MAX1535A Datasheet, PDF (26/39 Pages) Maxim Integrated Products – Highly Integrated Level 2 SMBus Battery Charger

Highly Integrated Level 2 SMBus
Battery Charger
Multiple bits may be set depending on the value of the
thermistor (e.g., a thermistor that is 450Ω causes both
the THERMISTOR_HOT and the THERMISTOR_UR bits
to be set). The thermistor may be replaced with fixed-
value resistors in battery packs that do not require the
thermistor as a secondary fail-safe indicator. In that
case, it is the responsibility of the battery pack elec-
tronics to manipulate the resistance to obtain correct
charger behavior.
AC Adapter Detection and Power Source
Selection
The MAX1535A includes a hysteretic comparator that
detects the presence of an AC power adapter. The
MAX1535A automatically delivers power to the system
load from an appropriate available power source. When
the adapter is present, the open-drain ACOK output
becomes high impedance and the P-channel source
switch (P3 in Figure 1) is turned on by PDS, thereby
powering the system. The switch threshold at ACIN is
2.048V. Use a resistive voltage-divider from the
adapter’s output to the ACIN pin to set the appropriate
detection threshold. When charging, the battery is iso-
lated from the system load with the P-channel load
switch (P2 in Figure 1), which is switched off by PDL.
When the adapter is absent, the drive to the switches
changes state in a fast break-before-make sequence.
PDL begins to turn on 7.5µs after PDS begins to turn off.
The threshold for selecting between the PDL and PDS
switches is set based on the voltage difference
between the DCIN and the BATT pins. If this voltage
difference drops below 100mV, the PDS is switched off
and PDL is switched on. Under these conditions, the
MAX1535A is completely powered down. The PDL
switch is kept on with a 100kΩ pulldown resistor when
the AC adapter is removed.
The drivers for PDL and PDS are fully integrated. The
positive bias inputs for the drivers connect to the SRC
pin and the negative bias inputs connect to a negative
regulator referenced to SRC. With this arrangement, the
drivers can swing from SRC to approximately 10V
below SRC.
DC-to-DC Converter
The MAX1535A employs a synchronous step-down DC-
to-DC converter with a P-channel high-side MOSFET
switch and an N-channel low-side synchronous rectifi-
er. The MAX1535A features a pseudofixed-frequency,
current-mode control scheme with cycle-by-cycle cur-
rent limit. The off-time is dependent upon V_DCIN,
V_BATT, and a time constant, with a minimum t_OFF of
300ns. The MAX1535A can also operate in discontinu-
ous conduction mode for improved light-load efficien-
cy. The operation of the DC-to-DC controller is deter-
mined by the following four comparators as shown in
the functional diagram in Figure 9:
• IMIN. Compares the control signal (LVC) against
100mV (typ). When LVC voltage is less than 100mV,
the comparator output is low and a new cycle can-
not start.
• CCMP. Compares LVC against the charge-current
feedback signal (CSI). The comparator output is
high and the high-side MOSFET on-time is terminat-
ed when the CSI voltage is higher than LVC.
• IMAX. Compares CSI to 2V (corresponding to 10A
when R2 = 10mΩ). The comparator output is high
and the high-side MOSFET on-time is terminated
when CSI voltage is higher than the threshold. A
new cycle cannot start until the IMAX comparator
output goes low.
• ZCMP. Compares CSI to 100mV (corresponding to
500mA when R2 = 10mΩ). The comparator output is
high and both MOSFETs are turned off when CSI
voltage is lower than the threshold.
CCV, CCI, CCS, and LVC Control Blocks
The MAX1535A controls input current (CCS control
loop), charge current (CCI control loop), or charge volt-
age (CCV control loop), depending on the operating
condition. The three control loops, CCV, CCI, and CCS,
are brought together internally at the lowest voltage
clamp (LVC) amplifier. The output of the LVC amplifier
is the feedback control signal for the DC-to-DC con-
troller. The minimum voltage at CCV, CCI, or CCS
appears at the output of the LVC amplifier and clamps
the other two control loops to within 0.3V above the
control point. Clamping the other two control loops
close to the lowest control loop ensures fast transition
with minimal overshoot when switching between differ-
ent control loops (see the Compensation section).
Continuous-Conduction Mode
With sufficient charge current, the MAX1535A inductor
current never reaches zero, which is defined as contin-
uous-conduction mode. The regulator switches at
400kHz (nominal) if it is not in dropout (V_BATT < 0.88 ×
V_DCIN). The controller starts a new cycle by turning on
the high-side P-channel MOSFET and turning off the
low-side N-channel MOSFET. When the charge-current
feedback signal (CSI) is greater than the control point
(LVC), the CCMP comparator output goes high and the
controller initiates the off-time by turning off the high-
side P-channel MOSFET and turning on the low-side N-
channel MOSFET. The operating frequency is governed
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