SNVAA22 June 2021 LP8867-Q1 , TMP61-Q1

4 LP886x-Q1 ISET Implementation

Some LED Drivers don’t have a built $T_{SET}$ pin to allow for easy connection of a thermistor divider for the temperature sensing. The circuit below provides a discrete solution for using a current sensing pin with LED Drivers that don’t have the $T_{SET}$ option. The output current of the LED outputs is controlled with external $R_{ISET}$ resistor. $R_{ISET}$ value for the target LED current per channel can be calculated usingEquation 5 .

Equation 5.

I_{L E D} = 2000 \times \frac{V_{B G}}{R_{I S E T}}

where

$V_{B G}$ = 1.2 V
$R_{ISET}$ is current setting resistor, kΩ
$I_{LED}$ is output current OUTx pin, mA

Equation 6. For example, if

R_{ISET}

is set to 20kΩ,

I_{LED}

will be 120 mA per channel.

The simulation in Figure 4-1 shows a discrete circuit that performs a derating current from 70ºC to 85ºC on the LED Low Side Current Limit.

Figure 4-1 ISET Temperature-Based LED Current Dimming Schematic

Simulations results are shown in Figure 4-2:

Figure 4-2 Simulation Results

The discrete implementation gives complete control of the output current to the LED Driver. The current derates from 60mA to 15mA from 70 to 85ºC. The linearity of the TMP61-Q1 ensures a reliable solution for accurate current output profiles.