4

FN7370.1

December 21, 2005

Block Diagram

Detailed Description

The ISL97631 uses a constant frequency, current mode

control scheme to provide excellent line and load

regulation. It can drive up to 6 LEDs in series or 15 LEDs in

parallel/series configuration, with efficiencies of up 85%.

The ISL97631 operates from an input voltage of 2.7V to

5.5V and can boost up to 27V.

Steady-State Operation

The ISL97631 operates with constant frequency PWM. The

switching frequency is around 1.35MHz. Depending on the

input voltage, inductance, number of LEDs and the LED

current, the converter operates in either continuous

conduction mode or discontinuous conduction mode. Both

are normal. The forward current of the LED is set using the

the equation:

Shut-Down

When taken low the ENAB pin places the ISL97631 into

power down mode. When in power down, the supply current

is reduced to less than 1µA.

Dimming Control

PWM DIMMING

The ENAB pin also doubles as a brightness control. There

are two different possible dimming control methods. The

first dimming method is controlled through the duty-cycle of

the ENAB input PWM waveform, which can operate at

frequencies up to 1kHz. For frequencies greater than 1kHz,

see Analog Dimming. The LEDs operate at either zero or

full current. This is the PWM dimming control method. The

relationship between the average LED current and the

duty-cycle (D) of the ENAB pin’s waveform is as follows:

FIGURE 7. ISL97631 BLOCK DIAGRAM

FET

Driver

PWM Logic

Controller

Current

Sense

GM

Amplifier

1.2MHz Oscillator and Ramp

Generator

Bandgap

Reference

Generator

95mV

GM Amp

Compensation

PWM

Comparator

Vin

Enable

LX

GND

FB

ISL97631

Vout

-

FET

Driver

PWM Logic

Controller

Current

Sense

GM

Amplifier

1.2MHz Oscillator and Ramp

Generator

Bandgap

Reference

Generator

95mV

GM Amp

Compensation

PWM

Comparator

Vin

Enable

LX

GND

FB

ISL97631

Vout

-

1.35MHz

Pin Descriptions

PIN

NUMBER

PIN

NAME

DESCRIPTION

1

VOUT Output Pin. Connect to the anode of the top

LED and the output filter capacitor .

2

GND

Ground Pin. Connect to local ground.

3

LX

Switching Pin. Connect to inductor.

4

ENAB Enable Pin. Connect to enable signal to turn-on

or off the device.

5

FB

Feedback Pin. Connect to the cathode of

bottom LED and the sense resistor.

6

VIN

Input Supply Pin. Connect to the input supply

voltage, the inductor and the input supply

decoupling capacitor.

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

=

(EQ. 1)

--------------- D

⋅

=

(EQ. 2)

ISL97631