ISL97634

8

FN6264.4

August 27, 2013

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

(see Figure 15) to adjust the LED current. As the DC dimming

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.

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

Equation 7:

The analog dimming circuit can be tailored to a desired relative

applied).

TABLE 1.

PART NO.

OVP

MAX ILED

ISL97634IRT14Z

14V

70mA

ISL97634IRT18Z

18V

50mA

ISL97634IRT26Z

26V

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 15. 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)

R

2

V

Dim_max

V

FB

–

() R

1

•

[]

V

FB

1F

min

–

()

•

[]

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

=

(EQ. 8)