Data Sheet

ADP8140

Rev. B | Page 15 of 23

REDUCING THE LED CURRENT WITH THE DIM PIN

The DIM pin scales the output current when either an analog

voltage or a PWM signal is applied to it. The response of the

ADP8140 to the DIM pin depends on the condition of the

MODE pin.

If the MODE pin is connected to GND or a 30.1 kΩ resistor at

startup, the DIM pin functions as an analog voltage input. As

such, a DIM voltage of 2 V or greater does not impact the output

current. A DIM voltage of 0 V reduces the output current to as

low as 125 μA (see Figure 15). Any DIM voltage between 0 V

and 2 V linearly scales the output current.

Figure 24. Reducing the Output Current by Applying an Analog Voltage to

the DIM Pin

If MODE is connected to REG or to a 52.3 kΩ resistor at

startup, a PWM duty cycle applied to the DIM pin is internally

filtered and used to scale the output currents. Set the DIM pin

frequency between 140 Hz and 40 kHz. A lower frequency

PWM signal gives the ADP8140 more information about the

applied duty cycle and leads to better resolution of the duty

cycle, which translates into smaller LED current step sizes on

the current sink digital-to-analog converters (DACs).

Figure 25. Reducing the Average Output Current by Pulse Width Modulating

the Current Sinks with the DIM Pin

When DIM = 0 V, or 0% duty cycle, the minimum output current

of the Typical Performance Characteristics section.

DIMMING LEDS WITH THE VT PIN

The VT pin has two modes of operation, depending on the

configuration of the MIN pin.

If MIN is connected to REG at startup, a PWM input to the VT

pin pulses the LED current sinks. If the PWM input is high, the

current goes to the value dictated by the ISET and DIM pins.

If the PWM input is low, the current becomes the minimum

current as defined in Figure 15. This small current allows the

LEDs to be slightly biased and minimizes the voltage difference

between the off and on states, which greatly reduces the

response time of the power delivery loop.

In this mode, the 7.75 V SINKx clamp is disabled. When the

clamp is disabled, the voltages on the SINKx pins rise as high as

15.1 V (typical) during the LED off time.

If MIN < 2.2 V, the voltage on the VT pin linearly scales the

LED current. VT voltages greater than 2 V (typical) produce

100% of the programmed ISET current. When VT is less than

2 V, the output current is reduced 1% per 20 mV. If the voltage

on the VT pin is below the voltage on the MIN pin, the power

supply to the LEDs is disabled. If the VT pin voltage rises above

the MIN threshold, plus some hysteresis, the power supply is

reenabled. If both the VT pin and the DIM pin are used for

analog dimming, the pin that gives the lower LED current is

used to set the LED current.

To implement thermal protection of the LEDs, the VT pin is

connected to an external negative temperature coefficient

(NTC) resistor. This NTC resistor is typically placed on the LED

heat sink. Selecting the value of the NTC and the resistor in the

network shapes the slope of the VT voltage in response to the

LED temperature. A resistor divider on the MIN pin sets the

level at which the power stage (FB_OUT and COMP) are

disabled.

Figure 26. Using an External NTC to Implement LED Thermal Protection

FAULT PROTECTIONS

To ensure the safety of the LEDs, the ADP8140 IC, and the

power source, the ADP8140 includes a comprehensive array of

detection and protection features.



Power supply overvoltage protection



LED overtemperature protection



LED short-circuit protection



LED open-circuit protection



IC overtemperature protection



Shorted ISET protection



Open ISET and EN protection

These features are summarized in the flowchart shown in

Figure 27.

SCALE

OUTPUT(%)

DIM (V)

DIM

MODE

100%

2V

MODE = GND

ANALOG CONTROL

SCALE

OUTPUT(%)

DIM DUTY CYCLE (%)

DIM

MODE

100%

100%

SCALE PWM

SCALE

OUTPUT(%)

VT (V)

MIN

NTC

VT

100%

2V

MIN