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LTC4071EMS8E#PBF

P1-P3P4-P6P7-P9P10-P12P13-P15P16-P18
LTC4071
10
4071fc
applicaTions inForMaTion
adapter voltage (V
WALL
) is 12V and the maximum charge
current is calculated as:
I
MAX _CHARGE
=
V
WALL
− V
BAT _ MIN
(
)
R
IN
=
12V − 3.2V
(
)
162
Ω
= 54mA
Figure 3. 2-Cell Battery Charger
Care must be taken in selecting the input resistor. Power
dissipated in R
IN
under full charge current is given by the
following equation:
P
DISS
=
V
WALL
− V
BAT _ MIN
(
)
2
R
IN
=
12V − 3.2V
(
)
2
162Ω
= 0.48W
The charge current decreases as the battery voltage
increases. If the battery voltage is 40mV less than the
programmed float voltage the LTC4071 consumes only
550nA of current, and all of the excess input current flows
into the battery. As the battery voltage reaches the float
voltage, the LTC4071 shunts current from the wall adapter
and regulates the battery voltage to V
FLOAT
= V
CC
. The
more shunt current the LTC4071 sinks, the less charge
current the battery gets. Eventually, the LTC4071 shunts
all the current flowing through R
IN
; up to the maximum
shunt current. The maximum shunt current in this case,
with no NTC adjustment is determined by the input resistor
and is calculated as:
I
SHUNT _MAX
=
V
WALL
− V
FLOAT
(
)
R
IN
=
12V − 4.1V
(
)
162Ω
= 49mA
At this point the power dissipated in the input resistor is
388mW.
The LTC4071 can also be used to regulate series-connected
battery stacks as illustrated in Figure 3. Here two LTC4071
devices are used to charge two batteries in series. A
single resistor sets the maximum charge/shunt current.
LTC4071
BAT
R
IN
GND
Li-Ion
BATTERY
V
CC
+
1µF
4071 F03
LTC4071
BAT
GND
Li-Ion
BATTERY
V
CC
+
1µF
WALL
ADAPTER
General Charging Considerations
The LTC4071 uses a different charging methodology from
previous chargers. Most Li-Ion chargers terminate the
charging after a period of time. The LTC4071 does not have
a discrete charge termination. Extensive measurements
on Li-Ion cells show that the cell charge current drops
to very low levels with the shunt charge control circuit
effectively terminating the charge. For improved battery
lifetime choose 4.0V or 4.1V float voltage.
The battery disconnect function requires some care in se-
lecting the input supply compliance for charging a battery
while powering a load at V
CC
. The internal battery discon-
nect switch remains off while charging the battery through
the body diode of the internal switch until V
CC
exceeds
V
LBC_VCC
. If the source voltage compliance is not greater
than V
LBC_VCC
, then the battery will never re-connect to
V
CC
and the system load will not be able to run on battery
power. Users may detect that the battery is connected by
monitoring the NTCBIAS pin as it will periodically pulse
high once V
CC
has risen above V
LBC_VCC
, and stops pulsing
once V
CC
falls below V
LBD
.
The simplest application of the LTC4071 is shown in Fig-
ure 2. This application requires only an external resistor
to program the charge/shunt current. Assume the wall
Figure 2. Single-Cell Battery Charger
4071 F02
LTC4071
BAT
R
IN
= 162Ω, 0.5W
GND
Li-Ion
BATTERY
V
CC
+
1µF
WALL
ADAPTER
LTC4071
11
4071fc
applicaTions inForMaTion
The GND pin of the top device is simply connected to
the V
CC
pin of the bottom device. Care must be taken in
observing the HBO status output pin of the top device as
this signal is no longer ground referenced. Likewise for
the control inputs of the top device; tie ADJ and LBSEL
of the top device to the local GND or V
CC
pins. Also, the
wall adapter must have a high enough voltage rating to
charge both cells.
NTC Protection
The LTC4071 measures battery temperature with a negative
temperature coefficient thermistor thermally coupled to the
battery. NTC thermistors have temperature characteristics
which are specified in resistance-temperature conversion
tables. Internal NTC circuitry protects the battery from
excessive heat by reducing the float voltage for each
10°C rise in temperature above 40°C (assuming a Vishay
thermistor with a B
25/85
value of 3490).
