NCP5007
http://onsemi.com
15
TYPICAL APPLICATIONS CIRCUITS
Standard Feedback
The standard feedback provides constant current to the
LEDs, independently of the V
bat
supply and number of
LEDs in series. Figure 28 depicts a typical application to
supply 13 mA to the load.
Figure 28. Basic DC Current Mode Operation with
Analog Feedback
EN
4
5
V
out
V
bat
R1
NCP5007
L1
22 mH
V
bat
C1
4.7 mF
D6 D5 D4 D3
U1
3
D2
V
bat
2
GND
GND
FB
1
D1
MBR0530
15 W
GND
GND
C2
1.0 mF
LED LED LED LED LED
GND
PWM Operation
The analog feedback pin 1 provides a way to dim the LED
by means of an external PWM signal as depicted in
Figure 29. Taking advantage of the high internal impedance
presented by the FB pin, one can set up a simple R/C network
to accommodate such a dimming function. Two modes of
operation can be considered:
• Pulsed mode, with no filtering
• Averaged mode with filtering capacitor
Although the pulsed mode will provide a good dimming
function, it will yield high switching transients which are
difficult to filter out in the control loop. As such this first
approach is not recommended. The output current depends
upon the duty cycle of the signal presented to the node pin 1:
this is very similar to the digital control shown in Figure 30.
The average mode yields a noise−free operation since the
converter operates continuously, together with a very good
dimming function. The cost is an extra resistor and one extra
capacitor, both being low cost parts.
