HT7833 drop voltage confusion

Question

I am designing a sensor based on ESP32-C3-13 module and sending sensor value over ESPNOW to another ESP32-C3-13. The circuit has HT7833 LDO and supplied by one 3.7 V 2500 mAh LiPO. During the send process around 3.5 V battery voltage level input to HT7833, I am facing ~600 mV drop. When I check HT7833 datasheet 600 mV voltage drop occurs in condition of 2.5 V ≤ Vo ≤ 2.7V

In order to investigate the issue I checked battery level over scope. On C1 it is showing level around 3.5 V and on C2 voltage level at around 2.8 V during Tx.

In order to understand the root of the problem I dropped C3 and tried to measure it again. Result has not changed. Then, I dropped all the capacitors and left only ESP32 and HT7833, result has not change either.

Have I interpreted the values wrong?

Please find screenshots and circuit diagram attached into the message.

Thank you for your help.

Orkun Gedik

Answer 1

You have made the appropriate measurements.

The battery’s internal resistance (plus that of the connections) appears to be too high to support the transmit mode current, at least in the measured charge condition, and the regulator is behaving as expected. The datasheet covers other output voltages than 3.3, but in your case the worst-case dropout voltage at 500mA is 500mV. So, for the regulator to regulate at 3.3V out the input voltage should not fall below 3.8V under load, regardless of battery charge conditions.

Even a perfect linear regulator would require 3.3V input, so if your battery voltage drops below that at the end of discharge it cannot regulate. A better LDO with a dropout of perhaps 50 or 100mV at the maximum load current and a battery that supplies more than 3.4V under full load and near end of charge could work. A quality 2500mAh cell at less than 0.2C should be able to do that over mot of its useful discharge curve.

Note: I’ve glossed over the difference between the actual maximum current draw (including variations from unit-to unit) and the arbitrary 500mA Holtek used for testing, but to do a solid design you should have more detailed specifications on all 3 major components.