Datasheet

Data Sheet ADPD1080/ADPD1081
Rev. B | Page 33 of 74
DETERMINING THE AVERAGE CURRENT
The ADPD1080/ADPD1081 drive an LED in a series of short
pulses. Figure 41 shows the typical ADPD1080/ADPD1081
configuration of an LED pulse burst sequence.
I
LED_MAX
3µs
19µs
16110-041
Figure 41. Typical LED Pulse Burst Sequence Configuration
In this example, the LED pulse width, t
LED_PULSE
, is 3 µs, and the
LED pulse period, t
LED_PERIOD
, is 19 µs. The LED being driven is a
pair of green LEDs driven to a 250 mA peak. The goal of C
VLED
is to buffer the LED between individual pulses. In the worst case
scenario, where the pulse train shown in Figure 41 is a continuous
sequence of short pulses, the V
LEDx
supply must supply the
average current. Therefore, calculate I
LED_AVERAGE
as follows:
I
LED_AVERAGE
= (t
LED_PULSE
/t
LED_PERIOD
) × I
LED_MAX
(1)
where:
I
LE D _ AV ER AG E
is the average current needed from the V
LEDx
supply
during the pulse period, and it is also the V
LEDx
supply current
rating.
I
LED_MAX
is the peak current setting of the LED.
For the values shown in Equation 1, I
LED_AVERAGE
= 3/19 ×
I
LED_MAX
. For typical LED timing, the average V
LEDx
supply
current is 3/19 × 250 mA = 39.4 mA, indicating that the V
LEDx
supply must support a dc current of 40 mA.
DETERMINING C
VLED
To determine the C
VLED
capacitor value, determine the
maximum forward-biased voltage, V
FB_LED_MAX
, of the LED in
operation. The LED current, I
LED_MAX
, converts to V
FB_LED_MAX
as
shown in Figure 42. In this example, 250 mA of current through
two green LEDs in parallel yields V
FB_LED_MAX
= 3.95 V. A ny
series resistance in the LED path must also be included in this
voltage. When designing the LED path, keep in mind that small
resistances can add up to large voltage drops due to the LED
peak current being large. In addition, these resistances can be
unnecessary constraints on the V
LEDx
supply.
1.0
1.5
2.0
2.5
3.0
3.5
4.0
4.5
LED FORWARD-BIAS VOLTAGE DROP (V)
LED DRIVER CURRENT SETTING (mA)
TWO 528nm LEDs
ONE 850nm LED
16110-042
Figure 42. Example of the Average LED Forward-Bias Voltage Drop as a
Function of the LED Driver Current Setting
To correctly size the C
VLED
capacitor, do not deplete it during the
pulse of the LED to the point where the voltage on the capacitor
is less than the forward bias on the LED. Calculate the minimum
value for the VLEDx bypass capacitor by
( )
6.0+−
×
=
FB_LED_MAXLED_MIN
LED_MAXLED_PULSE
VLED
VV
It
C
(2)
where:
t
LED_PULSE
is the LED pulse width.
I
LED_MAX
is the maximum forward-biased current on the LED
used in operating the devices.
V
LED_MIN
is the lowest voltage from the V
LEDx
supply with no load.
V
FB_LED_MAX
is the maximum forward-biased voltage required on
the LED to achieve I
LED_MAX
.
The numerator of the C
VLED
equation sets up the total discharge
amount in coulombs from the bypass capacitor to satisfy a
single programmed LED pulse of the maximum current. The
denominator represents the difference between the lowest
voltage from the V
LEDx
supply and the LED required voltage.
The LED required voltage is the voltage of the anode of the LED
such that the compliance of the LED driver and the forward-
biased voltage of the LED operating at the maximum current is
satisfied. At a 250 mA drive current, the compliance voltage of
the driver is 0.6 V. For a typical ADPD1080 example, assume
that the lowest value for the V
LEDx
supply is 4.75 V and that the
peak current is 250 mA for two 528 nm LEDs in parallel. The
minimum value for C
VLED
is then equal to 3.75 µF.
C
VLED
= (3 × 10
−6
× 0.250)/(4.75 – (3.95 + 0.6)) = 3.75 µF (3)
As shown in Equation 3, as the minimum supply voltage drops
close to the maximum anode voltage, the demands on C
VLED
become more stringent, forcing the capacitor value higher. It is
important to insert the correct values into Equation 1, Equation 2,
and Equation 3. For example, using an average value for
V
LED
_
MIN
instead of the worst case value for V
LED
_
MIN
can cause a
serious design deficiency, resulting in a C
VLED
value that is too small
and that causes insufficient optical power in the application.
Therefore, adding a sufficient margin on C
VLED
is strongly
recommended. Add additional margin to C
VLED
to account for
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