CAT32

9

Doc No. 5001, Rev. AC

LED DIMMING WITH PWM SIGNAL

PWM brightness control provides the widest dimming

range (greater than 20:1). By turning the LEDs ON and

OFF using the control signal the LEDs operate at either

zero or full current, but their average current changes

with the PWM signal duty cycle. Typically, a 5kHz to

40kHz PWM signal is used. PWM dimming with the

CAT32 can be accomplished two different ways.

The

SHDN pin can be driven directly or a resistor can be

SHDN pin is used,

increasing the duty cycle will increase the LED bright-

ness. Using this method, the LEDs can be dimmed and

turned off completely using the same control signal. A

0% duty cycle signal will turn off the CAT32, reducing the

total quiescent current to near zero.

decrease the brightness. Using this method, the LEDs

RR

V

V

PWM

SET

MAX

=×

−









015

1

.

In addition to providing the widest dimming range, PWM

brightness control also ensures the “purest” white LED

color over the entire dimming range. The true color of a

white LED changes with operating current, and is the

“purest” white at a specific forward current, usually

15mA or 20mA. If the LED current is less than or more

than this value, the emitted light becomes more blue.

Applications involving color LCDs can find the blue tint

objectionable.

When a PWM control signal is used to drive the

SHDN

pin of the CAT32, the LEDs are turned off and on at the

PWM frequency. The current through them alternates

between full current and zero current, so the average

current changes with duty cycle. This ensures that when

the LEDs are on, they can be driven at the appropriate

current to give the purest white light. LED brightness

varies linearly with the PWM duty cycle.

LED DIMMING WITH A LOGIC SIGNAL

For applications that need to adjust the LED brightness

the minimum LED current value (when the NMOS is

OFF):

R

V

I

MIN

LED MIN

=×













225

01

.

()

R

when the external NMOS switch is turned ON.

R

V

I

INCR

LED Increase

=×













225

01

.

()

LED DIMMING WITH A DC VOLTAGE

R

VV

I

I

ADJ

MAX

LED MAX

LEAD MIN

=×

−

−













225

01

.

()

(

)

CAT32

SHDN

5

PWM

CAT32

RSET

4

PWM

RPWM

RSET

CAT32

RSET

PWM

4

10k

Ω

RSET

RPWM

0.1µF

CAT32

RSET

VDC

4

RSET

RADJ

CAT32

RSET

4

RMIN

RINCR

LOGIC

SIGNAL

Figure 3: LED Dimming Circuits