AAT3670 View Datasheet(PDF) - Analog Technology Inc
Part Name
Description
MFG CO.
'AAT3670' PDF : 32 Pages View PDF
and USB low (USBL). The USBH mode can be set
as high as 900mA; however for most applications
utilizing a USB port as the source supply, 500mA is
the typical default USBH value and USBL is sub-
sequently set for 100mA. In the USBL fast charge
mode, the constant charging current is set to 20
percent of the programmed USBH. More simply
put, the USBL low fast charge level = USBH divid-
ed by five. The USBH or USBL modes may be
externally selected by USB select pin (USBSEL).
In the USBH mode, the battery cell preconditioning
or trickle charge current is fixed at 10 percent of the
programmed fast charge constant current. In the
USBL mode, the trickle charge current is only
reduced to 50 percent of the programmed fast
charge constant current level.
When the USBSEL pin is connected to a logic high
level, the USBH level will be active. Conversely,
when USBSEL is pulled to a logic low level (ground)
the USBL level will be used for fast charging. Refer
to Table 2 for the recommended RUSBSET value to
program the desired USB input constant current
charge levels.
USB Charge Reduction
In many instances, product system designers have
an issue of not knowing the real properties of a
potential USB port to be used to supply power to
the battery charger. Typical powered USB ports
commonly found on desktop and notebook PCs
should supply up to 500mA. In the event a USB
port being used to supply the charger is unable to
provide the programmed fast charge current, or if
the system under charge must also share supply
current with other functions causing an overload to
the USB port, the AAT3670 will automatically
reduce USB fast charge current to maintain port
integrity and protect the host system.
The USB charge reduction system becomes active
when the voltage on the USB input falls below the
USB charge reduction threshold (VCHRUSB), which is
typically 4.5V. Regardless of which USB charge
function is selected (USBH or USBL), the charge
reduction system will reduce the fast charge current
level in a linear fashion until the voltage sensed on
the USB input recovers above the charge reduction
threshold voltage. The USB charge reduction
threshold (VCHRUSB) may be externally set to a value
lower than 4.5V by placing a resistor divider network
22
AAT3670
1.6A Dynamic Battery Charger
and Power Manager
between VUSB and ground with the center connect-
ed to the CHRUSB pin. The USB charge reduction
feature may be disabled by shorting the CHRUSB
pin directly to the USB input pin.
The following equation may be used to approximate
a USB charge reduction threshold below 4.5V:
Eq.
2:
VUSBCHR
=
2.0V
(R2/[R2 +
R1])
where R1 and R2 < 1MΩ
VUSB
USB
R1
CHRUSB
R2
1.0M
VCHR = 2.0V
800k
Figure 3: Internal Equivalent Circuit for the
CHRUSB Pin.
USB Input Charge Inhibit and Resume
The AAT3670 under-voltage lockout (UVLO) and
power-on reset feature will function when the USB
input pin voltage level drops below the UVLO
threshold. At this point the charger will suspend
charging. When power is re-applied to the USB pin
or the UVLO condition recovers, the system charge
control will assess the state of charge on the battery
cell and will automatically resume charging in the
appropriate mode for the condition of the battery.
End of Charge Termination
The AAT3670 provides a user-programmable
charge termination current at the end of the charge
cycles. When the battery cell voltage as sensed by
the BATS pin reaches 4.2V, the charge control will
transition from constant current fast charge mode to
constant voltage mode. In constant voltage mode,
the battery cell voltage will be regulated at 4.2V.
The charge current will drop as the battery reaches
its full charge capacity. When the charge current
3670.2007.02.1.1
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