MAX686 datasheet(14/16 Pages) MAXIM

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DAC-Controlled Boost/Inverter

LCD Bias Supply with Internal Switch

______________________________________________________________________________________

R6 turns it off when LCDON goes high. R6 and R7 can

be the same value. Choose R7 such that the minimum

base current is greater than 1/50 of the collector current.

For example, assume VOUT(MIN) = 12.5V and ILCD =

10mA and then determine R7 as follows:

≤ 50 x (12.5 - 0.7) / 10mA = 59kΩ

Remember that the LCD voltage, VOUTSW, is the regu-

lated output voltage minus the drop across the PNP

switch (300mV typ).

Connecting VIN to VCC

The MAX686 (VCC, VDD) and the inductor (VIN) can be

powered from the same source as long as the +5.5V

VCC maximum limit is not violated. To ensure stability,

connect VIN and VDD directly to the source, connect

VCC to the source through a 100

Ω resistor (R8), and

bypass VCC with a 1µF ceramic capacitor as shown in

Figure 8. Since the supply current is very small, the

voltage drop across R8 is insignificant and does not

degrade performance. The RC isolates VCC from the

switching noise created by the inductor and internal

power switch.

Although, in many cases, the MAX686 and the inductor

are powered from the same source, it is often advanta-

geous in battery-powered applications to power the

MAX686 IC (VCC, VDD) from an available regulated sup-

ply and to power the inductor (VIN) directly from a bat-

tery. The MAX686 requires a +2.7V to +5.5V supply at

VCC, but the inductor can be powered from voltages as

low as 0.8V, significantly increasing usable battery life.

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 MAX686 evaluation kit or equivalent PC

board-based design. Breadboards or proto-boards

should never be used when prototyping switching regu-

lators.

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 resis-

tance, and radiated noise, with preference given to the

feedback circuit, the ground circuit, and LX. Place R1

and R2 as close to the feedback pin as possible. Place

the bypass capacitors as close to the pins as possible.

Refer to the MAX686 evaluation kit data sheet for an

example of proper board layout.

MAX686

VDD

VIN = 2.7V

TO 5.5V

DACOUT

MBR0530L

VOUT

100

Ω

µF

µH

µF

VCC

Figure 8. Using a Common Supply-Voltage Source

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