ADPD4100 datasheet(17/101 Pages) AD

17 / 101 page

Data Sheet

ADPD4100/ADPD4101

Rev. 0 | Page 17 of 101

Either side of each LED driver output pair, but not both, can be

driven in any of the 12 available time slots. Up to four LED

driver outputs can be enabled in any time slot using the LED_

DRIVESIDE1_x, LED_DRIVESIDE2_x, LED_DRIVESIDE3_x,

and LED_DRIVESIDE4_x bits. The current is set on a per

driver, per time slot basis using the LED_CURRENT1_x, LED_

CURRENT2_x, LED_CURRENT3_x, and LED_CURRENT4_x

bits. Each driver can be programmed from 1.5 mA to 200 mA

with a monotonic 7-bit setting, as shown in Figure 20. Each

setting from 1 to 127 increases the LED drive current by ~1.5 mA.

Setting LED_CURRENTx_x = 0 disables that particular driver.

Although each driver can be programmed to 200 mA and up to

four LED drivers can be enabled in any time slot, there is a

limitation of a total of 400 mA of combined LED driver current

that can be provided in any time slot. It is up to the user to

program the LED drivers such that this 400 mA limit is not

exceeded. If the 400 mA limit is exceeded by the user settings,

priority is given, in the following order, to LED1x, LED2x,

LED3x, and LED4x. For example, if the user settings have

LED1A set to 150 mA, LED2B set to 150 mA, and LED3A set to

150 mA in a single time slot, LED1A and LED2B both provide

150 mA. However, LED3A is limited to 100 mA to maintain the

400 mA total LED drive current limit for the device.

250

200

150

100

112

LED_CURRENTx_x SETTING

Figure 20. LED Driver Current vs. LED_CURRENTx_x Setting

LED Driver Protection from High Inductance

If the total inductance in the path between the LED and the

ADPD4100/ADPD4101 LED driver pin (LEDxA or LEDxB)

and in the path between the LED and the LED supply voltage

(VLEDx) is significant due to the use of long wires and multiple

connectors, connect a reverse biased protection diode to a

suitable high supply voltage such as VLEDx at the LED driver pin

used. That is, connect a reverse biased protection diode

between the LED driver pin used and VLEDx.

LED Driver Protection from LED Driver Pin Overvoltage

In typical designs, no external components are needed on the

LED driver. However, in some cases where the LED driver pin

voltage has the possibility to be pulled above 3.6 V, an external

NPN bipolar junction transistor (BJT) type transistor can be

connected to the LED driver pin, as shown in Figure 21. This

additional transistor serves as a protection of the LED driver

pin from exceeding the maximum allowable LEDxx pin voltage

of 3.6 V.

Protecting the LED driver pins from overvoltage is required

when (VLEDx − LED turn on voltage) > 3.6 V, or when the

voltage source has a shunt resistance of less than 10 MΩ and the

supply voltage is above 3.6 V. For example, VLEDx of 6 V can be

used without pulling the LED driver pins up past 3.6 V for low

leakage LEDs such as some green and blue LEDs. Typically,

when VLEDx < 6 V, the protection transistor is not necessary.

LEDxx

VLED1

VBASE

CVLED

NOTES

1. CVLED IS THE BYPASS CAPACITOR.

2. VBASE IS THE BASE VOLTAGE.

Figure 21. LED Driver Overvoltage Protection Circuit

The NPN BJT selection must follow these guidelines:

•

The current capacity must match the maximum LED

driver current on the LED. The maximum LED driver

current is 200 mA for one LED driver and 400 mA for

multiple LED drivers connected to the same LED.

•

The voltage rating of the transistor must exceed the supply

being used on the load.

•

The base emitter voltage must be ≤0.9 V at the maximum

LED driver current on the LED.

•

Lower collector emitter voltages at the maximum LED

driver current on the LED provide more operating room

for the load being driven.

DETERMINING CVLED

To determine the bypass capacitor (CVLED) value, determine the

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

operation. The maximum LED current, ILED_MAX, converts to

VFB_LED_MAX as shown in Figure 22. In this example, 125 mA of

current through two green LEDs in parallel yields VFB_LED_MAX =

3.5 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.

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