Datasheet

bq24155
www.ti.com SLUS942 –FEBRUARY 2010
PWM Controller in Charge Mode
The bq24155 provides an integrated, fixed 3-MHz frequency voltage-mode controller with Feed-Forward function
to regulate charge current or voltage. This type of controller is used to improve line transient response, thereby,
simplifying the compensation network used for both continuous and discontinuous current conduction operation.
The voltage and current loops are internally compensated using a Type-III compensation scheme that provides
enough phase margin for stable operation, allowing the use of small ceramic capacitors with low ESR. The
device operates between 0% to 99.5% duty cycles.
The bq24155 has back to back common-drain N-channel FETs at the high side and one N-channel FET at low
side. The input N-FET (Q1) prevents battery discharge when VBUS is lower than VAUXPWR. The second
high-side N-FET (Q2) is the switching control switch (see Figure 11). A charge pump circuit is used to provide
gate drive for Q1, while a bootstrap circuit with an external bootstrap capacitor is used to supply the gate drive
voltage for Q2.
Cycle-by-cycle current limit is sensed through the FETs Q2 and Q3. The threshold for Q2 is set to a nominal
2.3-A peak current. The low-side FET (Q3) also has a current limit that decides if the PWM Controller will operate
in synchronous or non-synchronous mode. This threshold is set to 100mA and it turns off the low-side N-channel
FET (Q3) before the current reverses, preventing the battery from discharging. Synchronous operation is used
when the current of the low-side FET is greater than 100mA to minimize power losses.
Battery Charging Process
At the beginning of precharge, while battery voltage is below the V
(SHORT)
threshold, the bq24155 applies a
short-circuit current, I
(SHORT)
, to the battery.
When the battery voltage is above V
(SHORT)
and below V
(OREG)
, the charge current ramps up to fast charge
current, I
O(CHARGE)
, or a charge current that corresponds to the input current of I
(IN_LIMIT)
. The slew rate for fast
charge current is controlled to minimize the current and voltage over-shoot during transient. Both the input
current limit (default at 100 mA), IIN_LIMIT, and fast charge current, I
O(CHARGE)
, can be set by the host. Once the
battery voltage reaches the regulation voltage, V
(OREG)
, the charge current is tapered down as shown in
Figure 13. The voltage regulation feedback occurs by monitoring the battery-pack voltage between the CSOUT
and PGND pins. The regulation voltage is adjustable (3.5 V to 4.44 V) and it is programmed through I
2
C
interface.
The bq24155 monitors the charging current during the voltage regulation phase. Once the termination threshold,
ITERM, is detected and the battery voltage is above the recharge threshold, the bq24155 terminates charge. The
termination current level is programmable. To disable the charge current termination, the host can set the charge
termination bit (I_Term) of charge control register to 0, see the I
2
C section for details.
A new charge cycle is initiated when one of the following conditions is detected:
• The battery voltage falls below the V
(OREG)
– V
(RCH)
threshold.
• VBUS Power-on reset (POR), if battery voltage is below the V
(LOWV)
threshold.
• CE bit toggle or RESET bit is set (host controlled)
Safety Timer in Charge Mode
At the beginning of charging process, the bq24155 starts a 32-minute timer (T32min) that can be stopped by any
write-action performed by host through I
2
C interface. Once the 32-minute timer is stopped, a 32-second timer
(T32sec) is automatically started. The 32-second timer can be reset by host using I
2
C interface. Writing "1" to
reset bit of TMR_RST in control register resets the 32-second timer and TMR_RST is automatically set to "0"
after the 32-second timer is reset. If the 32-second timer expires, the charge is terminated and charge
parameters are reset to default values. Then the 32-minute timer starts and the charge resumes.
During normal charging process, the bq24155 is normally in 32-second mode with host control, and 32-minute
mode without host control using I
2
C interface. The process repeats until the battery is fully charged. If the
32-minute timer expires, bq24155 turns off the charger, enunciates FAULT on the STATx bits of status register,
and sends the 128µs interrupt pulse. This function prevents battery over charge if the host fails to reset the
safety timer. The safety timer flow chart is shown in Figure 14. Fault condition is cleared by POR and fault status
bits can only be updated after the status bits are read out by the host.
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