MAX1873SEEE Maxim Integrated Products, MAX1873SEEE Datasheet - Page 11

• Manufacturer Part Number: MAX1873SEEE
• Description: Battery Management Switch-Mode Li+ Charger
• Manufacturer: Maxim Integrated Products

Specifications of MAX1873SEEE

• Product: Charge Management
• Battery Type: Li-Ion, Li-Polymer, NiCd, NiMH
• Operating Supply Voltage: 6 V to 28 V
• Supply Current: 4 mA
• Maximum Operating Temperature: + 85 C
• Minimum Operating Temperature: - 40 C
• Package / Case: QSOP-16
• Charge Safety Timers: No
• Mounting Style: SMD/SMT
• Temperature Monitoring: No
• Uvlo Start Threshold: 0.05 V
• Uvlo Stop Threshold: 0.38 V

IOUT is an analog voltage output that is proportional to
the actual charge current. With the aid of a microcon-
troller, the IOUT signal can facilitate gas-gauging, indi-
cate percent of charge, or charge-time remaining. The
equation governing this output is:
where V
BATT pins, and ICHG is the charging current. IOUT can
drive a load capacitance of 5nF.
For Li+ batteries, VADJ sets the per-cell battery-regula-
tion voltage limit. To set the VADJ voltage, use a resis-
tive-divider from REF to GND (Figure 1). For a battery
voltage of 4.2V per cell, use resistors of equal value
(100kΩ each) in the VADJ voltage-divider. To set other
battery-regulation voltages, see the remainder of this
section.
The per-cell battery regulation voltage is a function of
Li+ battery chemistry and construction and is usually
clearly specified by the manufacturer. If this is not
clearly specified, be sure to consult the battery manu-
facturer to determine this voltage before charging any
Li+ battery. Once the per-cell voltage is determined,
the VADJ voltage is calculated by the equation:
where V
(for the total series-cell stack), N is the number of Li+
battery cells, and V
Set V
that the total divider resistance (R1+ R2) is near 200kΩ.
R2 can then be calculated as follows:
Since the full range of VADJ (from 0 to VREF) results in
a ±5.263% adjustment of the battery-regulation limit
(3.979V to 4.421V), the resistive-divider’s accuracy
need not be as tight as the output-voltage accuracy.
Using 1% resistors for the voltage-divider still provides
±0.75% battery-voltage-regulation accuracy.
Setting the Battery-Regulation Voltage
VADJ
CSB
BATTR
V
V
V
R
by choosing R1. R1 should be selected so
IOUT
VADJ
OUT
and V
2
=
is the desired battery-regulation voltage
Charge-Current Monitor Output
[
=
=
V
=
VADJ
20
______________________________________________________________________________________
BATT
20
[
REF
9 5
(
.
(
R
V
(
CSB
CSB
V
is the reference voltage (4.2V).
/
are the voltages at the CSB and
(
BATTR
V
REF
Simple Current-Limited Switch-Mode
Design Procedure
×
−
I
V
CHG
BATT
−
)
/
V
N
VADJ
)
]
)
−
or
(
9
)
V
]
REF
×
R
1
)
The charging current ICHG is sensed by the current-
sense resistor R
also adjusted by the voltage at ICHG/EN. If ICHG/EN is
connected to REF (the standard connection), the
charge current is given by:
In some cases, common values for R
the desired charge-current value. It may also be desir-
able to reduce the 0.2V CSB-to-BATT sense threshold
to reduce power dissipation. In such cases, the
ICHG/EN input may be used to reduce the charge-cur-
rent-sense threshold. In those cases the equation for
charge current becomes:
The input-source current limit, I
current sense resistor, R
between CSSP and CSSN. The equation for the source
current is:
This limit is typically set to the current rating of the input
power source or AC adapter to protect the input source
from overload. Short CSSP and CSSN to DCIN if the
input-source current-limit feature is not used.
The inductor value may be selected for more or less
ripple current. The greater the inductance, the lower
the ripple current. However, as the physical size is kept
the same, larger inductance value typically results in
higher inductor series resistance and lower inductor
saturation current. Typically, a good tradeoff is to
choose the inductor such that the ripple current is
approximately 30% to 50% of the DC average charging
current. The ratio of ripple current to DC charging cur-
rent (LIR) can be used to calculate the inductor value:
where f
300kHz) and I
inductor current is given by:
Li+ Charger Controller
SW
L
I
Setting the Charging-Current Limit
CHG
=
is the switching frequency (nominally
{
[
CHG
V
V
Setting the Input-Current Limit
=
BATT
DCIN MAX
CSB
I
0 2 .
CHG
I
IN
is the charging current. The peak
V V
(
[
between CSB and BATT, and is
V
(
= 0 1 . /
= 0 2 . /
DCIN MAX
ICH EN
)
CSS
V R
×
/
V R
(
f
SW
CSS
, (Figure 1) connected
/
Inductor Selection
CSB
V
IN
)
×
REF
−
, is set by the input-
I
CHG
V
BATT
)
CSB
/
R
×
CSB
may not allow
]
LIR
}
/
]
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