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ISL97642

TFT-LCD DC/DC with Integrated Amplifiers

Intersil 

ISL97642

VBOOST

LX

0.9V

PG_LDOP +

-

+

-

GMP

LDO_ON

36V

ESD

CLAMP

RBP

700Ω

DRVP

FBP

RP1

RP2

20kΩ

1: Np

0.1µF

CP (TO 36V)

0.1µF

VON (TO 35V)

CON

FIGURE 18. VON FUNCTIONAL BLOCK DIAGRAM

LX

0.1µF

PG_LDON -

+

0.4V

LDO_OFF

VREF

FBN

RN2

20kΩ

CP (TO -26V)

0.1µF

1: Nn

-

+

GMN

36V

ESD

CLAMP

RN1

DRVN

RBN

700Ω

VOFF (TO -20V)

COFF

FIGURE 19. 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 ISL97642 ranges from +15V to

+36V. A fault comparator is also included for monitoring the

output voltage. The undervoltage 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 ISL97642 ranges

from -5V to -25V. A fault comparator is also included for

monitoring the output voltage. The undervoltage threshold is

set at 200mV above the 0.2V reference level.

Set-up Output Voltage

Refer to the “Typical Application Circuit” on page 18. The

output voltages of VON, VOFF and VLOGIC are determined

by Equations 9 and 10:

VON

=

VR

E

F

×

⎛

⎜1

⎝

+

RR-----11---21-⎠⎟⎞

(EQ. 9)

VOFF

=

VR

E

F

N

+

R-----2---2-

R21

×

(VR

E

FN

–

VRE

F

)

(EQ. 10)

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 20, or the linear regulator can control

only one stage charge pump and regulate the final charge

pump output, as shown in Figure 21.

VIN

CHARGE PUMP

OR VBOOST OUTPUT

700Ω

DRVP

Q11

NPN

CASCODE

TRANSISTOR

ISL97642

VON

FBP

FIGURE 20. CASCODE NPN TRANSISTOR CONFIGURATION

FOR HIGH CHARGE PUMP OUTPUT VOLTAGE

(>36V)

13

FN6436.0

June 18, 2007

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