NCP5005
http://onsemi.com
15
TYPICAL APPLICATIONS CIRCUITS
Standard Feedback
The standard feedback provides a constant current to the
LED, independently of the V
bat
supply and number of LED
associated 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
1
5
V
out
V
bat
R1
NCP5005
L1
22 mH
V
bat
C1
4.7 mF
D6 D5 D4 D3
U1
4
D2
V
bat
2
GND
GND
FB
3
D1
MBR0530
15 W
GND
GND
C2
1.0 mF
LWT67C
GND
LWT67C LWT67C LWT67C LWT67C
PWM Operation
The analog feedback Pin 3 provides a way to dim the
LED by means of an external PWM signal as depicted in
Figure 29. By optimizing the internal high 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, from a human eye standpoint, it will continuously
start and stop the converter, yielding high transients . These
transients might generate spikes difficult to filter out in the
rest of the application, a situation not recommended. The
output current depends upon the duty cycle of the signal
presented to the node Pin 3: this is very similar to the digital
control discussed in Figure 31.
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.
