7

FN6264.3

March 7, 2008

only rated for the required voltage range can therefore be

used, which will optimize the component costs in some cases.

Shutdown

When PWM/EN is taken low the ISL97634 enters into the

power-down mode where the supply current is reduced to

less than 1µA. The device resumes normal when the

PWM/EN goes high.

Components Selection

The input capacitance is typically 0.22µF. The output

capacitor should be in the range of 0.22µF to 1µF. X5R or

X7R type of ceramic capacitors of the appropriate voltage

rating are recommended.

When choosing an inductor, make sure the average and

peak current ratings are adequate by using Equations 3, 4 and

5 (80% efficiency assumed):

Where:

•

• L is the inductance in H

The ISL97634 supports a wide range of inductance values

(10µH to ~82µH). For lower inductor values or lighter loads,

the boost inductor current may become discontinuous. For

high boost inductor values, the boost inductor current will be

in continuous mode.

In addition to the inductor value and switching frequency, the

input voltage, number of LEDs and the LED current also

affects whether the converter operates in continuous

conduction or discontinuous conduction mode.

Both operating modes are allowed and normal. The

discontinuous conduction mode yields lower efficiency due

to higher peak current.

Compensation

The product of the output capacitor and the load create a

pole while the inductor creates a right half plane zero. Both

of these attributes degrade the phase margin but the

ISL97634 has internal compensation network that ensures

the device operates reliably under the specified conditions.

The internal compensation and the highly integrated

functions of the ISL97634 make it a design friendly device to

be used in high volume, high reliability applications.

Applications

Analog Dimming

Analog dimming is usually not recommended because of the

brightness non-linearity at low levels dimming. However,

some systems are EMI or noise sensitive that analog

dimming may be more suitable than PWM dimming under

those situations. The ISL97632 is part of the same family as

the ISL97634 and has been designed with a serial interface

to give access to 32 separate dimming levels. Alternatively

analog dimming can be achieved by applying a variable DC

current. As the DC dimming signal voltage increases above

voltage drop on RSET decreases. Thus, the LED current

decreases as shown in Equation 6:

brightness will increase.

The DC dimming signal voltage can be a variable DC voltage

from a POT, a DCP (Digitally Controlled Potentiometer), or a

DC voltage generated by filtering a high frequency PWM

control signal.

may be calculated for any desired “relative brightness” (F)

using Equation 7:

TABLE 1.

PART NO.

OVP

MAX NO. OF

LEDS

MAX ILED

ISL97634IRT14Z

14V

3

70mA

ISL97634IRT18Z

18V

4

50mA

ISL97634IRT26Z

26V

7

30mA

I

LAVG

I

⋅

0.8 V

IN

⋅

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

=

(EQ. 3)

I

LPK

I

LAVG

1

2

---

I

L

Δ

⋅

+

=

(EQ. 4)

I

L

Δ

V

IN

V

OUT

V

IN

–

()

⋅

LV

⋅⋅

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

=

(EQ. 5)

I

LED

V

FB

R

1

R

2

+

() V

⋅

–

⋅

R

⋅

SET

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

=

(EQ. 6)

FIGURE 14. ANALOG DIMMING CONTROL APPLICATION

CIRCUIT

ISL97634

C1

LX

PWM

FB

GND

L1

22µH

C2

0.22µF

4.75

Ω

LEDs

1µF

LX

3.3V

R1

R2

3.3k

V

Dim

R

2

R

1

------- V

FB

1

R

1

R

2

-------

F

–

+

⎝⎠

⎜⎟

⎛⎞

⋅⋅

=

(EQ. 7)

ISL97634