ISL97645A

FN6353 Rev 1.00

Page 14 of 16

October 21, 2010

Start-Up Sequence

threshold value, Boost converter starts up, and gate pulse

modulator circuit output holds until VDPM is charged to

1.215V. Note that there is a DC path in the boost converter

from the input to the output through the inductor and diode,

hence the input voltage will be seen at output with a forward

voltage drop of diode before the part is enabled. If this voltage

is not desired, the following circuit can be inserted between

input and inductor to disconnect the DC path when the part is

disabled.

.

back plate of an LCD display. This plate is capacitively coupled

to the pixel drive voltage which alternately cycles positive and

negative at the line rate for the display. Thus the amplifier must

be capable of sourcing and sinking capacitive pulses of

current, which can occasionally be quite large (a few 100mA

for typical applications).

400mA. This limit level, which is roughly the same for sourcing

and sinking, is included to maintain reliable operation of the

part. It does not necessarily prevent a large temperature rise if

the current is maintained. (In this case the whole chip may be

shut down by the thermal trip to protect functionality.) If the

display occasionally demands current pulses higher than this

limit, the reservoir capacitor will provide the excess and the

amplifier will top the reservoir capacitor back up once the pulse

has stopped. This will happen on the µs time scale in practical

voltage will have settled again before the next line is

processed.

Fault Protection

ISL97645A provides the overall fault protections including over

current protection and over-temperature protection.

An internal temperature sensor continuously monitors the die

temperature. In the event that die temperature exceeds the

thermal trip point, the device will shut down and disable itself.

The upper and lower trip points are typically set to +140°C and

+100°C respectively.

TABLE 6. VGH_M STATUS TABLE

VDPM

RESET

VGH

VGH_M

COMMENT

x

x

x

<2.5V

GROUND

Will be grounded if VIN is above

a logic threshold. Could occur at

power-up or power-down

>VLOR

<1.215V

x

>2.5V

GROUND

Start-up only condition: If either

VDPM < 1.215V, GND VGHM

x

<1.215V

High

>2.5V

GROUND

>VLOR

>1.215V

High

>2.5V

Switching

controlled by

VFLK

x

x

Low

>2.5V

VGH

Power-down state. Could occur

at power-up if part starts with

VGH > 2.5V

FIGURE 18. CIRCUIT TO DISCONNECT THE DC PATH OF

BOOST CONVERTER

INPUT

ENABLE

TO INDUCTOR