MAX1535A Datasheet, PDF (15/39 Pages) Maxim Integrated Products – Highly Integrated Level 2 SMBus Battery Charger

Highly Integrated Level 2 SMBus
Battery Charger
Table 1. Summary of Operating States
INPUT
CONDITIONS _{AC PRESENT}
DCIN
V_DCIN > 7.5V
THM
X
BATT
X
V_DD
X
X = Don’t care.
POWER FAIL
V_DCIN < V_BATT +
0.3V
X
V_BATT > V_DCIN -
0.3V
X
OPERATING STATES
BATTERY
BATT UNDERVOLTAGE V_DD UNDERVOLTAGE
X
X
X
V_THM < 0.91 × V_DD
X
X
X
V_BATT < 2.5V
X
X
X
V_DD < 2.5V
Setting Input-Current Limit
The total input current, from a wall cube or other DC
source, is the sum of the system supply current and the
current required by the charger. The MAX1535A
reduces the source current by decreasing the maximum
charge current when the input current exceeds the set
input current limit. This technique does not truly limit the
input current. As the system supply current rises, the
available charge current drops proportionally to zero.
Thereafter, the total input current can increase without
limit.
An internal amplifier compares the differential voltage
between CSSP and CSSN to a scaled voltage set by
the InputCurrent() command over the SMBus. The total
input current is the sum of the device supply current,
the charger input current, and the system load current.
The device supply current is minimal (6mA, max) in
comparison to the charger current and system load.
The total input current can be estimated as follows:
[ ] ^{( ) ( ) I}INPUT ^{= I}LOAD ^{+ I}CHARGE ^{× V}BATT ^{/ V}IN ^{× η}
carefully calculate its power rating. Take into account
variations in the system’s load current and the overall
accuracy of the sense amplifier. Note that the voltage
drop across R1 contributes additional power loss,
which reduces efficiency.
System currents normally fluctuate as portions of the
system are powered up or put to sleep. Without input-
current regulation, the input source must be able to
deliver the maximum system current and the maximum
charger-input current. By using the input-current-limit
circuit, the output-current capability of the AC wall
adapter can be lowered, reducing system cost.
LDO Regulator
An integrated low dropout (LDO) linear regulator pro-
vides a 5.4V supply derived from DCIN, which can
deliver at least 10mA of load current. The LDO powers
the gate driver of the low-side N-channel MOSFET in
the DC-to-DC converter. See the MOSFET Drivers sec-
tion. The LDO also biases the 4.096V reference and
most of the control circuitry. Bypass LDO to GND with a
1µF ceramic capacitor.
where η is the efficiency of the DC-to-DC converter
(typically 85% to 95%).
The MAX1535A allows for a 16-bit InputCurrent() com-
mand that translates to a 1mA LSB and a 65.535A full-
scale current using a 10mΩ current-sense resistor (R1
in Figure 1). Equivalently, the InputCurrent() value sets
the voltage across CSSP and CSSN inputs in 10µV per
LSB increments. The MAX1535A ignores the first 7
LSBs and uses the next 6 bits to control the input-cur-
rent DAC. The input-current range of the MAX1535A is
from 256mA to 11.004A. All codes requesting input cur-
rent above 11.004A result in an input-current setting of
11.004A. All codes requesting input current between
1mA to 256mA result in an input-current setting of
256mA. The default input-current-limit setting at POR is
256mA. When choosing the current-sense resistor R1,
V_DD Supply
The V_DD input provides power to the SMBus interface
and the thermistor comparators. Connect V_DD to LDO,
or apply an external supply to V_DD to keep the SMBus
interface active while the supply to DCIN is removed.
Operating Conditions
Table 1 is a summary of the following four MAX1535A
operating states:
• AC present. When DCIN is greater than 7.5V, the AC
adapter is considered to be present. In this condi-
tion, both the LDO and REF function properly and
battery charging is allowed. The AC_PRESENT bit
(bit 15) in the ChargerStatus() register is set to 1.
• Power fail. When DCIN is less than BATT + 0.3V, the
part is in the power-fail state since the charger does
not have enough input voltage to charge the battery.
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