LD48 MPD [MicroPower Direct, LLC], LD48 Datasheet - Page 3

• Manufacturer Part Number: LD48
• Description: Compact, Wired Constant Current DC/DC LED Drivers
• Manufacturer: MPD [MicroPower Direct, LLC]

PWM Output Current Control
PWM Control Signals
PWM Dimming Application
A simple method of achieving digital (or PWM) dimming is by using a 555 timer
to apply a series of pulses to the V
ates over a supply voltage range of 4.5 VDC to 15VDC. Here it is connected
to the 12 VDC output of the SR7805 switching regulator (this is also the V
of the LED driver). Care should be taken to minimize ripple at the V
Excess ripple could cause timing errors.
The timer is connected for astable (free run) operation. The frequency is set
by R
it reaches the level of
discharge through D
voltage drops to
MicroPower Direct
1
, R
2
and C
4
. The timing capacitor (C
1 /3
V
1
CC
and R
2 /3
, the discharge pin goes high and C
•
V
CC
2
292 Page Street Ste D Stoughton, MA 02072
to the internal discharge transistor. When the C
, the discharge pin (pin 7) goes low and C
ADJ
input, as illustrated above. The 555 oper-
4
) charges through R
4
begins to charge
1
and D
CC
2
. When
input.
4
will
•
IN
4
TEL: [phone]
again. The formulas for calculating the frequency and duty cycle are included
in the MPD application note “Driving LEDs”.
The diodes (D
bypasses R
essentially blocking R
allow for duty cycles over a range of approximately 5% to 95%. If manual
adjustment is desired, a potentiometer may be substituted for R
adjustment of the circuit).
The size of C
sible. In order to avoid excessive current fl ow through the internal discharge
transistor, it is recommended that R
www.micropowerdirect.com
2
4
while C
1
is generally not critical, but it should be as low leakage as pos-
and D
•
FAX: [phone]
4
2
2
is charging. Diode D
during the charge period. Theoretically, this circuit will
) allow duty cycles below 50% to be set. Diode D
An LED operates at its maximum effi ciency when
operated at the rated drive current specifi ed by
the manufacturer. Operating an LED at lower than
its rated forward current not only decreases the
system effi ciency; but may cause color (or wave-
length) shifting. In illumination applications, this
could cause visible changes to lighting.
A preferred method is using pulse width modula-
tion (PWM). As shown at left, the output current
is adjusted by applying a PWM signal to the V
input. By varying the signal duty cycle the average
output current is adjusted up or down. To avoid
visible fl icker, the PWM signal should be greater
than 100 Hz.
For duty cycles (DPWM) between 0 and 1, the
output current is derived by the formula:
The V
tor transistor (as shown). The diode and resistor
suppress high amplitude negative spikes that may
be caused by the drain-source capacitance of the
transistor. Negative spikes on the control input of
the unit could cause errors in output current or
erratic operation.
The V
output of a microcontroller. Again, any high amplitude
negative spikes that may be caused by the drain-
source capacitance of the FET must be supressed.
ADJ
ADJ
input can also be driven by the open drain
input may be driven via an open collec-
1
be at least 5 k .
•
I
NOM
E-Mail: [e-mail]
2
Available free at the MPD
of this circuit in “Driving LEDs”.
is optional (but recommended),
=
I
MAX
Get a full explanation
X D
PWM
2
website
(with some
ADJ
1