ADPD1081 datasheet(33/74 Pages) AD

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Data Sheet

ADPD1080/ADPD1081

Rev. C | 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.

ILED_MAX

3µs

19µs

Figure 41. Typical LED Pulse Burst Sequence Configuration

In this example, the LED pulse width, tLED_PULSE, is 3 µs, and the

LED pulse period, tLED_PERIOD, is 19 µs. The LED being driven is a

pair of green LEDs driven to a 250 mA peak. The goal of CVLED

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 VLEDx supply must

supply the average current. Therefore, calculate ILED_AVERAGE as

follows:

ILED_AVERAGE = (tLED_PULSE/tLED_PERIOD) × ILED_MAX

(1)

where:

ILED_AVERAGE is the average current needed from the VLEDx supply

during the pulse period, and it is also the VLEDx supply current

rating.

ILED_MAX is the peak current setting of the LED.

For the values shown in Equation 1, ILED_AVERAGE = 3/19 ×

ILED_MAX. For typical LED timing, the average VLEDx supply

current is 3/19 × 250 mA = 39.4 mA, indicating that the VLEDx

supply must support a dc current of 40 mA.

DETERMINING CVLED

To determine the CVLED capacitor value, determine the

maximum forward-biased voltage, VFB_LED_MAX, of the LED in

operation. The LED current, ILED_MAX, converts to VFB_LED_MAX as

shown in Figure 42. In this example, 250 mA of current through

two green LEDs in parallel yields VFB_LED_MAX = 3.95 V. Any

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 VLEDx supply.

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

LED DRIVER CURRENT SETTING (mA)

TWO 528nm LEDs

ONE 850nm LED

Figure 42. Example of the Average LED Forward-Bias Voltage Drop as a

Function of the LED Driver Current Setting

To correctly size the CVLED 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

(

−

FB_LED_MAX

LED_MIN

LED_MAX

LED_PULSE

VLED

(2)

where:

tLED_PULSE is the LED pulse width.

ILED_MAX is the maximum forward-biased current on the LED

used in operating the devices.

VLED_MIN is the lowest voltage from the VLEDx supply with no load.

VFB_LED_MAX is the maximum forward-biased voltage required on

the LED to achieve ILED_MAX.

The numerator of the CVLED 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 VLEDx 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 VLEDx supply is 4.75 V and

that the peak current is 250 mA for two 528 nm LEDs in

parallel. The minimum value for CVLED is then equal to 3.75 µF.

CVLED = (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 CVLED

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

VLED_MIN instead of the worst case value for VLED_MIN can cause a

serious design deficiency, resulting in a CVLED value that is too small

and that causes insufficient optical power in the application.

ADPD1081 Datasheet(PDF) 33 Page - Analog Devices

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