BQ24618_15 Datasheet, PDF (17/43 Pages) Texas Instruments – System Power Selector and Low

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bq24618
SLUSA55B – OCTOBER 2010 – REVISED APRIL 2015
8.3 Feature Description
8.3.1 Battery Voltage Regulation
The bq24618 uses a high-accuracy voltage band gap and regulator for the high charging voltage accuracy. The
charge voltage is programmed through a resistor divider from the battery to ground, with the midpoint tied to the
VFB pin. The voltage at the VFB pin is regulated to 2.1 V, giving the following equation for the regulation voltage:
VBA _T
= 2.1
V
´
é
êë¹⁺
R2
R1
ù
úû
,
where
• R2 is connected from VFB to the battery.
• R1 is connected from VFB to GND.
(1)
8.3.2 Battery Current Regulation
The ISET1 input sets the maximum fast-charging current. Battery charge current is sensed by resistor R_SR
connected between SRP and SRN. The full-scale differential voltage between SRP and SRN is 100 mV. Thus,
for a 10-mΩ sense resistor, the maximum charging current is 10 A. The equation for charge current is:
I_CHARGE
=
V_ISET1
20 ´ R_SR
(2)
V_ISET1, the input voltage range of ISET1, is from 0 V to 2 V. The SRP and SRN pins are used to sense voltage
across R_SR using the default value of 10 mΩ. However, resistors of other values can also be used. A larger
sense resistor gives a larger sense voltage and a higher regulation accuracy, but at the expense of higher
conduction loss.
8.3.3 Input Adapter Current Regulation
The total input from an AC adapter or other DC source is a function of the system supply current and the battery
charging current. System current normally fluctuates as portions of the systems are powered up or down. Without
DPM, the source must be able to supply the maximum system current and the maximum charger input current
simultaneously. By using DPM, the battery charger reduces the charging current when the input current exceeds
the input current limit set by ACSET. The current capability of the AC adaptor can be lowered, reducing system
cost.
Similar to sensing battery regulation current, adaptor current is sensed by resistor R_AC connected between ACP
and ACN. Its maximum value is set by ACSET using Equation 3:
I_DPM
=
V_ACSET
20 ´ R_AC
(3)
V_ACSET, the input voltage range of ACSET, is from 0 V to 2 V. The ACP and ACN pins are used to sense voltage
across R_AC using the default value of 10 mΩ. However, resistors of other values can also be used. A larger
sense resistor gives a larger sense voltage and a higher regulation accuracy, but at the expense of higher
conduction loss.
8.3.4 Precharge
On power up, if the battery voltage is below the V_LOWV threshold, the bq24618 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 by the voltage on the ISET2 pin, V_ISET2, according to Equation 4.
I_PRECHARGE =
V_ISET2
100 ´ R_SR
(4)
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