Design procedure – Rainbow Electronics MAX1873 User Manual
Page 11
Charge-Current Monitor Output
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
CSB
and V
BATT
are the voltages at the CSB and
BATT pins, and ICHG is the charging current. IOUT can
drive a load capacitance of 5nF.
Design Procedure
Setting the Battery-Regulation Voltage
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
(100k
Ω 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
BATTR
is the desired battery-regulation voltage
(for the total series-cell stack), N is the number of Li+
battery cells, and V
REF
is the reference voltage (4.2V).
Set V
VADJ
by choosing R1. R1 should be selected so
that the total divider resistance (R1+ R2) is near 200k
Ω.
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 Charging-Current Limit
The charging current ICHG is sensed by the current-
sense resistor R
CSB
between CSB and BATT, and is
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
CSB
may not allow
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:
Setting the Input-Current Limit
The input-source current limit, I
IN
, is set by the input-
current sense resistor, R
CSS
, (Figure 1) connected
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.
Inductor Selection
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
SW
is the switching frequency (nominally
300kHz) and I
CHG
is the charging current. The peak
inductor current is given by:
L
V
V
V
V
f
I
LIR
BATT
DCIN MAX
BATT
DCIN MAX
SW
CHG
=
−
[
]
{
}
Ч
Ч
Ч
[
]
(
)
(
)
/
I
V R
IN
CSS
= 0 1
.
/
I
V V
V
R
CHG
ICH EN
REF
CSB
=
(
)
0 2
.
/
/
/
I
V R
CHG
CSB
= 0 2
.
/
R
V
V
V
R
VADJ
REF
VADJ
2
1
=
−
(
)
[
]
×
/
V
V
N
V
VADJ
BATTR
REF
=
(
)
[
]
−
(
)
9 5
9
.
/
V
V
V
or
V
R
I
IOUT
CSB
BATT
OUT
CSB
CHG
=
−
(
)
=
×
(
)
20
20
MAX1873
Simple Current-Limited Switch-Mode
Li+ Charger Controller
______________________________________________________________________________________
11