LP8553: LP8553: Issues with Low Input Voltage application

Part Number: LP8553 I have my circuit connected in the same way as the Typical Application for Low Input Voltage on Page 2 of the datasheet as I am using 5V input on Vbatt, Vin, Vldo. I have a 160Hz PWM input to the PWM input pin. However, I am seeing an UVLO fault in register 02h as bit 0 is set to 1. I have not changed any default settings. The output voltage shows 10.1V, current is at about 140mA (shown from power supply), but there is no output PWM through the LED string. Are there any settings I have to set in the EEPROM or other registers to get the low input voltage application to function? Any ideas as to why the LEDs aren't turned on?

Hi Brandon, For the VLDO, you need to connect a cap to ground, but you have connected to the "Vin" this LDO is internal output, please modify the schematic and try again. BR Sean

In the datasheet in shows connecting the VLDO to Vin as I have in my schematic. Since VBATT is 5V in my schematic, I connected VLDO & VIN together to VBATT.

Hi Sir, Yes, that is ok, but you can try with a higher input voltage source and check the fault regs since you have a UVLO fault now, And when you try with higher supply voltage, please don't connect the “VLDO" pin to Vin. BR Sean

Okay so essentially you're asking me to connect the chip in the same way as shown here?

Also, do I need to connect a 5V source to VLDO (with a cap to ground)?

Hi Sir, Yes, please try with a higher input voltage, and since the supply voltage will exceed 5V, so don't connect the "VLDO" to the "Vin" and just connect a cap between "VLDO" and ground. BR Sean

Why does the data sheet show 5V on the VLDO pin? Does that indicate that the VLDO pin outputs 5V?

Yes, you are correct, this is an internal LDO which output 5V for internal power supply. BR Sean

Hi, I am still in process of having my boards modded to try and separate VLDO and VIN. However, this would still not solve my problem in that I only have 5V available to power this driver. Would there be any other reasons my schematic could have issues?

Hi Brandon, Since the problem is UVLO, you need to increase the power supply voltage or you need to decrease the LED counts or decrease the output current. BR Sean