ISL97642 Просмотр технического описания (PDF) - Intersil

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ISL97642

TFT-LCD DC/DC with Integrated Amplifiers

Intersil 

ISL97642

FBB

CINT

DRVN

FBN

VREF REFERENCE

GENERATOR

OSCILLATOR

SLOPE COMP OSC

COMPENSATION

PWM

Σ

LOGIC

CONTROLLER

GM

AMPLIFIER

BUFFER

UVLO

COMPARATOR

THERMAL

SHUTDOWN

CURRENT

AMPLIFIER

CURRENT REF

CURRENT

LIMIT COMPARATOR

SS

SHUTDOWN

AND START-UP

CONTROL

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----O----O-----S----T-

VIN

=

------1-------

1–D

(EQ. 1)

Where D is the duty cycle of switching MOSFET.

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 60kΩ is recommended.

The boost converter output voltage is determined using

Equation 2:

VBOOST

=

R-----1----+-----R-----2-

R1

×

VREF

(EQ. 2)

10

FN6436.0

June 18, 2007

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