ADPD103 datasheet(24/53 Pages) AD

24 / 53 page

Data Sheet

ADPD103

Rev. B | Page 23 of 52

LED DRIVER PINS AND LED SUPPLY VOLTAGE

The LEDx1, LEDx2, and LEDx3 pins have an absolute maximum

voltage rating of 3.6 V.Any voltage exposure over this rating affects

the reliability of the device operation and, in certain circumstances,

causes the device to cease proper operation. The voltage of the

LEDx pins must not be confused with the supply voltages for the

LED themselves (VLEDx). VLEDx is the voltage applied to the anode of

the external LED, whereas the LEDx pin is the input of the internal

current driver, and the pins are connected to the cathode of the

external LED.

LED DRIVER OPERATION

The LED driver for the ADPD103 is a current sink requiring

0.2 V of compliance above ground to maintain the programmed

current level. Figure 24 shows the basic schematic of how the

ADPD103 connects to an LED through the LED driver. The

Determining the Average Current and the Determining CVLED

sections define the requirements for the bypass capacitor

(CVLED) and the supply voltages of the LEDs (VLEDx).

ADPD103

LEDx

LGND

VLEDx

SUPPLY

CVLED

Figure 24. VLEDx Supply Schematic

DETERMINING THE AVERAGE CURRENT

The ADPD103 drives an LED in a series of short pulses. Figure 25

shows the typicalADPD103 configuration of a pulse burst

sequence.

ILED_MAX

3µs

19µs

Figure 25. 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 25 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_PEAK

(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_PEAK is peak current setting of the LED.

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

ILED_PEAK. 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, IFB_LED_MAX, converts to VFB_LED_MAX as shown in

Figure 26. In this example, 250 mAof 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

very 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

100

150

200

250

LED DRIVER CURRENT SETTING (mA)

TWO 528nm LEDs

ONE 850nm LED

Figure 26. Example of the Average LED Forward-Biased Voltage Drop as a

Function of the Driver Current

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.

To calculate the minimum value for the VLEDx bypass capacitor, use

the following equation:

)

(

−

MAX

LED

MIN

LED

MAX

LED

LED_PULSE

VLED

(2)

where:

tLED_PULSE is the LED pulse width.

IFB_LED_MAX is the maximum forward-biased current on the LED

used in operating the device.

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_PEAK.

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