BQ24650 Datasheet, PDF (13/33 Pages) Texas Instruments – Synchronous Switch-Mode Battery Charge Controller for Solar Power With Maximum Power Point Tracking

bq24650
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SLUSA75 – JULY 2010
If the solar panel or other input source cannot provide the total power of the system and bq24650 charger, the
input voltage drops. Once the voltage sensed on the MPPSET pin drops below 1.2V, the charger maintains the
input voltage by reducing the charge current. If the MPPSET pin voltage is forced below 1.2V, the bq24650 stays
in the input voltage regulation loop while the output current is zero. The STAT1 pin is LOW and STAT2 pin is
HIGH.
The voltage at the MPPSET pin is regulated to 1.2V, giving Equation 2 for the regulation voltage:
VMPPSET = 1.2 V ´
é R3 ù
êë¹⁺ _R4 úû
(2)
The MPPSET pin is also used as charge enable control. If the voltage on MPPSET is pulled down below 75mV,
charge is disabled. Charge resumes if the voltage on MPPSET goes back above 175mV.
BATTERY CURRENT REGULATION
Battery current is sensed by resistor R_SR connected between SRP and SRN. The full-scale differential voltage
between SRP and SRN is fixed at 40mV. Thus, for a 20-mΩ sense resistor, the charging current is 2A. For
charging current, refer to Equation 3:
40 mV
I_CHARGE = _RSR
(3)
BATTERY PRECHARGE
On power-up, if the battery voltage is below the V_LOWV threshold, the bq24650 applies the precharge current to
the battery. This feature is intended to revive deeply discharged cells. If the V_LOWV threshold is not reached within
30 minutes of initiating precharge, the charger turns off and a FAULT is indicated on the status pins.
The precharge current is determined as 1/10 of the fast charge current according to the following equation:
4 mV
I_PRECHARGE = _RSR
(4)
CHARGE TERMINATION AND RECHARGE
The bq24650 monitors the charging current during the voltage regulation phase. Termination is detected while
the voltage on the VFB pin is higher than the VRECH threshold and the charge current is less than the I_TERM
threshold (1/10 of fast charge current), as calculated in Equation 5:
4 mV
I_TERM = _RSR
(5)
A new charge cycle is initiated when one of the following conditions occurs:
• The battery voltage falls below the recharge threshold
• A power-on-reset (POR) event occurs
• MPPSET falls below 75mV to reset charge enable
The TERM_EN pin may be taken LOW to disable termination. If TERM_EN is pulled above 1.6V, the bq24650
allows termination.
POWER UP
The bq24650 uses a SLEEP comparator to determine the source of power on the VCC pin, since VCC can be
supplied either from a battery or an adapter. If the VCC voltage is greater than the SRN voltage, and all other
conditions are met for charging, the bq24650 then attempts to charge a battery (see the Enabling and
Disabling Charging section). If SRN voltage is greater than VCC, indicating that a battery is the power source,
the bq24650 enters low quiescent current (<15µA) SLEEP mode to minimize current drain from the battery.
If VCC is below the UVLO threshold, the device is disabled, and VREF LDO turns off.
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