MAX17005A/MAX17006A/MAX17015A
1.2MHz, Low-Cost,
High-Performance Chargers
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The PWM filter accepts the digital signal with a frequency
from 128Hz to 500kHz. Zero duty cycle shuts down the
MAX17005A/MAX17006A/MAX17015A, and 99.5% duty
cycle corresponds to full scale (60mV) across CSIP
and CSIN.
Choose a current-sense resistor (RS2) to have a suffi-
cient power-dissipation rating to handle the full-charge
current. The current-sense voltage can be reduced to
minimize the power-dissipation period. However, this can
degrade accuracy due to the current-sense amplifier’s
input offset (0.25mV typ). See
Typical Operating
Characteristics
to estimate the charge-current accuracy
at various set points.
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 controller
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 down. The
input-current-limit circuit reduces the power require-
ment 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-to-DC converter
(typically 85% to 95%).
In the MAX17005A/MAX17006A/MAX17015A, the volt-
age across CSSP and CSSN is constant at 60mV.
Choose the current-sense resistor, RS1, to set the input
current limit. For example, for 4A input current limit,
choose RS1 = 15mΩ. For the input current-limit set-
tings, which cannot be achievable with standard sense
resistor values, use a resistive voltage-divider between
CSSP and CSSN to tune the setting (Figure 4).
To minimize power dissipation, first choose RS1
according to the closest available value. For conve-
nience, choose Ra = 6kΩ and calculate Rb from the
above equation.
Choose a current-sense resistor (RS1) to have a suffi-
cient power rating to handle the full system current. The
current-sense resistor can be reduced to improve effi-
ciency, but this degrades accuracy due to the current-
sense amplifier’s input offset (0.15mV typ). See
Typical
Operating Characteristics
to estimate the input current-
limit accuracy at various set points.
Automatic Power-Source Selection
The MAX17005A/MAX17006A/MAX17015A use an
external charge pump to drive the gate of an n-channel
adapter selection switch (N3 and Q1a). In Figure 1,
when the adapter is present, BST is biased 5V above
V
ADAPTER
so that N3 and Q1a are on, and Q1b is off.
As long as the adapter is present, even though the
charger is off, the power stage forces a refresh pulse
to the BST charge pump every 5ms.
When the adapter voltage is removed, the charger
stops generating BST refresh pulses and N4 forces N2
off, Q1b turns on and supplies power to the system
from the battery.
In Figure 1, D1 must have low forward-voltage drop and
low reverse-leakage current to ensure sufficient gate
drive at N3 and Q1a. A 100mA, low reverse-leakage
Schottky diode is the right choice.
Analog Input Current-Monitor Output
Use IINP to monitor the system-input current, which is
sensed across CSSP and CSSN. The voltage at IINP is
proportional to the input current:
where I
INPUT
is the DC current supplied by the AC
adapter, G
IINP
is the transconductance of the sense
amplifier (2.8mA/V typ), and R
IINP
is the resistor con-
nected between IINP and ground. Typically, IINP has a
0 to 3.5V output-voltage range. Leave IINP unconnected
when not used.
