Rainbow Electronics MAX8730 - Manual (Page 18)

MAX8730
Low-Cost Battery Charger
18 ______________________________________________________________________________________
The MAX8730 includes a foldback feature, which
reduces the Schottky requirement at low battery volt-
ages. See the Foldback Current Section.
Setting Input-Current Limit
The total input current, from a wall adapter or other DC
source, is the sum of the system supply current and the
current required by the charger. When the input current
exceeds the set input current limit, the MAX8730
decreases the charge current to provide priority to sys-
tem load current. System current normally fluctuates as
portions of the system are powered up or put to sleep.
The input-current-limit circuit reduces the power
requirement of the AC wall adapter, which reduces
adapter cost. As the system supply rises, the available
charge current drops linearly to zero. Thereafter, the
total input current can increase without limit.
The total input current is the sum of the device supply cur-
rent, the charger input current, and the system load cur-
rent. The total input current can be estimated as follows:
where η is the efficiency of the DC-DC converter (typi-
cally 85% to 95%).
CLS sets the maximum voltage across the current-
sense resistor RS1, which determines the input current
limit. The full-scale differential voltage between CSSP
and CSSN is 75mV (5A for RS1 = 15m
Ω). Set CLS
according to the following equation:
The input range for CLS is 1.1V to V REF . Choose a cur-
rent-sense resistor (RS1) to have a sufficient power rat-
ing to handle the full system current. The current-sense
resistor may be reduced to improve efficiency, but this
degrades accuracy due to the current-sense amplifier’s
input offset (±3mV). See the Typical Operating Charac-
teristics to estimate the input current-limit accuracy at
various set points. The input current-limit error amplifier
(GMS) is compensated at the CCS pin; see the Com-
pensation section.
Input-Current Measurement
IINP monitors the system-input current sensed across
CSSP and CSSN. The voltage of IINP is proportional to
the input current according to the following equation:
V IINP = I INPUT x RS1 x G IINP x R 10
where I INPUT is the DC current supplied by the AC
adapter, G IINP is the transconductance of IINP
(2.8µA/mV typ), and R 10 is the resistor connected
between IINP and ground. Connect a 0.1µF filter
capacitor from IINP to GND to reduce ripple. IINP has a
0 to 4.5V output-voltage range. Connect IINP to GND if
it is not used.
The MAX8730 provides a short-circuit latch to protect
against system overload or short. The latch is set when
V
IINP rises above 4.2V, and disconnects the adapter
from the system by turning PDS off (PDL does not
change). The latch is reset by bringing SRC below
UVLO (remove and reinsert the adapter). Choose a fil-
ter capacitor that is large enough to provide appropri-
ate debouncing and prevent accidental faults, yet
results in a response time that is fast enough to ther-
mally protect the MOSFETs. See the System Short
Circuit section.
IINP can be used to measure battery-discharge current
(see Figure 1) when the adapter is absent. To disable
IINP and reduce battery consumption to 10µA, drive
INPON to low. Charging is disabled when INPON is
low, even if the adapter is present.
AC-Adapter Detection and
Power-Source Selection
The MAX8730 includes a hysteretic comparator that
detects the presence of an AC power adapter and
automatically selects the appropriate power source.
When the adapter is present (V
ASNS
> V
BATT
-
-100mV) the battery is disconnected from the system
load with the p-channel (P3) MOSFET. When the
adapter is removed (V
ASNS < V BATT - -270mV), PDS
turns off and PDL turns on with a 5µs break-before-
make sequence.
The ACOK output can be used to indicate the presence
of the adapter. When V
ACIN > 2.1V and V ASNS > V BATT
- 100mV, ACOK becomes low. Connect a 10k
Ω pullup
resistor between LDO and ACOK. Use a resistive volt-
age-divider from the adapter’s output to the ACIN pin to
set the appropriate detection threshold. Since ACIN
has a 6V absolute maximum rating, set the adapter
threshold according to the following equation:
Relearn Mode
The MAX8730 can be programmed to perform a relearn
cycle to calibrate the battery’s fuel gauge. This cycle
consists of isolating the battery from the charger and dis-
charging it through the system load. When the battery
V
V
ADAPTER THRESHOLD
ADAPTER MAX
_
_ >
3
V I x RS x
V
mV
CLS LIMIT
REF =
1
75
I I
I x V
V x
INPUT LOAD
CHARGE BATTERY
IN
= +
η