NCP5604A, NCP5604B

http://onsemi.com

11

and 23 mA when Q1 is ON. The concept can use either a

DC drive or a pulsed mode to dynamically dim the light out

of the LED.

A different analog approach, but more complex solution,

can be derived from either a DC or a pulsed voltage

associated with a current mirror built with low cost discrete

devices (see Figure 14). The associated filter (R2/R3/C1)

provides a continuous voltage to the current mirror, thus a

digitally controlled continuous output current. Generally

speaking, the PWM frequency could be either in the low

end 20 Hz to 200 Hz band, or above the audio band (25 kHz

and beyond) to make sure the dimming can be adjusted

from zero to 100% without any audible noise. As a matter

of fact, the NCP5604A has been designed to guarantee

200 kHz PWM at the ENABLE pin.

The current mirror can be largely improved by using an

external operational amplifier to get a very stable and

temperature independent current, but such a solution could

turn out to be too expensive and, generally speaking, the

basic structure given, Figure 13, gives good results since

the current depends mostly upon the quality of resistor R2.

Figure 13. Basic Analog Dimming Control

VBAT

C2P

1

2

2

3

4

9

GND

EN

IREF

AGND

PGND

C2N

C3P

C3N

VOUT

ENABLE

Q1

High Out

Figure 14. Basic Analog PWM Dimming Control

VBAT

C2P

1

2

2

3

4

9

GND

EN

IREF

AGND

PGND

C2N

C3P

C3N

VOUT

ENABLE

GND

BC846BDW

15 k

R2

GND

12 k

R1

PWM

V = 3.3 V