EL7584

FN7317 Rev 2.00

Page 9 of 16

February 4, 2005

Applications Information

The EL7584 is high efficiency multiple output power solution

designed specifically for thin-film transistor (TFT) liquid

crystal display (LCD) applications. The device contains one

high current boost converter and two low power charge

The boost converter contains an integrated N-channel

MOSFET to minimize the number of external components.

The converter output voltage can be set from 5V to 18V with

independently regulated to positive and negative voltages

using external resistors. Output voltages as high as 40V can

be achieved with additional capacitors and diodes.

Boost Converter

The boost converter operates in constant frequency pulse-

width-modulation (PWM) mode. Quiescent current for the

EL7584 is only 5mA when enabled, and since only the low

side MOSFET is used, switch drive current is minimized.

90% efficiency is achieved in most common application

operating conditions.

A functional block diagram with typical circuit configuration is

shown on previous page. Regulation is performed by the

PWM comparator which regulates the output voltage by

comparing a divided output voltage with an internal

reference voltage. The PWM comparator outputs its result to

the PWM logic. The PWM logic switches the MOSFET on

and off through the gate drive circuit. Its switching frequency

is external adjustable with a resistor from timing control pin

1.2MHz operating frequency range.

Start-Up

MOSFET is controlled by the start-up oscillator, which

generates fixed duty-ratio of 0.5 - 0.7 at a frequency of

several hundred kilohertz. This will boost the output voltage,

providing the initial output current load is not too great

(<250mA).

takes over the control. The duty ratio will be decided by the

multiple-input direct summing comparator, Max_Duty signal

(about 90% duty-ratio), and the Current Limit Comparator,

whichever is the smallest.

The soft-start is provided by the current limit comparator. As

the internal 12µA current source charges the external soft-

start capacitor, the peak MOSFET current is limited by the

voltage on the capacitor. This in turn controls the rising rate

of output voltage.

The regulator goes through the start-up sequence as well

after the EN signal is pulled to HI.

Steady-State Operation

When the output reaches the preset voltage, the regulator

operates at steady state. Depending on the input/output

condition and component, the inductor operates at either

continuous-conduction mode or discontinuous-conduction

mode.

In the continuous-conduction mode, the inductor current is a

triangular waveform and LX voltage a pulse waveform. In the

discontinuous-conduction mode, the inductor current is

completely ‘dried-out’ before the MOSFET is turned on

again. The input voltage source, the inductor, and the

MOSFET and output diode parasitic capacitors forms a

resonant circuit. Oscillation will occur in this period. This

oscillation is normal and will not affect the regulation.

At very low load, the MOSFET will skip pulse sometimes.

This is normal.

Current Limit

The MOSFET is current limited to <1.75Amps (nominal).

the following formula:

where:

•

decided by:

• D is the MOSFET turn-on radio and is decided by:

L

2

-------

–



---------



=

I

L

L

---------

D

-------



=

D

------------------------

=