Datasheet

ISET
ISET
CHARGE
K
R
I
=
bq24278
www.ti.com SLUSB04 –JUNE 2012
PWM Controller in Charge Mode
The bq24278 provides an integrated, fixed 1.5 MHz frequency voltage-mode controller with to power the system
and supply the charge current. The converter is internally compensated and provides enough phase margin for
stable operation, allowing the use of small ceramic capacitors with very low ESR.
The bq24278 input scheme prevents battery discharge when the supply voltage is lower than V
BAT
. The high-side
N-MOSFET (Q1) switches to control the power delivered to SYS. The DRV LDO supplies the gate drive for the
internal MOSFETs. The high-side MOSFET is supplied by a boot strap circuit with external boot-strap capacitor
(BST).
The input is protected from short circuit by a cycle-by-cycle current limit that is sensed through the high-side
MOSFET. The threshold is set to a nominal 5-A peak current. The input also utilizes an input current limit that
limits the current from the power source.
Battery Charging Process
When the battery is deeply discharged or shorted, the bq24278 applies a 50mA current to charge the battery
voltage up to acceptable charging levels. During this time, the battery FET is linearly regulated to maintain the
system output regulation at V
SYS(REG)
. Once the battery rises above V
SHORT
, the charge current increases to the
fastcharge current setting. The SYS voltage is regulated to V
SYS(REG)
while the battery is linearly charged through
the battery FET. Additionally, the thermal regulation loop reduces the charge current to maintain the die
temperature at safe levels. Under normal conditions, the time spent in this region is a very short percentage of
the total charging time, so if the charge current is reduced, the reduced charge rate does not have a major
negative effect on total charge time. If the current limit for the SYS output is reached (limited by the input current
limit, or V
IN_DPM
), the charge current is reduced to provide the system with all the current that is needed. If the
charge current is reduced to 0mA, pulling further current from SYS causes the output to fall to the battery voltage
and enter supplement mode (see the “Dynamic Power Path Management" section for more details).
Once the battery is charged enough to where the system voltage begins to rise above V
SYSREG
(depends on the
charge current setting), the battery FET is turned on fully and the battery is charged with the programmed charge
current programmed using the ISET input, I
CHARGE
. The slew rate for fast charge current is controlled to minimize
the current and voltage over-shoot during transient. The charge current is programmed by connecting a resistor
from ISET to GND. The value for R
ISET
is calculated using Equation 1.
(1)
Where I
CHARGE
is the programmed fast charge current and K
ISET
is the programming factor found in the Electrical
Characteristics table.
The charge current is regulated to I
CHARGE
until the battery is charged to the regulation voltage. Once the battery
voltage is close to the regulation voltage, V
BATREG
, the charge current is tapered down as shown in Figure 2 while
the SYS output remains connected to the battery. The voltage regulation feedback occurs by monitoring the
battery-pack voltage between the BAT and PGND pins.
The bq24278 monitors the charging current during the voltage regulation phase. Once the termination threshold,
I
TERM
, is detected and the battery voltage is above the recharge threshold, the bq24278 terminates charge and
turns off the battery charging FET and begins battery detection. The system output is regulated to the V
BAT(REG)
voltage and supports the full current available from the input and the battery supplement mode (see the
“Dynamic Power Path Management” section for more details) is still available.
A charge cycle is initiated when one of the following conditions is detected:
1. The battery voltage falls below the V
BAT(REG)
-V
RCH
threshold.
2. V
IN
Power-on reset (POR)
3. CE toggle
4. Toggle Hi-Impedance mode (using CD)
If the battery voltage is ever greater than V
BAT(REG)
, the PWM converter is turned off and the battery is discharged
to V
BAT(REG)
. This prevents further overcharging the battery and allows the battery to discharge to safe operating
levels.
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