EL7641ILTZ datasheet(13/19 Pages) RENESAS

13 / 19 page

EL7640, EL7641, EL7642

FN7415 Rev 2.00

Page 13 of 19

February 22, 2006

FIGURE 20. VOFF FUNCTIONAL BLOCK DIAGRAM

The VON power supply is used to power the positive supply of

the row driver in the LCD panel. The DC/DC consists of an

external diode-capacitor charge pump powered from the

inductor (LX) of the boost converter, followed by a low dropout

linear regulator (LDO_ON). The LDO_ON regulator uses an

external PNP transistor as the pass element. The onboard

LDO controller is a wide band (>10MHz) transconductance

amplifier capable of 5mA output current, which is sufficient for

up to 50mA or more output current under the low dropout

condition (forced beta of 10). Typical VON voltage supported by

EL7640, EL7641 and EL7642 ranges from +15V to +36V. A

fault comparator is also included for monitoring the output

voltage. The under-voltage threshold is set at 25% below the

1.2V reference.

The VOFF power supply is used to power the negative supply

of the row driver in the LCD panel. The DC/DC consists of an

external diode-capacitor charge pump powered from the

inductor (LX) of the boost converter, followed by a low dropout

linear regulator (LDO_OFF). The LDO_OFF regulator uses an

external NPN transistor as the pass element. The onboard

LDO controller is a wide band (>10MHz) transconductance

amplifier capable of 5mA output current, which is sufficient for

up to 50mA or more output current under the low dropout

condition (forced beta of 10). Typical VOFF voltage supported

by EL7640, EL7641 and EL7642 ranges from -5V to -25V. A

fault comparator is also included for monitoring the output

voltage. The under-voltage threshold is set at 200mV above

the 0.2V reference level.

Set-up Output Voltage

Refer to Typical Application Diagram, the output voltages of

VON, VOFF and VLOGIC are determined by the following

equations:

Where VREF = 1.2V, VREFN = 0.2V.

High Charge Pump Output Voltage (>36V)

Applications

In the applications where the charge pump output voltage is

over 36V, an external NPN transistor needs to be inserted in

between the DRVP pin and the base of pass transistor Q3 as

shown in Figure 21, or the linear regulator can control only one

stage charge pump and regulate the final charge pump output

as shown in Figure 22.

36V

ESD

CLAMP

GMN

LDO_OFF

1: Nn

FBN

DRVN

0.1µF

CP (TO -26V)

RBN

COFF

VOFF (TO -20V)

RN1

RN2

20k



VREF

PG_LDON

0.4V

700



VON

VREF 1

R12

R11

----------









VOFF

VREFN

R22

R21

----------

VREFN VREF

–





VIN

OR VBOOST

CHARGE PUMP

OUTPUT

700



Q11

FBP

DRVP

NPN

CASCODE

TRANSISTOR

EL764X

VON

FIGURE 21. CASCODE NPN TRANSISTOR CONFIGURATION

FOR HIGH CHARGE PUMP OUTPUT VOLTAGE

(>36V)

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