MAX1535A Datasheet, PDF (13/39 Pages) Maxim Integrated Products – Highly Integrated Level 2 SMBus Battery Charger

Highly Integrated Level 2 SMBus
Battery Charger
Detailed Description
The MAX1535A includes all the functions necessary to
charge Li+, NiMH, and NiCd smart batteries. A high-
efficiency, synchronous-rectified, step-down DC-to-DC
converter is used to implement a precision constant-
current, constant-voltage charger with input current lim-
iting. The DC-to-DC converter uses an external P-chan-
nel MOSFET as the buck switch and an external
N-channel MOSFET as the synchronous rectifier to con-
vert the input voltage to the required charge current
and voltage. The charge current and input current-limit
sense amplifiers have low input-offset errors and can
use small-value sense resistors.
The MAX1535A features a voltage-regulation loop
(CCV) and two current-regulation loops (CCI and CCS).
The loops operate independently of each other. The
CCV voltage-regulation loop monitors BATT to ensure
that its voltage never exceeds the voltage set by the
ChargeVoltage() command. The CCI battery current-
regulation loop monitors current delivered to BATT to
ensure that it never exceeds the current limit set by the
ChargeCurrent() command. The charge current-regula-
tion loop is in control as long as the BATT voltage is
below the set point. When the BATT voltage reaches its
set point, the voltage-regulation loop takes control and
maintains the battery voltage at the set point. A third
loop (CCS) takes control and reduces the charge cur-
rent when the sum of the system load and the input cur-
rent to the charger exceeds the power-source current
limit set by the InputCurrent() command. The
MAX1535A also allows the user to clamp the pro-
grammed charge current and charge voltage. This fea-
ture effectively avoids damage to the battery if the
charger was programmed with invalid data.
Based on the presence or absence of the AC adapter,
the MAX1535A automatically selects the appropriate
source for supplying power to the system. A P-channel
load switch controlled from the PDL output and a similar
P-channel source switch controlled from the PDS output
are used to implement this function. The MAX1535A can
be programmed by a microcontroller (µC) to perform a
relearning, or conditioning, cycle in which the battery is
isolated from the charger and completely discharged
through the system load. When the battery reaches
100% depth of discharge, it is recharged to full capacity
(contact the battery-pack manufacturers for the 100%
depth of discharge threshold).
The circuit shown in Figure 1 demonstrates a typical
application for smart-battery systems.
Setting Charge Voltage
The SMBus specification allows for a 16-bit
ChargeVoltage() command that translates to a 1mV LSB
and a 65.535V full-scale voltage; therefore, the
ChargeVoltage() code corresponds to the output volt-
age in millivolts. The MAX1535A ignores the first 4 LSBs,
and uses the next 11 bits to control the voltage DAC.
The charge voltage range of the MAX1535A is 0 to
19.200V. All codes requesting charge voltage greater
than 19.200V result in a voltage setting of 19.200V. All
codes requesting charge voltage below 1.024V result in
a voltage set point of zero, which terminates charging.
The VMAX pin can be used to set an upper limit to the
charge voltage. This feature supercedes the value set
with the ChargeVoltage() command when charge volt-
age is greater than V_CHARGE__MAX. The voltage range
of VMAX is from 0 to V_REF. The maximum charge volt-
age can be related to the voltage on VMAX using the
following equation:
V_{CHARGE _ MAX}
=
5V
V
×
V_VMAX
where V_VMAX is the voltage on the VMAX pin.
Setting Charge Current
The SMBus specification allows for a 16-bit
ChargeCurrent() command that translates to a 1mA
LSB and a 65.535A full-scale current using a 10mΩ
current-sense resistor (R2 in Figure 1). Equivalently, the
ChargeCurrent() value sets the voltage across CSIP
and CSIN inputs in 10µV per LSB increment. The
MAX1535A ignores the first 7 LSBs and uses the next 6
bits to control the current DAC. The charge-current
range of the MAX1535A is 0 to 8.064A using a 10mΩ
current-sense resistor. All codes requesting charge
current above 8.064A result in a current setting of
8.064A. All codes requesting charge current between
1mA to 128mA result in a current setting of 128mA. The
default charge-current setting at power-on reset (POR)
is also 128mA.
The IMAX pin can be used to set an upper limit to the
charge current. This feature supercedes the value set
with the ChargeCurrent() command when charge cur-
rent is greater than I_CHARGE__MAX. The voltage range of
IMAX is from 0 to V_REF. The maximum charge current
can be related to the voltage on IMAX using the follow-
ing equation:
I_{CHARGE _ MAX}
=
5A
V
×
V_IMAX
where V_IMAX is the voltage on the IMAX pin.
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