W-52

8

40

562

30

750

25

909

20

1.13 k

15

1.50 k

10

2.26 k

5

4.53 k

For other LED current values, use the following equation

R

SET

255

0.1 V

I

LED

Most white LEDs are driven at maximum currents of

15 mA to 20 mA. Some higher power designs will use two

parallel strings of LEDs for greater light output, resulting in

30 mA to 40 mA (two strings of 15 mA to 20 mA) flowing

into the LED pin.

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 5 kHz to 40 kHz PWM signal

is used. PWM dimming with the W-52 can be

accomplished two different ways.

The SHDN pin can be driven directly or a resistor can be

added to drive the RSET pin. If the SHDN pin is used,

increasing the duty cycle will increase the LED brightness.

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 W-52, reducing the total quiescent

current to near zero.

If the RSET pin is used, increasing the duty cycle will

decrease the brightness. Using this method, the LEDs are

dimmed using RSET and turned off completely using

SHDN. If the RSET pin is used to provide PWM dimming,

R

PWM

R

SET

V

MAX

0.15 V

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 15 mA or

20 mA. 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 W-52, 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 in

discrete steps, a logic signal can be used. RMIN sets the

minimum LED current value (when the NMOS is OFF):

R

MIN

255

0.1 V

I

LED(MIN)

the external NMOS switch is turned ON.

R

INCR

255

0.1 V

I

LED(Increase)

LED Dimming with a DC Voltage

R

ADJ

225

V

MAX

0.1 V

I

LED(MAX)

I

LEAD(MIN)

PCB Layout Guidelines

The W-52 is a high frequency switching regulator and

therefore proper PCB board layout and component

placement can minimize noise and radiation and increase

efficiency. To maximize efficiency, the W-52 design has

fast switch rise and fall times. To prevent radiation and high

frequency resonance problems minimize the length and area

of all traces connected to the SW pin and use a ground plane

under the switching regulator.

The switch, schottky output diode and output capacitor

signal path should be kept as short as possible. The ground

GND pin and not be shared with other components.

Figure 15. LED Dimming Circuits

W-52

5

PWM

W-52

RSET

4

PWM

W-52

RSET

4

W-52

RSET

PWM

10 k

0.1 F

4

W-52

RSET

Logic

Signal

4

SHDN