Datasheet

MAX8903A–E/G/H/J/N/Y
2A 1-Cell Li+ DC-DC Chargers
for USB and Adapter Power
16
Maxim Integrated
die temperature. The feedback signal requiring the
smallest current controls the average output current in
the inductor. This scheme minimizes total power dissi-
pation 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 battery
and the system voltage (V
SYS
) is slightly above V
SYSMIN
as shown in Figure 4. The battery charger independently
controls the battery charging current. V
SYSMIN
is set to
either 3.0V or 3.4V based on the version of MAX8903_.
See Table 6.
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 Mode (DCM)
As shown in Table 2, the DC input supports both AC
adapters (up to 2A) and USB (up to 500mA). With the
DCM logic input set high, the DC input is in adapter
mode and the DC input current limit is set by the resis-
tance from IDC to GND (R
IDC
). Calculate R
IDC
accord-
ing to the following equation:
R
IDC
= 6000V/I
DC-MAX
With the DCM logic input set low, the DC input current
limit is internally programmed to 500mA or 100mA as
set by the IUSB logic input. With the IUSB logic input
set high, the DC input current limit is 500mA and the
DC input delivers current to SYS through the step-down
regulator. With the IUSB logic input set low, the DC
input current limit is 100mA. In this 100mA mode, the
step-down regulator is turned off and its high-side
switch operates as a linear regulator with a 100mA cur-
rent limit. The linear regulator’s output is connected to
LX and its output current flows through the inductor into
CS and finally to SYS.
The DCM pin has an internal diode to DC as shown in
Figure 1. To prevent current from flowing from DCM
through the internal diode and to the DC input, DCM
cannot be driven to a voltage higher than DC. The
C O M PO N EN T
( F I G U R ES 2 A N D 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 ( M AX 8903A/M AX 8903C /M AX 8903D /M AX 8903H /M AX 8903J) or
22µF ( M AX 8903B/M AX 8903E /M AX 8903G/M AX 8903Y ) cer am i c cap aci tor
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 100kΩ
THM Negative TC thermistor Philips NTC thermistor, P/N 2322-640-63103, 0kΩ ±5% at +25°C
R
T
THM pullup resistor 10kΩ
R
IDC
D C i np ut cur r ent- l i m i t p r og r am m i ng r esi stor 3kΩ ±1%, for 2A limit
R
ISET
Fast-charge current programming resistor 1.2kΩ ±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