SNVAA22 June 2021 LP8867-Q1 , TMP61-Q1

3 LP886x-Q1 TSET Implementation

The LP886x-Q1 has an optional feature to decrease automatically LED current when LED overheating is detected with an external PTC sensor. An example of the behavior is shown in Figure 3-1. When the PTC temperature reaches T1, the LP886x-Q1 starts to decrease the LED current. When the LED current has reduced to 17.5% of the nominal value, current turns off until temperature returns to the operation range.

Figure 3-1 Temperature-Based LED Current Dimming Functionality

When external NTC (PTC) is connected, the $T_{SENSE}$ pin current decreases LED output current. Temperature T1 and de-rate slope are defined by external resistors as explained below.

Figure 3-2 TSET Temperature-Based LED Current Dimming Schematic

Parallel resistance of the PTC sensor $R_{T}$ and resistor R4 is calculated by formula:

Equation 1.

R_{| |} = \frac{R_{T} \times R 4}{R_{T} + R 4}

T_{SET}

voltage can be calculated with Equation 2.

Equation 2.

V_{T S E T} = V_{D D} \times \frac{R 3}{R 1 + R 3}

T_{SENSE}

pin current is calculated by Equation 3.

Equation 3.

I_{T S E N S E} = \frac{V_{T S E T} - V_{D D} \times \frac{R_{| |}}{R_{| |} + R 2}}{R_{| |} + R 5 - \frac{{R_{| |}}^{2}}{R_{| |} + R 2}}

where

$V_{DD}$ is the bias voltage of the resistor group. It is recommended to connect with chip's internal LDO output (pin 2)

I_{SET}

pin current designed by

R_{ISET}

is:

Equation 4.

I_{S E T_S C A L E D} = \frac{V_{B G}}{R_{I S E T}}

Using:

R1	5kΩ
RT	6.5kΩ
R2	10kΩ
R3	PTC
R4	DNP
R5	1kΩ
R_ISET	2.4kΩ

The output current thermal foldback profile in Figure 3-3 is achieved:

Figure 3-3 Current Foldback Profile with TMP61