Rainbow Electronics MAX8903A - Manual (Page 14)

MAX8903A
2A 1-Cell Li+ DC-DC Charger for USB*
and Adapter Power
14 ______________________________________________________________________________________
step-down output current programmed at IDC, maxi-
mum charger current programmed at ISET, and maxi-
mum die temperature. The feedback signal requiring
the smallest current controls the average output current
in the inductor. This scheme minimizes total power dis-
sipation for battery charging and allows the battery to
absorb any load transients with minimum system volt-
age disturbance.
If the battery voltage is below V SYSMIN , the charger
does not directly connect the system voltage to the bat-
tery. V
SYS pin is held at a fixed point slightly above
V
SYSMIN
, and does not track the battery. The battery
charger independently controls the battery charging
current. V
SYSMIN is set to 3.0V in the MAX8903A, for
other V SYSMIN values, please contact the factory.
After the battery charges to 50mV above V
SYSMIN
, the
system voltage is connected to the battery. The battery
fast-charge current then controls the step-down con-
verter to set the average inductor current so that both
the programmed input current limit and fast-charge cur-
rent limit are satisfied.
DC-DC Step-Down Control Scheme
A proprietary hysteretic current PWM control scheme
ensures fast switching and physically tiny external com-
ponents. The feedback control signal that requires the
smallest input current controls the center of the peak
and valley currents in the inductor. The ripple current is
internally set to provide 4MHz operation. When the
input voltage decreases near the output voltage, very
high duty cycle occurs and, due to minimum off-time,
4MHz operation is not achievable. The controller then
provides minimum off-time, peak current regulation.
Similarly, when the input voltage is too high to allow
4MHz operation due to the minimum on-time, the con-
troller becomes a minimum on-time, valley current regu-
lator. In this way, ripple current in the inductor is always
as small as possible to reduce ripple voltage on SYS for
a given capacitance. The ripple current is made to vary
with input voltage and output voltage in a way that
reduces frequency variation. However, the frequency
still varies somewhat with operating conditions. See the
Typical Operating Characteristics .
DC Input—USB mode
When powering from DC with DCM set to logic-low, the
DC input is set to USB mode. The input current limit from
DC is then internally set to 500mA max if IUSB is high
and 100mA max if IUSB is low. For the 500mA case, the
DC input continues to operate as a step-down regulator
to minimize thermal heating. For the 100mA case, the
COMPONENT
(FIGURES 2 AND 3)
FUNCTION PART
C DC , C USB Input filter capacitor 4.7µF ceramic capacitor
C VL VL filter capacitor 1.0µF ceramic capacitor
C SYS SYS output bypass capacitor 10µF ceramic capacitor
C BAT Battery bypass capacitor 10µF ceramic capacitor
C
CT
Charger timing capacitor 0.15µF low TC ceramic capacitor
R PU (X4) Logic output pullup resistors 100k Ω
THM Negative TC thermistor
Phillips NTC thermistor, P/N 2322-640-63103, 0k Ω
±5% at +25°C
R T THM pullup resistor 10k Ω
R IDC DC input current-limit programming resistor 3k Ω ±1%, for 2A limit
R ISET Fast-charge current programming resistor 1.2k Ω ±1%, for 1A charging
L1 DC input step-down inductor 1µH inductor with I SAT > 2A
Table 1. External Components List for Figures 2 and 3
I BAT x R ON
4.2V
4.4V
V BAT
V SYSMIN
V SYS
Figure 4. SYS Tracking V BAT to the Minimum System Voltage