DAC-Controlled Boost/Inverter

LCD Bias Supply with Internal Switch

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9

Figure 3. Alternative Negative Output Voltage Application Circuit

MAX686

D1

MBR0530L

D2

MBR0530L

L1

22

µH

R4

2

Ω

NEGATIVE

OUTPUT

VOLTAGE

UP

POL

SHDN

DN

GND

PGND

LX

DACOUT

LCDON

FB

REF

POK

ISET

0.1

µF

2.2

µF

2.2

µF

R3

R2

R1

ON/OFF

DAC CONTROL

470pF

0.1

µF

15

µF

Detailed Description

The MAX686 is a step-up converter that contains an

internal N-channel MOSFET switch to convert a +0.8V

to +27.5V battery voltage to a higher positive or a nega-

tive voltage. Figure 1 shows the MAX686 configured to

produce a positive output voltage. Figure 2 shows the

MAX686 configured with one additional diode and

capacitor to produce a negative output voltage. Figure

3 shows an alternative method for developing negative

output voltages. Set the output voltage with an external

resistor-divider network. Adjust the output voltage with

the internal digital-to-analog converter (DAC). The

MAX686’s current-limited pulse-frequency-modulation

(PFM) control scheme has programmable current limit-

ing and provides high efficiency over a wide range of

load conditions.

Boost Control Scheme (POL = GND)

A combination of peak current limiting and a pair of one-

shots controls the MAX686 switching. During the on-

cycle, the internal switch closes, and current through

the inductor ramps up until either the fixed 10µs maxi-

mum on-time expires (at low input voltages) or the

switch peak current limit is reached. The peak current

250mA (ISET = GND) (see the section

Setting the Peak

Inductor Current Limit).

After the on-cycle terminates, the switch turns off, and

the inductor charges the output capacitor through the

diode. If the output is out of regulation after the mini-

mum off-time has transpired, another on-cycle begins.

If the output is within regulation when the minimum off-

time transpires, the off-cycle extends until the output

falls out of regulation, at which point an on-cycle starts.

less than 1V and the switch current limit is exceeded),

the MAX686 operates in initial power-up mode, and the

minimum off-time increases to 5µs to provide soft-start.

The switching frequency, which depends on the load,

the input voltage, and the output voltage, can be as

high as 300kHz.

In inverting operation, the MAX686 regulates the volt-

is reversed. The minimum off-time changes to 3.5µs for

negative output voltages. When the output is well below

limit is exceeded), initial power-up is assumed, and the

minimum off-time increases to 5µs to provide soft-start.