BQ24740_07 Datasheet, PDF (20/32 Pages) Texas Instruments – Host-controlled Multi-chemistry Battery Charger with Low Input Power Detect

bq24740
SLUS736 – DECEMBER 2006
www.ti.com
During the DCM mode the loop response automatically changes and has a single pole system at which the pole
is proportional to the load current, because the converter does not sink current, and only the load provides a
current sink. This means at very low currents the loop response is slower, as there is less sinking current
available to discharge the output voltage. At very low currents during non-synchronous operation, there may be
a small amount of negative inductor current during the 80 ns recharge pulse. The charge should be low enough
to be absorbed by the input capacitance.
Whenever the converter goes into 0% duty-cycle mode, and BTST – PH < 4 V, the 80-ns recharge pulse occurs
on LODRV, the high-side MOSFET does not turn on, and the low-side MOSFET does not turn on (no 80-ns
recharge pulse), and there is no discharge from the battery.
ISYNSET CONTROL (CHARGE UNDER-CURRENT)
In bq24740, ISYN is internally set as the charge current threshold at which the charger changes from
non-synchronous operation into synchronous operation. The low side driver turns on for only 80 ns to charge the
boost cap. This is important to prevent negative inductor current, which may cause a boost effect in which the
input voltage increases as power is transferred from the battery to the input capacitors. This can lead to an
over-voltage on the PVCC node and potentially cause some damage to the system. This programmable value
allows setting the current threshold for any inductor current ripple, and avoiding negative inductor current. The
minimum synchronous threshold should be set from ½ the inductor current ripple to the full ripple current, where
the inductor current ripple is given by
I _{RIPPLE_MAX}
2
v
I_SYN
v
I _{RIPPLE_MAX}
ǒ Ǔ ǒ Ǔ ǒ Ǔ _{VIN_MAX * VBAT_MIN}
V_{BAT_MIN}
V_{IN_MAX}
1
fs
and I_{RIPPLE_MAX} +
L_MIN
(4)
where
V_{IN_MAX}: maximum adapter voltage
V_{BAT_MIN}: minimum BAT voltage
f_S: switching frequency
L_MIN: minimum output inductor
The ISYNSET comparator, or charge under-current comparator, compares the voltage between SRP-BAT and
internal threshold on the cycle-to-cycle base. The threshold is set to 13 mV on the falling edge with 8 mV
hysteresis on the rising edge with 10% variation.
HIGH ACCURACY IADAPT USING CURRENT SENSE AMPLIFIER (CSA)
An industry standard, high accuracy current sense amplifier (CSA) is used to monitor the input current by the
host or some discrete logic through the analog voltage output of the IADAPT pin. The CSA amplifies the input
sensed voltage of ACP – ACN by 20x through the IADAPT pin. The IADAPT output is a voltage source 20 times
the input differential voltage. Once PVCC is above 5 V and ACDET is above 0.6V, IADAPT no longer stays at
ground, but becomes active. If the user wants to lower the voltage, they could use a resistor divider from IOUT
to AGND, and still achieve accuracy over temperature as the resistors can be matched 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 OVER VOLTAGE PROTECTION (ACOV)
ACOV provides protection to prevent system damage due to high input voltage. The controller enters ACOV
when ACDET > 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 ACDET voltage returns below 3.1 V. ACOV threshold is 130% of the adapter-detect
threshold.
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