MAX1620EEE datasheet(16/20 Pages) MAXIM

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Digitally Adjustable LCD Bias Supplies

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Setting the Maximum Output Voltage

(DAC Adjustment)

The DAC is adjustable from 0V to 1.5V in 32 steps, and

1LSB = 1.5V / 31. DAC adjustment of VOUT is provided

by adding R3 to the divider circuit (Figure 4). Be sure

that VOUT,MAX does not exceed the LCD panel rating.

For VOUT,MAX = 25V and VOUT,MIN = 12.5V, R3 is deter-

mined as follows:

R3 = R5 x (VFB) / (VOUT,MAX - VOUT,MIN)

= 2.2M

Ω x (1.5) / (25 - 12.5) = 264kΩ

The general form for VOUT as a function of the DAC out-

put (VDOUT) is:

VOUT = VOUT,MIN + (VFB - VDOUT) x R5 / R3

At power-up the DAC resets to mid-scale (10000), which

corresponds to VDOUT = 0.774V; therefore, the output

voltage after reset is as follows:

VOUT,RESET = VOUT,MIN + (1.5 - 0.774) x R5 / R3

Note that for a positive output voltage, VOUT increases

as VDOUT decreases. VOUT,MAX corresponds to VDOUT

= 0V, and VOUT,MIN corresponds to VDOUT = 1.5V.

For a negative output voltage, VOUT = VOUT,MIN +

(VFB - VDOUT) x R5 / R3. Assume VOUT,MAX = -25V and

VOUT,MIN = -12.5V; then determine R3 and VOUT,RESET

as follows:

R3 = R5 x (VFB - VDOUT,MAX) / (VOUT,MAX - VOUT,MIN)

= 2.5M

Ω x (0 - 1.5) / (-25 - -12.5) = 300kΩ

VOUT,RESET = -12.5 + (0 - 0.774) x (2.5M) /

(300k) = -18.95V

Note that for a negative output voltage,

VOUT increases

as VDOUT increases. VOUT,MAX corresponds to VDOUT

= 1.5V, and

VOUT,MIN corresponds to VDOUT = 0V.

Potentiometer Adjustment

The output can be adjusted with a potentiometer instead

of the DAC. Choose RPOT = 100kΩ, and connect it

between REF and GND. Connect R3 to the potentiome-

ter’s wiper, instead of to DOUT. The same design equa-

tions as above apply.

Controlling the LCD Using

POK and

LCDON

When voltage at POK is greater than 1V, the open-drain

LCDON output pulls low. LCDON withstands 27V; there-

fore, it can drive a PFET or PNP transistor to switch on

the MAX1620/MAX1621’s positive output. The following

represent three cases for using this feature:

1) As an off switch, to ensure that a positive boosted

output goes to 0V during shutdown. In this case,

connect POK to SHDN. Without this switch, the posi-

tive output falls to one diode-drop below the input

voltage (VBATT) in shutdown. LCDON is not needed

for negative outputs, which will fall to 0V in shut-

down anyway.

2) As an output sensing cutoff for positive outputs.

Connect POK to the feedback voltage divider to

sense the output voltage. The output is switched on

only when it reaches a set percentage of the set

voltage.

3) As an input sensing output cutoff for positive out-

puts. Connect POK to a voltage divider to sense the

input voltage. The output is switched on only when

the input reaches the set level (Figure 4).

To control the open-drain output LCDON by sensing

the input voltage, connect a resistor-divider (R1-R2,

Figure 4) from VBATT to POK. Choose R2 = 100k. For

example, if the minimum battery voltage is 5.3V, deter-

mine R1 as follows:

R1 = R2 x [(VBATT / VPOK) - 1]

= 100k x [(5.3 / 0.992) - 1] = 434k

Ω

LCDON can also be controlled via software (MAX1621,

Table 4).

Table 4. MAX1621 LCDON Output

Truth Table

POK Pin

LCDON Output

LCDON Bit

<1V

Floating

<1V

Floating

>1V

ON, pulls low

>1V

Floating

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