Rainbow Electronics MAX8903A - Manual (Page 16)

MAX8903A
2A 1-Cell Li+ DC-DC Charger for USB*
and Adapter Power
16 ______________________________________________________________________________________
Soft-Start
To prevent input transients that can cause instability in
the USB or AC adapter power source, the rate of
change of the input current and charge current is limit-
ed. When an input source is valid, SYS current is
ramped from zero to the set current-limit value in typi-
cally 50µs. This also means that if DC becomes valid
after USB, the SYS current limit is ramped down to zero
before switching from the USB to DC input. At some
point, SYS is no longer able to support the load and
may switch over to BAT. The switchover to BAT occurs
when V
SYS
< V
BATT
. This threshold is a function of the
SYS capacitor size and SYS load. The SYS current limit
then ramps from zero to the set current level and SYS
supports the load again as long as the SYS load current
is less than the set current limit.
When the charger is turned on, the charge current ramps
from 0A to the ISET current value in typically 1.0ms.
Charge current also soft-starts when transitioning to fast-
charge from prequal, when the input power source is
switched between USB and DC, and when changing the
USB charge current from 100mA to 500mA with the IUSB
logic input. There is no di/dt limiting, however, if R
ISET
is
changed suddenly using a switch.
Battery Charger
While a valid input source is present, the battery charg-
er can attempt to charge the battery with a fast-charge
current determined by the resistance at the ISET pin:
R ISET = 1200/I CHG-MAX
Monitoring Charge Current
The voltage from ISET to GND is a representation of the
battery charge current and can be used to monitor the
current charging the battery. A voltage of 1.5V repre-
sents the maximum fast-charge current.
If necessary, the charge current is reduced automati-
cally to prevent the SYS voltage from dropping.
Therefore, a battery never charges at a rate beyond the
capabilities of a 100mA or 500mA USB input, or over-
loads an AC adapter. See Figure 5.
POWER SOURCE
DOK UOK
DCM IUSB USUS
DC STEP-DOWN
OUTPUT
CURRENT LIMIT
USB INPUT
CURRENT LIMIT
MAXIMUM
CHARGE
CURRENT**
AC Adapter at DC Input L X H X X 6000/R IDC
Lesser of
1200/R ISET and
6000/R IDC
L X L L L 100mA
Lesser of
1200/R ISET and
100mA
L X L H L 500mA
Lesser of
1200/R ISET and
500mA
USB Power at DC Input
L X L X H USB suspend
USB input off. DC
input has priority.
0
H L X L L 100mA
Lesser of
1200/R ISET and
100mA
H L X H L 500mA
Lesser of
1200/R ISET and
500mA
USB Power at USB Input,
DC Unconnected
H L X X H USB suspend 0
DC and USB Unconnected H H X X X
No DC input
No USB input 0
Table 2. Input Limiter Control Logic
** Charge current cannot exceed the input current limit. Charge may be less than the maximum charge current if the total SYS load
exceeds the input current limit.
X = Don’t care.