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BQ25504 View Datasheet(PDF) - Texas Instruments

Part NameDescriptionManufacturer
BQ25504 Ultra Low-Power Boost Converter With Battery Management for Energy Harvester Applications Texas-Instruments
Texas Instruments Texas-Instruments
BQ25504 Datasheet PDF : 36 Pages
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8.4 Device Functional Modes
The bq25504 has three functional modes: cold-start operation, main boost charger enabled and thermal
shutdown. The cold start circuitry is powered from VIN_DC. The main boost charger circuitry is powered from
VSTOR while the boost power stage is powered from VIN_DC. Details of entering and exiting each mode are
explained below.
8.4.1 Cold-Start Operation (VSTOR < VSTOR_CHGEN, VIN_DC > VIN(CS) and PIN > PIN(CS))
Whenever VSTOR < VSTOR_CHGEN, VIN_DC VIN(CS) and PIN > PIN(CS), the cold-start circuit is on. This
could happen when there is not input power at VIN_DC to prevent the load from discharging the battery or during
a large load transient on VSTOR. During cold start, the voltage at VIN_DC is clamped to VIN(CS) so the energy
harvester's output current is critical to providing sufficient cold start input power, PIN(CS) = VIN(CS) X IIN(CS).
The cold-start circuit is essentially an unregulated, hysteretic boost converter with lower efficiency compared to
the main boost charger. None of the other features function during cold start operation. The cold start circuit's
goal is to charge VSTOR higher than VSTOR_CHGEN so that the main boost charger can operate. When a
depleted storage element is initially attached to VBAT, as shown in Figure 12 and the harvester can provide a
voltage > VIN(CS) and total power at least > PIN(CS), assuming minimal system load or leakage at VSTOR and
VBAT, the cold start circuit can charge VSTOR above VSTOR_CHGEN. Once the VSTOR voltage reaches the
VSTOR_CHGEN threshold, the IC
1. first performs an initialization pulse on VRDIV to reset the feedback voltages,
2. then disables the charger for 32 ms (typical) to allow the VIN_DC voltage to rise to the harvester's open-
circuit voltage which will be used as the input voltage regulation reference voltage until the next MPPT
sampling cycle and
3. lastly performs its first feedback sampling using VRDIV, approximately 64 ms after the initialization pulse.
Figure 12. Charger Operation After a Depleted Storage Element is Attached and Harvester is Available
The energy harvester must supply sufficient power for the IC to exit cold start. Due to the body diode of the
PFET connecting VSTOR and VBAT, the cold start circuit must charge both the capacitor on CSTOR up to the
VSTOR_CHGEN and the storage element connected to VBAT up to VSTOR_CHGEN less a diode drop. When a
rechargeable battery with an open protector is attached, the intial charge time is typically short due to the
minimum charge needed to close the battery's protector FETs. When large, discharged super capacitors with
high DC leakage currents are attached, the intial charge time can be signficant.
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