Rainbow-electronics MAX1765 Bedienungsanleitung PDF herunterladen (Seite 16)

MAX1765

used if ISET is employed to reduce the peak inductor

current (see Setting the Switch Current Limit and Soft-

Start). For high efficiency, choose an inductor with a

high-frequency core material to reduce core losses. To

minimize radiated noise, use a toroid or shielded induc-

tor. See Table 3 for suggested components and Table

4 for a list of component suppliers.

Output Diode

To assist startup with input voltages below 1.1V or

when V

OUT

is set for >4V, use a Schottky diode—such

as a 1N5817, MBR0520L or equivalent—between LX

and POUT (Figure 2). The Schottky diode carries cur-

rent after the synchronous rectifier turns off. Thus, its

current rating only needs to be 500mA. Connect the

diode as close to the IC as possible. Do not use ordi-

nary rectifier diodes; their slow switching speeds and

long reverse-recovery times render them unacceptable.

For input voltages over 1.8V, the Schottky diode may

improve light-load efficiency.

Input and Output Filter Capacitors

Choose input and output filter capacitors that will ser-

vice the input and output peak currents with accept-

able voltage ripple. Choose input capacitors with

working voltage ratings over the maximum input volt-

age and output capacitors with working voltage ratings

higher than the output. A 100µF, 100mΩ, low equiva-

lent-series-resistance (ESR) tantalum output capacitor

is recommended for most applications. At the output of

the linear regulator (OUTL), use a 4.7µF ceramic

capacitor for stability at loads up to 500mA.

The input filter capacitor reduces peak currents drawn

from the input source and also reduces input switching

noise. The input voltage source impedance determines

the required size of the input capacitor. When operat-

ing directly from one or two NiMH cells placed close to

the MAX1765, use a single 33µF low-ESR input filter

capacitor.

The Sanyo POSCAP, Panasonic SP/CB, and Kemet

T510 are good low-ESR capacitors. Low-ESR tantalum

capacitors offer a good trade-off between price and

performance. Do not exceed the ripple current ratings

of tantalum capacitors. Avoid aluminum electrolytic

capacitors; their high ESR typically results in higher

output ripple voltage.

Bypass Capacitors

Bypass REF to GND with 0.22µF. Also, bypass OUT to

GND with a 0.68µF ceramic capacitor, and connect

OUT to POUT with a 4.7Ω resistor. Each of these com-

ponents should be placed as close to its respective IC

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

Layout Considerations

High switching frequencies and large peak currents

make PC board layout a critical part of design. Poor

design will cause excessive EMI and ground bounce,

both of which can cause instability or regulation errors

by corrupting the voltage and current feedback signals.

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. Connect the

inductor from the battery to the LX pins as close to the

IC as possible.

Keep the voltage feedback network very close to the

IC, within 0.2in (5mm) of the FB pins. Keep noisy

traces, such as those from the LX pin, away from the

voltage feedback networks and guarded from them

using grounded copper. Refer to the MAX1765 EV kit

for a full PC board example.

Applications Information

Use in a Typical Wireless

Phone Application

The MAX1765 is ideal for use in digital cordless and

PCS phones. The PA is connected directly to the step-

up converter output for maximum voltage swing and

power efficiency (Figure 10). The internal linear regula-

tor is used for postregulation to generate low-noise

power for DSP, control, and RF circuitry. The following

equations may be used to estimate the typical available

output current under conditions other than those listed

here:

800mA, Low-Noise, Step-Up DC-DC Converter

with 500mA Linear Regulator

16 ______________________________________________________________________________________

Figure 7. Feedback Connections

MAX1765

INLOUTL POUT

OUT

FBL FB

GND PGND

STEP-UP

OUTPUT

LINEAR-

REGULATOR

OUTPUT

1 2 ... 11 12 13 14 15 16 17 18 19

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