Low-Cost, Multichemistry Battery-
Charger Building Block
MAX172
Use IINP to monitor the system input current being
Tꢂboe 10 Ceoo-CVunt PrVgrꢂmming Tꢂboe
sensed across CSSP and CSSN. The output voltage
range is 0 to 3V. The voltage of IINP is proportional to
the output current by the equation:
CELL
< 0.20V
CELL CꢀꢄNT
V
2
3
4
CELLS
0.40V < V
< V
-0.5V
LDO
CELLS
V
= I
× RS1 × G × R10
IINP
6
)
(
IINP
SOURCE
V
- 0.25V < V
< V
LDO
CELLS LDO
where I
is the DC current being supplied by the
SOURCE
AC adapter power, G
is the transconductance of
IINP
The input source current is the sum of the device cur-
rent, the charger input current, and the load current.
The device current is minimal (6mA) in comparison to
the charge and load currents. The actual source cur-
rent required is determined as follows:
IINP (1µS typ), and R10 is the resistor connected
between IINP and ground.
In the typical application circuit, duty cycle affects the
accuracy of V
(Figure 3). AC load current also
IINP
affects accuracy (Figure 4).
I
= I
+
I
× V
/ V × η
3
(
)
(
)
( )
SOURCE
LOAD
CHARGE
BATT
IN
Connect IINP pin to ground if it is not used.
LDO Regulator
where η is the efficiency of the DC-DC converter (85%
to 95% typ).
LDO provides a 5.4V supply derived from DCIN and
can deliver up to 15mA of current. The MOSFET drivers
are powered by DLOV and BST, which must be con-
nected to LDO as shown in Figure 1. LDO also supplies
the 4.096V reference (REF) and most of the control cir-
cuitry. Bypass LDO with a 1µF capacitor.
V
determines the reference voltage of the GMS
CLS
error amplifier. Sense resistor RS1 sets the maximum
allowable source current. Calculate the maximum cur-
rent as follows:
I
= V
/ 20 × RS1
4
(
)
( )
SOURCE_MAX
CLS
DC-to-DC Converter
The MAX1772 employs a buck regulator with a boot-
strapped NMOS high-side switch and a low-side NMOS
synchronous rectifier.
Once the input current limit is reached, the charging
current is tapered back until the input current is below
the desired threshold.
DC-DC CVntrVooer
The control scheme is a constant off-time variable fre-
quency, cycle-by-cycle current mode. The off-time is
When choosing the current-sense resistor, note that the
voltage drop across this resistor causes further power
loss, reducing efficiency.
constant for a given BATT voltage. It varies with V
BATT
operation; a maximum on-time of 10ms allows the con-
troller to achieve >99% duty cycle with continuous con-
duction. Figure 5 shows the controller functional
diagram.
AC Adapter Detection
Connect the AC adapter voltage through a resistive
divider to ACIN to detect when AC power is available,
as shown in Figure 1. ACOK is an open-drain output
and is high when ACIN is less than REF/2.
MꢀSFET DriverU
The low-side driver output DLO swings from 0 to DLOV.
DLOV is usually connected through a filter to LDO. The
high-side driver output DHI is bootstrapped off LX and
Current Measurement
Use ICHG to monitor the battery-charging current
being sensed across CSIP and CSIN. The output volt-
age range is 0 to 3V. The voltage of ICHG is proportion-
al to the output current by the equation:
swings from V
to V
. When the low-side driver
BST
LX
turns on, BST rises to one diode voltage below DLOV.
Filter DLOV with a resistor-capacitor (RC) circuit whose
cutoff frequency is about 50kHz. The configuration in
Figure 1 introduces a cutoff frequency of around
48kHz:
V
= I
× RS2 × G × R9
ICHG
5
(
)
ICHG
ICHG
where I
is the battery-charging current, G
is
ICHG
ICHG
the transconductance of ICHG (1mS typ), and R9 is the
resistor connected between ICHG and ground.
ꢀ
ꢀ
f = 1/2πRC = 1 / (2π 33Ω 0.1µF) = 48kHz (7)
Connect ICHG pin to ground if it is not used.
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MAXIM [ MAXIM INTEGRATED PRODUCTS ]
