28V, Low-Power, High-Voltage,

Boost or Inverting DC-DC Converter

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9

increasing usable battery life. Using separate supplies

by isolating it from the switching transients, allowing a

smaller, less-expensive input filter capacitor to be used

in many applications. If input voltages below 2V will be

common, reducing the inductor to 22µH may improve

performance in this voltage range, at the potential cost

of some decrease in maximum load current and effi-

ciency.

In the negative configuration shown in Figure 3, the

inverting charge pump injects current into LX with each

cycle. The amount of charge injected increases at

limit threshold. Resistor R3 increases the usable input

voltage range by limiting the peak injected current. The

2

Ω resistor shown provides a usable input voltage

input voltage range, R3 may be increased or

decreased as necessary, with a resulting efficiency

change of roughly 0.5%/

Ω.

Layout Considerations

Proper PC board layout is essential due to high current

levels and fast switching waveforms that radiate noise.

It is recommended that initial prototyping be performed

using the MAX629 evaluation kit or equivalent PC

board-based design. Breadboards or proto-boards

should never be used when prototyping switching reg-

ulators.

It is important to connect the GND pin, the input

bypass-capacitor ground lead, and the output filter-

capacitor ground lead to a single point (star ground

configuration) to minimize ground noise and improve

regulation. Also, minimize lead lengths to reduce stray

capacitance, trace resistance, and radiated noise, with

preference given to the feedback circuit, the ground

circuit, and LX. Place R1 and R2 as close to the feed-

back pin as possible. Place the input bypass capacitor

Refer to the MAX629 evaluation kit data sheet for an

example of proper board layout.

Figure 4. Adjustable Output Voltage

MAX629

R2

( ) ARE FOR NEGATIVE OUTPUT VOLTAGE CONFIGURATIONS.

R1

FB

GND

(REF)