639-1469

USA: (847) 639-6400

Coilcraft

(803) 626-3123

USA: (803) 946-0690

(800) 282-4975

AVX

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