The LTC4071 uses a ratio of resistor values to measure
battery temperature. The LTC4071 contains an internal
fixed resistor voltage divider from NTCBIAS to GND with
four tap points; NTC
TH1
–NTC
TH4
. The voltages at these
tap points are periodically compared against the voltage at
the NTC pin to measure battery temperature. To conserve
power, the battery temperature is measured periodically
by biasing the NTCBIAS pin to V
CC
about once every 1.5
seconds.
The voltage at the NTC pin depends on the ratio of NTC
thermistor value, R
NTC
, and a bias resistor, R
NOM
. Choose
R
NOM
equal to the value of the thermistor at 25°C. R
NOM
is 10k for a Vishay NTHS0402N02N1002F thermistor with
a B
25/85
value of 3490. R
NOM
must be connected from
NTCBIAS to NTC. The ratio of the NTC pin voltage to the
NTCBIAS voltage when it is pulsed to V
CC
is:
R
NTC
R
NTC
+ R
NOM
( )
When the thermistor temperature rises, the resistance
drops; and the resistor divider between R
NOM
and the
thermistor lowers the voltage at the NTC pin.
An NTC thermistor with a different B
25/85
value may also
be used with the LTC4071. However the temperature trip
points are shifted due to the higher negative temperature
coefficient of the thermistor. To correct for this difference
add a resistor, R
FIX
, in series with the thermistor to shift
the ratio:
FIX
NTC
R
FIX
+ R
NTC
+ R
NOM
( )
Up to the internal resistive divider tap points: NTC
TH1
through NTC
TH4
. For a 100k thermistor with a B
25/85
value of 3950, e.g. NTHS0402N01N1003F, at 70°C (with
R
NOM
= 100k) choose R
FIX
= 3.92k. The temperature trip
points are found by looking up the curve 1 thermistor R/T
values plus R
FIX
that correspond to the ratios for NTC
TH1
= 36.5%, NTC
TH2
= 29%, NTC
TH3
= 22.8%, and NTC
TH4
= 17.8%. Selecting R
FIX
= 3.92k results in trip points of
39.9°C, 49.4°C, 59.2°C and 69.6°C.
Another technique may be used without adding an ad-
ditional component. Instead decrease R
NOM
to adjust the
NTC
TH
thresholds for a given R/T thermistor profile. For
example, if R
NOM
= 88.7k (with the same 100k thermis-
tor) then the temperature trip points are 41.0°C, 49.8°C,
58.5°C and 67.3°C.
When using the NTC features of the LTC4071 it is important
to keep in mind that the maximum shunt current increases
as the float voltage, V
FLOAT_EFF
drops with NTC conditioning.
Reviewing the single-cell battery charger application with
a 12V wall adapter in Figure 2; the input resistor should be
increased to 165Ω such that the maximum shunt current
does not exceed 50mA at the lowest possible float voltage
due to NTC conditioning, V
FLOAT_MIN
= 3.8V.
Thermal Considerations
At maximum shunt current, the LTC4071 may dissipate up
to 205mW. The thermal dissipation of the package should
be taken into account when operating at maximum shunt
current so as not to exceed the absolute maximum junc-
tion temperature of the device. With θ
JA
of 40°C/W, in the
MSOP package, at maximum shunt current of 50mA the
junction temperature rise is about 8°C above ambient.
With θ
JA
of 76°C/W in the DFN package, at maximum
shunt current of 50mA the junction temperature rise is
about 16°C above ambient. The junction temperature, T
J
,
is calculated depending on ambient temperature, T
A
, power
LTC4071
12
4071fc
applicaTions inForMaTion
dissipation, PD (in W), and θ
JA
is the thermal impedance
of the package (in °C/W):
T
J
= T
A
+ (PD × θ
JA
).
The application shown in Figure 4 illustrates how to prevent
triggering the low-battery disconnect function under large
pulsed loads due to the high ESR of thin-film batteries.
