LTC4066/LTC4066-1
22
4066fc
APPLICATIONS INFORMATION
Any time a battery is connected to the BAT pin and the
SHDN pin is low, the BAT pin current can be monitored
with the following equation:
I
V
R
BAT
ISTAT
ISTAT
= • 1000
where |I
BAT
| is the absolute value of the BAT pin current,
V
ISTAT
is the voltage on the I
STAT
pin and R
ISTAT
is the total
resistance from the I
STAT
pin to ground.
The POL pin has two states: high impedance and strong
pull-down. High impedance indicates that current is fl ow-
ing from BAT to OUT (ideal diode function) and strong
pull-down indicates that current is fl owing from OUT to
BAT (charging). If an external ADC is used to convert the
I
STAT
voltage, then the POL pin can be thought of as a
sign bit.
When the ideal diode function is operating, the I
STAT
pin
cannot monitor ideal diode load currents less than about
1mA. For any ideal diode load current less than 1mA, the
I
STAT
pin will source a constant current of approximately
1μA. However, when the battery charger function is operat-
ing, the I
STAT
pin will continue to source one-thousandth
of the battery charge current even if the charge current
drops to less than 1mA.
When choosing the value of R
ISTAT
, two details must be
considered. For the battery charger function, the value of
R
ISTAT
programs the charge current below which the CHRG
pin transitions to its high impedance state (see CHRG
Status Output Pin). Furthermore, the available common
mode range on the I
STAT
pin needed to maintain an accurate
ratio between I
BAT
and I
ISTAT
is limited. When charging, the
I
STAT
pin voltage should not exceed approximately V
OUT
– 0.5V. When the ideal diode is functioning, the I
STAT
pin
voltage should not exceed approximately V
BAT
– 0.5V (for
the typical minimum operating voltage for the ideal diode
this value would be 2.8V – 0.5V = 2.3V). Typically, it is this
second case that is the limiting situation since V
BAT
is typi-
cally lower than V
OUT
(while charging) and transient ideal
diode loads tend to be greater than typical charge currents
(causing a higher voltage on the I
STAT
pin). Therefore,
choosing a value of R
ISTAT
based on the CHRG detection
current may limit the maximum ideal diode load current
that can be sensed accurately. Consider an example:
a) Desired charge current = 850mA
b) Desired CHRG detection current = 100mA
c) Maximum transient ideal diode current = 1.5A
Calculate:
a) R
PROG
= (1V/850mA) • 50,000 = 59k
b) R
ISTAT
= 100V/100mA = 1k
c) V
ISTAT(MAX)
= 1.5A/1000 • 1k = 1.5V
In this example, there is no common mode problem because
the maximum I
STAT
voltage (1.5V) is well below the 2.3V
minimum. However, if, instead of 100mA, the desired CHRG
detection current was lowered to 40mA, then the desired
R
ISTAT
resistor would increase to 2.5k (100V/40mA) and
the maximum I
STAT
voltage would increase to 3.75V (as-
suming no change in the 1.5A maximum ideal diode cur-
rent). Therefore, ideal diode currents greater than 920mA
(2.3V/2.5k • 1000) might not be reported accurately. To
calculate the maximum ideal diode current that will be
reported accurately:
I
VV
R
DMON MAX
BAT
ISTAT
()
–.
=
05