MAX1535 Maxim Integrated Products, MAX1535 Datasheet - Page 26

• Manufacturer Part Number: MAX1535
• Description: Highly Integrated Level 2 Smbus Battery Charger
• Manufacturer: Maxim Integrated Products

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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.
The MAX1535 includes a hysteretic comparator that
detects the presence of an AC power adapter. The
MAX1535 automatically delivers power to the system
load from an appropriate available power source. When
the adapter is present, the open-drain ACOK output
becomes low impedance and the P-channel source
switch (P1 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
MAX1535 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.
The MAX1535 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 MAX1535 features a pseudofixed-frequency,
current-mode control scheme with cycle-by-cycle cur-
rent limit. The off-time is dependent upon V
V
300ns. The MAX1535 can also operate in discontinuous
26
BATT
AC Adapter Detection and Power Source
______________________________________________________________________________________
, and a time constant, with a minimum t
DC-to-DC Converter
Selection
OFF
DCIN
of
,
conduction mode for improved light-load efficiency.
The operation of the DC-to-DC controller is determined
by the following four comparators as shown in the func-
tional diagram in Figure 9:
• IMIN. Compares the control signal (LVC) against
• CCMP. Compares LVC against the charge-current
• IMAX. Compares CSI to 2V (corresponding to 10A
• ZCMP. Compares CSI to 100mV (corresponding to
The MAX1535 controls input current (CCS control loop),
charge current (CCI control loop), or charge voltage
(CCV control loop), depending on the operating condi-
tion. 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 controller.
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 low-
est control loop ensures fast transition with minimal
overshoot when switching between different control
loops (see the Compensation section).
With sufficient charge current, the MAX1535 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
V
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
DCIN
100mV (typ). When LVC voltage is less than 100mV,
the comparator output is low and a new cycle can-
not start.
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.
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.
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 controller starts a new cycle by turning on
Continuous-Conduction Mode
BATT
< 0.88 ×