LP8863-Q1: Power Loss＠Load Dump

Hi Kishimoto-san,

If Vin is higher than Vout, Vout level will be Vin-diode drop = Vout. So, Vout level can be much higher than target adapted level for a given LED Vf triggering OVP fault.

There are 2 types of boost OVP fault on LP8863. Boost OVP low and Boost OVP high. 

Boost OVP low is triggered when FB voltage is 0.25~0.3V higher than desired value(typ 1.2V). This corresponds to boost output voltage higher than target value by R1/R2*0.25(or 0.3). where R1= upper FB divider resistor, R2= lower FB divider resistor. Once Boost OVP low fault is detected, boost will stop switching temporarily until OVP condition disappears while all other functions work normally. Boost OVP low fault can be detected within ~100us since OVP condition occurs.

Boost OVP high is triggered when FB voltage is ~0.6V higher than desired value. Calculation method for boost output for this condition is same as boost OVP low fault. Once Boost OVP high fault is detected, boost enters fault recovery mode which stops boost and LED drivers, so device will be in standby mode.

First Boost OVP high fault can be detected within 500~600ms since OVP condition occurs and this detection time is reduced to 50ms for fault recovery attempt.

So, if only Boost OVP low condition happens, it won't affect normal operation, but Boost OVP high fault will affect it.

For worst case Vin and Vout, boost voltage higher than target value will be 40-0.5(diode)-24 = 15.5V by rough calculation, and this will be high enough to trigger Boost OVP high fault. But if load dump condition only occurs less than 500ms, Boost OVP high fault won't be detected as a fault and device will continue normal operation.

Status register and INT pin operation for both OVP faults can be disabled in register control of LP8863.

Regarding power loss during load dump condition,

for high LED current, Vout will be close to max due to high Vf of LEDs and voltage difference between Vin and Vout will be minimized --> low headroom voltage, low power loss on LED driver

for low LED current, Vout will be much lower than Vin due to low Vf of LEDs, so headroom voltage will be high, but power loss will be low due to low LED current .

for mid LED current, this is the calculation for LED driver power loss.

40V - 0.5V - 30V(Typ LED Vf) = 9.5V

Iled = 50mA(not max to be typ Vf)

Power loss = 9.5*0.05*6= 2.85W. This power number is pretty high, but duration of load dump condition will be very short, and junction to top case thermal parameter is only 0.3C/W(this can vary with PCB design).

In short, this apps condition will work and many customers use LP8863 for similar apps conditions.

Hope this helps,

Hi Kishimoto-san,

θJC is not a common parameter to estimate junction temperature. Actually, die temp can be directly measured by internal temp sensor, and it will work up to 150C, and this will be high enough margin to support all load conditions at various conditions. The bigger concerns will be temperature of SW FET where high SW current flows and high power loss occurs at. This power loss will be usually OK at low freq like a few hundred kHz, but may be critical at higher SW freq near max freq 2MHz.

So, selecting high power/high SW speed FET and diode will be important for high SW freq..

Hope this helps,