MAX629CSLASHD datasheet(9/12 Pages) MAXIM

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usable battery life. Using separate supplies for VCC and

VIN also reduces noise injection onto VCC by isolating it

from the switching transients, allowing a smaller, less-

expensive input filter capacitor to be used in many appli-

cations. If input voltages below 2V will be common, reduc-

ing the inductor to 22μH may improve performance in this

voltage range, at the potential cost of some decrease in

maximum load current and efficiency.

In the negative configuration shown in Figure 3, the invert-

ing charge pump injects current into LX with each cycle.

The amount of charge injected increases at higher VIN,

and may prematurely trip the internal currentlimit thresh-

old. Resistor R3 increases the usable input voltage range

by limiting the peak injected current. The 2Ω resistor

shown provides a usable input voltage range beyond VIN

= 15V. In applications with a different input voltage range,

R3 can 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 regulators. It is impor-

tant 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 cir-

cuit, the ground circuit, and LX. Place R1 and R2 as close

to the feedback pin as possible. Place the input bypass

capacitor as close as possible to VCC and GND.

Refer to the MAX629 evaluation kit data sheet for an

example of proper board layout.

Figure 4. Adjustable Output Voltage

MAX629

VOUT

VBIAS

( ) ARE FOR NEGATIVE OUTPUT VOLTAGE CONFIGURATIONS.

GND

(REF)

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MAX629

28V, Low-Power, High-Voltage,

Boost or Inverting DC-DC Converter

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