Rainbow-electronics MAX1621 Manual de usuario descargar pdf (Pagina 15)

MAX1620/MAX1621

Digitally Adjustable LCD Bias Supplies

______________________________________________________________________________________ 15

START

CONDITION

MOST

SIGNIFICANT

ADDRESS BIT

LEAST

SIGNIFICANT

ADDRESS BIT

SLAVE PULLS

SDA LOW

MAX1621 DRIVES SDA

D4POK D3 D2 D1 D0

SLAVE

ACKNOWLEDGE

MOST

SIGNIFICANT

DATA BIT

SCL

SDA

R/W BIT

Figure 9. MAX1621 Serial-Interface Read Example

Design Procedure

__________and Component Selection

The MAX1620/MAX1621 output voltage can be adjusted

manually or via a digital interface. In addition, positive

bias voltage can be switched with LCDON using an

external PFET or PNP transistor.

Output Adjustment

Setting the Minimum Output Voltage

The minimum output voltage is set with a resistor-divider

(R4-R5, Figure 4) from V

OUT

to AGND. The FB threshold

voltage is 1.5V. Choose R4 to be 300kΩ so that the cur-

rent in the divider is about 5µA. Determine R5 as follows:

R5 = R4 x (V

OUT,MIN

- V

) / V

For example, if V

OUT,MIN

= 12.5V:

R5 = 300kΩ x (12.5 - 1.5) / (1.5) = 2.2MΩ

Mount R4 and R5 close to the FB pin to minimize para-

sitic capacitance.

For a negative output voltage, the FB threshold voltage

is 0V, and R4 is placed between FB and REF (Figures 5

and 6). Again, choose R4 to be 300kΩ so that the cur-

rent in the divider is about 5µA. Then determine R5 as

follows:

R5 = R4 x V

OUT,MIN

/ V

REF



For example, if V

OUT,MIN

= -12.5V:

R5 = 300kΩ x (12.5) / (1.5) = 2.5MΩ

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