BC3770 Datasheet, PDF (21/45 Pages) Freescale Semiconductor, Inc – 2.0 A Switch-Mode Charger

5.3.10.3 Battery Remote Sensing
In order for the device to exactly regulate the + terminal on the battery cell as close to a preset V_{BAT_REG} as possible, the BATREG and
BATSNSN pins are used. In the real application, there may be some voltage drop between the CHGOUT pin and the + terminal, which
comes from parasitic resistance, due to the PCB trace and a charge current. This voltage drop makes the VBAT_REG not regulate at the
target regulation voltage. To reduce the charging time, place those two pins, BATREG and BATSNSN, as close to each + and - terminal
on the battery pack as possible.
5.3.10.4 Adaptive-Input Current Limit (AICL)
The AICL function prevents the current limited input supply voltage from sagging below a certain preset AICL threshold voltage (V_{AICL_TH}).
If the required input current of device for a certain programmed value of I_{IN_LIM} exceeds the maximum input current of the VBUS supply,
then the VBUS supply will collapse and the device won't function properly under these conditions. To keep the device functional with a
current and voltage limited VBUS source, the device in Start-up mode automatically starts incrementing the input current limit to either the
default or pre-programmed value until either the input current limit is detected or the VBUS voltage detects the AICL threshold, to keep
input supply voltage as a valid power source to provide the load for the application. The device allows the maximum current the input
supply can possibly provide without severely collapsing.
In general, the AICL function is enabled whenever the input current tries to exceed the input current limit, while charging the battery and/
or providing the system load. At the beginning of the charge cycle with discharged battery, the required input current could be lower than
the VBUS current limit. The input current increases as the battery voltage increases. Eventually, the input current may exceed the VBUS
input current limit. If this happens, the AICL function takes over and lowers the charge current below the programmed value to keep VBUS
around VAICL_TH.
Most of the time charge reduction occurs at the beginning of charge cycle when a low current limited AC adapter or USB port is connected
as a valid input source. During start-up the device detects the current limited supply by slowly stepping up the input current with a
programmable soft-start clock period. Each current step is around 25 mA. As the charge and system current are stepped up, the input
current also steps up in staircase fashion. Eventually, the input current will hit the current limit. The input falls to or below V_{AICH_TH}. When
this happens, the device steps down the input current by 25 mA at the next clock rising edge. This allows the input supply voltage to rise
above V_{AICL_TH} at the next soft start clock cycle. The charge current is stepped up again by 25 mA. This again allows the input current to
be exceed. The input reduction continues to step up and down the input current by 25 mA to maintain the input supply voltage as close
to V_{AICL_TH} as possible.
VB USIN
V_{AIC L_TH}
I_{IN_ LIM}
I_{I N_L I M}
IIN_ LIM
V_{B AT _RE G}
IC HG+SY S
OT P-Programmable
Soft Star t Clock Period
25mA
V _BAT
Figure 11. AICL Operation
BC3770
21
Analog Integrated Circuit Device Data
Freescale Semiconductor