NCP5007
V
bat
V
bat
U1
EN
C1
3
5
4
V
bat
4.7 mF
L1
22 mH
Average Network
R2
D1
GND
2
1
GND
FB
GND
V
out
R3
PWM
MBR0530
10 k
150 k
NCP5007
C2
1.0 mF
C3
100 nF
R4
5.6 k
GND
GND
GND
R1
10 W
D6
D5
D4
D3
D2
LED LED LED
Sense Resistor
LED
LED
NOTE: RC filter R2 and C3 is optional (see text)
Figure 29. Basic DC Current Mode Operation with PWM Control
To implement such a function, lets consider the feedback
input as an operational amplifier with a high impedance input
(reference schematic Figure 29). The analog loop will keep
going to balance the current flowing through the sense
resistor R1 until the feedback voltage is 200 mV. An extra
resistor (R4) isolates the FB node from low resistance to
ground, making possible to add an external voltage to this pin.
The time constant R2/C3 generates the voltage across C3,
added to the node pin 1, while R2/R3/R4/R1/C3 create the
discharge time constant. In order to minimize the pick up
noise at FB node, the resistors shall have relative medium
value, preferably well below 1.0 MW. Consequently, let
R2 = 150 k, R3 = 10 k and R4 = 5.6 k. In addition, the
feedback delay to control the luminosity of the LED shall be
acceptable by the user, 10 ms or less being a good
compromise. The time constant can now be calculated based
on a 400 mV offset voltage at the C3/R2/R3 node to force
zero current to the LED. Assuming the PWM signal comes
from a standard gate powered by a 3.0 V supply, running at
5.0 kHz, then full dimming of the LED can be achieved with
a 95% span of the Duty Cycle signal.
Digital Control
An alternative method of controlling the luminosity of the
LEDs is to apply a PWM signal to the EN pin (see
Figure 30). The output current depends upon the Duty
Cycle, but care must be observed as the DC−DC converter
is continuously pulsed ON/OFF and noise is likely to be
generated.
V
bat
U1
3
C1
5
4
Pulse
EN
V
bat
4.7 mF
L1
22 mH
GND
D1
2
1
GND
FB
GND
V
out
MBR0530
NCP5007
C2
1.0 mF
R1
5.6 W
D6
D5
D4
D3
D2
GND
GND
NOTE: Pulse width and frequency depends upon the application constraints.
Figure 30. Typical Semi−Pulsed Mode of Operation
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