BQ24751RHDRG4 Datasheet, PDF (23/38 Pages) Texas Instruments – Host-controlled Multi-chemistry Battery Charger

bq24751
www.ti.com............................................................................................................................................... SLUS734D – DECEMBER 2006 – REVISED MARCH 2009
and
I_{RIPPLE_MAX}
2
£ I_SYN
£ I_{RIPPLE_ MAX}
I_RIPPLE
=
(V_IN
-
V_BAT
)´
V_BAT
V_IN
´
1
f_s
L
=
V_IN
´
(1-
D
)´
D
´
1
f_s
L
(4)
where:
V_IN = adapter voltage
V_BAT = BAT voltage
f_{S =} switching frequency
L = output inductor
D = duty-cycle
I_{RIPPLE_MAX} Happens when the duty-cycle(D) is mostly near to 0.5 at given V_IN, f_s, and L.
The ISYNSET comparator, or charge undercurrent comparator, compares the voltage between SRP-SRN and
internal threshold. The threshold is set to 13 mV on the falling edge, with an 8-mV hysteresis on the rising edge
with a 10% variation.
High Accuracy IADAPT Using Current Sense Amplifier (CSA)
An industry-standard, high-accuracy current sense amplifier (CSA) is used by the host or some discrete logic to
monitor the input current through the analog voltage output of the IADAPT pin. The CSA amplifies the sensed
input voltage of ACP – ACN by 20x through the IADAPT pin. The IADAPT output is a voltage source 20 times
the input differential voltage. When PVCC is above 5 V and ACDET is above 0.6 V, IADAPT no longer stays at
ground, but becomes active. If the user wants to lower the voltage, they can use a resistor divider from IOUT to
AGND, and still achieve accuracy over temperature as the resistors can be matched according to their thermal
coefficients.
A 200-pF capacitor connected on the output is recommended for decoupling high-frequency noise. An additional
RC filter is optional, after the 200-pF capacitor, if additional filtering is desired. Note that adding filtering also
adds additional response delay.
Input Overvoltage Protection (ACOV)
ACOV provides protection to prevent system damage due to high input voltage. Once the adapter voltage is
above the programmable OVPSET voltage (3.1 V), charge is disabled, the adapter is disconnected from the
system by turning off ACDRV, and the battery is connected to the system by turning on BATDRV. ACOV is not
latched—normal operation resumes when the OVPSET voltage returns below 3.1 V.
Input Undervoltage Lock Out (UVLO)
The system must have 5 V minimum of PVCC voltage for proper operation. This PVCC voltage can come from
either the input adapter or the battery, using a diode-OR input. When the PVCC voltage is below 5 V, the bias
circuits REGN, VREF, and the gate drive bias to ACFET and BATFET stay inactive, even with ACDET above 0.6
V.
Note:
The bq24751 will not allow ACDRV to turn on in the unique condition that ACP<3V and
PVCC>UVLO. In this condition, the break-before-make protection latch gets stuck in a state that it
thinks the BATDRV is ON, and does not allow the ACDRV to turn on
Battery Overvoltage Protection
The converter stops switching when BAT voltage goes above 104% of the regulation voltage. The converter will
not allow the high-side FET to turn on until the BAT voltage goes below 102% of the regulation voltage. This
allows one-cycle response to an overvoltage condition, such as when the load is removed or the battery is
disconnected.
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