MAX686 查看數據表（PDF） - Maxim Integrated

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MAX686

DAC-Controlled Boost/Inverter LCD Bias Supply with Internal Switch

Maxim Integrated 

DAC-Controlled Boost/Inverter

LCD Bias Supply with Internal Switch

VIN = 0.8V TO 27.5V

15µF

VCC = 2.7V TO 5.5V

0.1µF

DAC CONTROL

ON/OFF

L1

22µH

VCC

LX

VDD

DACOUT

POL

LCDON

FB

MAX686

UP

DN

REF

SHDN

GND

POK

ISET

PGND

D2

MBR0530L

R4

2Ω

2.2µF

R3

D1

MBR0530L

R1

0.1µF

CF

470pF

R2

NEGATIVE

OUTPUT

VOLTAGE

|VOUT| ≤ (27.5V - VIN)

2.2µF

Figure 3. Alternative Negative Output Voltage Application Circuit

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

limit is selectable to either 500mA (ISET = VCC) or

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.

The MAX686 regulates the voltage on FB (VFB) to

1.25V. When the output is well below regulation (VFB is

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.

Inverting Control Scheme (POL = VCC)

In inverting operation, the MAX686 regulates the volt-

age on FB (VFB) to 0V, and the error amplifier’s polarity

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

negative output voltages. When the output is well below

regulation (VFB is 0.25V or more and the switch current

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

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

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