SLUAB26 March   2025 BQ25672 , BQ25798

 

  1.   1
  2.   Abstract
  3.   Trademarks
  4. 1Introduction
  5. 2Discrete VINDPM Auto-reset Circuit
  6. 3Summary
  7. 4References

1 Introduction

All NVDC power path battery chargers have output (that is, SYS pin) short circuit protection. In a properly designed application where the input power of the NVDC charger is sufficient, the charger provides the minimum system voltage (MINSYS) for the maximum system load even with a completely discharged battery pack. With no battery or completely discharged battery pack with open protector, the BQ25798 (or BQ25672) SYS voltage (VSYS) can fall below VSYS-SHORT = 2.0V if the system load is higher than the input power at VBUS supply or if the input voltage falls below the minimum input voltage threshold (VINDPM) of the charger. After seven attempts to restore VSYS, the BQ25798 forces high impedance (HiZ) mode (battery-only mode) by setting the EN_HIZ I2C register bit = 1 as shown in Figure 1-1. Oscilloscope CH1 (royal blue) is the VAC1 pin of the charger, CH2 (light blue) is the REGN pin, CH3 (pink) is SYS output and CH4 (green) is the simulated panel output voltage connected to VAC1 pin.

Charger with No Battery Entering VSYS Short when Input Voltage Falls Below VINDPM Threshold Figure 1-1 Charger with No Battery Entering VSYS Short when Input Voltage Falls Below VINDPM Threshold

With battery voltage (VBAT) higher than MINSYS voltage, the BQ25798 maximum power point tracking (MPPT) feature periodically resets the input voltage dynamic power path (VINDPM) threshold to a fixed percent of the panel open circuit voltage (VOC). This makes sure of the continued operation of the charger without collapsing the voltage of the panel due to too much current draw at a voltage that is not at the (MPP) voltage. Figure 1-2 shows an example of the MMPT feature in operation. CH2 (light blue) is battery charge current. When the panel output voltage drops below the VINDPM threshold, the charger stops. Once the MPPT timer expires, the charger recomputes the VINDPM threshold based on VOC measurement and the MPP percentage register. Then, the converter restarts and continues charging the battery, eventually with less current and the newly limited input power.

MPPT Sampling with MINSYS=3.5V, VBAT=3.7V, ICHG=1A, MPPT Period=30s Figure 1-2 MPPT Sampling with MINSYS=3.5V, VBAT=3.7V, ICHG=1A, MPPT Period=30s

Unfortunately, the periodic VINDPM reset of the MPPT feature is only enabled if VBAT is higher than MINSYS. With a discharged battery, the charger VINDPM threshold defaults to VOC minus 0.7V for low voltages or 1.4V for high voltages. At sunrise, if a cloud passes over the panel, then the output voltage can start high and drops below in between the VINDPM threshold and charger UVLO. This causes VSYS to collapse and the charger to enter HiZ mode, as shown in Figure 1-1. Charger HiZ mode can only be exited by an I2C write from a system processor that has no input voltage or waiting for the panel voltage to drop below UVLO (3.4V).