ISL97642 데이터 시트보기 (PDF) - Renesas Electronics

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ISL97642

TFT-LCD DC/DC with Integrated Amplifiers

Renesas Electronics 

ISL97642

FBB

CINT

DRVN

FBN

VREF REFERENCE

GENERATOR

OSCILLATOR

SLOPE COMP OSC

COMPENSATION

PWM



LOGIC

CONTROLLER

GM

AMPLIFIER

BUFFER

UVLO

COMPARATOR

THERMAL

SHUTDOWN

CURRENT

AMPLIFIER

SHUTDOWN

AND START-UP

CONTROL

CURRENT REF

CURRENT

LIMIT COMPARATOR

SS

VREF

+

-

UVLO

COMPARATOR

BUFFER

SS

+ 0.2V

-

0.4V

UVLO

COMPARATOR

BUFFER

LX

PGND

DRVP

FBP

FIGURE 16. BLOCK DIAGRAM

Boost Converter

The main boost converter is a current mode PWM converter

operating at a fixed frequency. The 1.2MHz switching

frequency enables the use of low profile inductor and

multilayer ceramic capacitors, which results in a compact, low

cost power system for LCD panel design.

The boost converter can operate in continuous or

discontinuous inductor current mode. The ISL97642 is

designed for continuous current mode, but it can also operate

in discontinuous current mode at light load. In continuous

current mode, current flows continuously in the inductor during

the entire switching cycle in steady state operation. The

voltage conversion ratio in continuous current mode is given by

Equation 1:

V-----B---V-O---I-O-N----S----T- = 1-----–-1----D---

(EQ. 1)

Figure 17 shows the block diagram of the boost controller.

It uses a summing amplifier architecture consisting of GM

stages for voltage feedback, current feedback and slope

compensation. A comparator looks at the peak inductor current

cycle by cycle and terminates the PWM cycle if the current limit

is reached.

An external resistor divider is required to divide the output

voltage down to the nominal reference voltage. Current drawn

by the resistor network should be limited to maintain the overall

converter efficiency. The maximum value of the resistor network

is limited by the feedback input bias current and the potential for

noise being coupled into the feedback pin. A resistor network in

the order of 60k is recommended. The boost converter output

voltage is determined using Equation 2:

VBOOST = R-----1--R--+---1--R-----2-  VREF

(EQ. 2)

Where D is the duty cycle of switching MOSFET.

FN6436 Rev 0.00

June 18, 2007

Page 10 of 19

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