Simple SOT23 Boost Controllers

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11

Diode Selection

The MAX1522/MAX1523/MAX1524s’ high switching fre-

quency demands a high-speed rectifier. Schottky

diodes are recommended for most applications

because of their fast recovery time and low forward

voltage. Ensure that the diode’s current rating is ade-

quate to withstand the diode’s RMS current:

Also, the diode reverse breakdown voltage must

Schottky diodes may not be practical because of this

voltage requirement. In these cases, use a high-speed

silicon rectifier with adequate reverse voltage. Another

consideration for high input voltages is reverse leakage

of the diode. This should be considered using the man-

ufacturer’s specification due to its direct influence on

system efficiency.

Layout Considerations

High switching frequencies and large peak currents

make PC board layout a very important part of design.

Good design minimizes excessive EMI on the feedback

paths and voltage gradients in the ground plane, both

of which can result in instability or regulation errors.

Connect the inductor, input filter capacitor, and output

filter capacitor as close together as possible, and keep

their traces short, direct, and wide. Connect their

ground pins at a single common node in a star-ground

configuration. The external voltage-feedback network

should be very close to the FB pin, within 0.2in (5mm).

Keep noisy traces (such as the trace from the junction

of the inductor and MOSFET) away from the voltage-

feedback network; also keep them separate, using

grounded copper. The MAX1522/MAX1523/ MAX1524

evaluation kit manual shows an example PC board lay-

out and routing scheme.

Generating Resistance

with PC Board Traces

If the output capacitor’s ESR is too low for proper regu-

lation, it can be increased artificially directly on the PC

board. For example, an additional 50m

Ω of ESR added

to the output capacitor provides best regulation. The

resistivity of a 10mil trace using 1oz copper is about

50m

Ω per inch. Therefore, a 10mil trace 1in long gener-

ates the required resistance.

II

I

DIODE RMS

OUT

PEAK