BQ24640 Datasheet, PDF (12/25 Pages) Texas Instruments – High-Efficiency Synchronous Switch-Mode Super Capacitor Charger

bq24640
SLUSA44 – MARCH 2010
DETAILED DESCRIPTIONS
V_OREG
(PROG)
I_CHARGE
(PROG)
Constant Current
Constant Voltage
Taper Current
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Time
Figure 10. Typical Charging Profile
OUTPUT VOLTAGE REGULATION
The bq24640 uses a high accuracy voltage regulator for the charging voltage. The charge voltage is
programmed via a resistor divider from the output to ground, with the midpoint tied to the VFB pin. The voltage at
the VFB pin is regulated to 2.1V, giving the following equation for the regulation voltage:
VOUT
= 2.1V
´
éêë_{1 +}
R2 ù
R1 ^úû
(1)
where R2 is connected from VFB to the output and R1 is connected from VFB to GND.
OUTPUT CURRENT REGULATION
The ISET input sets the maximum charging current. Output current is sensed by resistor R_SR connected between
SRP and SRN. The full-scale differential voltage between SRP and SRN is 100mV. Thus, for a 10mΩ sense
resistor, the maximum charging current is 10A. The equation for charge current is:
I_CHARGE =
V_ISET
20 ´ R _SR
(2)
The input voltage range of ISET is between 0 and 2V. The SRP and SRN pins are used to sense voltage across
R_SR with default value of 10mΩ. However, resistors of other values can also be used. A larger sense resistor will
give a larger sense voltage, a higher regulation accuracy; but, at the expense of higher conduction loss.
POWER UP
The bq24640 uses a SLEEP comparator to determine if the source of power on the VCC pin is a valid supply to
charge the capacitor. If the VCC voltage is above the UVLO threshold and greater than the SRN voltage, and all
other conditions are met, bq24640 will then start to charge (See Enabling and Disabling Charging). If the SRN
voltage is greater than VCC, the bq24640 enters a low quiescent current SLEEP mode to minimize current drain
from the capacitor (<15µA).
If VCC is below the UVLO threshold, the device is disabled.
ENABLE AND DISABLE CHARGING
The following conditions have to be valid before charge is enabled:
• CE is HIGH
• The device is not in Under-Voltage-Lockout (UVLO) mode, and not in VCCLOWV
• The device is not in SLEEP mode (i.e., VCC > SRN)
• The VCC voltage is lower than the AC over-voltage threshold (VCC < V_ACOV)
• 30ms delay is complete after initial power-up
• The REGN LDO and VREF LDO voltages are at the correct levels
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