Figure 5. 4.2V AC Line Charging, UL Leakage Okay
4071 F05
LTC4071
AC 110V
ADJNTC
BAT
GND
Li-Ion
BATTERY
NTCBIASFLOAT
V
CC
R1
= 249k R2 = 249k
LBSEL
+
SYSTEM
LOAD
MB4S
– +
DANGER! HIGH VOLTAGE
R3 = 249k R4 = 249k
DANGEROUS AND LETHAL POTENTIALS ARE PRESENT IN AC
LINE-CONNECTED CIRCUITS! BEFORE PROCEEDING ANY
FURTHER, THE READER IS WARNED THAT CAUTION MUST BE
USED IN THE CONSTRUCTION, TESTING AND USE OF AC
LINE-CONNECTED CIRCUITS. EXTREME CAUTION MUST BE
USED IN WORKING WITH AND MAKING CONNECTIONS TO
THESE CIRCUITS. ALL TESTING PERFORMED ON AC
LINE-CONNECTED CIRCUITS MUST BE DONE WITH AN
ISOLATION TRANSFORMER CONNECTED BETWEEN THE AC LINE
AND THE CIRCUIT. USERS AND CONSTRUCTORS OF AC
LINE-CONNECTED CIRCUITS MUST OBSERVE THIS PRECAUTION
WHEN CONNECTING TEST EQUIPMENT TO THE CIRCUIT TO
AVOID ELECTRIC SHOCK.
age recovers, as the capacity of the battery should provide
roughly 50 hours of use for an equivalent 0.1%•20mA =
20µA load. To prevent load pulses from tripping the low
battery disconnect, add a decoupling capacitor from V
CC
to
GND. The size of this capacitor can be calculated based on
how much margin is required from the LBD threshold as
well as the amplitude and pulse width of the load transient.
For a 1.0mAh battery with a state-of-charge of 3.8V, the
margin from LBD is 600mV with LBSEL tied to GND. For
a square-wave load pulse of 20mA with a pulse width of
5ms, the minimum size of the decoupling cap required to
hold V
CC
above LBD is calculated as follows:
C
BYPASS
=
20mA • 5ms
600mV
= 166.6µF
Take care to select a bypass capacitor with low leakage.
The LTC4071 can be used to charge a battery to a 4.2V
float voltage from an AC line with a bridge rectifier as
shown in the simple schematic in Figure 5. In this example,
Figure 4. Adding a Decoupling Capacitor
for Large Load Transients
Table 2 lists some thin-film batteries, their capacities
and their equivalent series resistance. The ESR causes
V
BAT
and V
CC
to droop as a product of the load current
amplitude multiplied by the ESR. This droop may trigger
the low-battery disconnect while the battery itself may
still have ample capacity. Adding a bypass capacitor to
V
CC
prevents large low duty cycle load transients from
pulling down on V
CC
.
Table 2. Low Capacity Li-Ion and Thin-Film Batteries
VENDOR P/N CAPACITY RESISTANCE V
MIN
CYMBET CBC012 12µAh 5k to 10k 3.0V
CYMBET CBC050 50µAh 1500Ω to 3k 3.0V
GS NanoTech N/A 500µAh 40Ω 3.0V
APS-Autec LIR2025 20mAh 0.75Ω 3.0V
APS-Autec LIR1025 6mAh 30Ω 2.75V
IPS MEC225-1P 0.13mAh 210Ω to 260Ω 2.1V
IPS MEC220-4P 0.4mAh 100Ω to 120Ω 2.1V
IPS MEC201-10P 1.0mAh 34Ω to 45Ω 2.1V
IPS MEC202-25P 2.5mAh 15Ω to 20Ω 2.1V
GM Battery GMB031009 8mAh 10Ω to 20Ω 2.75V
For example, given a 0.1% duty cycle 5ms load pulse of
20mA and a 1.0mAh IPS MEC201-10P solid-state thin-film
battery with an equivalent series resistance of 35Ω, the
voltage drop at V
CC
can be as high as 0.7V while the load
is on. However once the load pulse ends, the battery volt-
LTC4071
BAT
NTCBIAS
NTC
LBSEL
ADJ
R
IN
V
IN
GND
Li-Ion
V
CC
+
C
BYPASS
4071 F04
FLOAT
10k
NTHS0402N02N1002F
T
PULSED
I
LOAD
SYSTEM LOAD
P1-P3P4-P6P7-P9P10-P12P13-P15P16-P18

LTC4071EMS8E#PBF

Mfr. #:
Buy LTC4071EMS8E#PBF
Manufacturer:
Analog Devices / Linear Technology
Description:
Battery Management Li-Ion/Polymer Shunt Battery Charger System with Low Battery Disconnect
Lifecycle:
New from this manufacturer.
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