LTC4071
12
4071fb
applicaTions inForMaTion
dissipation, PD (in W), and θJA is the thermal impedance
of the package (in °C/W):
TJ = TA + (PD × θJA).
TheapplicationshowninFigure4illustrateshowtoprevent
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
ADJ
NTC
BAT
GND
Li-Ion
BATTERY
NTCBIAS
FLOAT
VCC
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.
of the battery should provide roughly 35 hours of use for
an equivalent 0.1%20mA = 20A load. To prevent load
pulses from tripping the low battery disconnect, add a
decoupling capacitor from VCC 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 0.7mAhr bat-
tery 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 VCC above LBD
is calculated as follows:
CBYPASS =
20mA 5ms
600mV
= 166.6F
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
showninthesimpleschematicinFigure5.Inthisexample,
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
VBAT and VCC 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
VCC prevents large low duty cycle load transients from
pulling down on VCC.
Table 2. Low Capacity Li-Ion and Thin-Film Batteries
VENDOR
P/N
CAPACITY
RESISTANCE
VMIN
CYMBET
CBC012
12Ah
5k to 10k
3.0V
CYMBET
CBC050
50Ah
1500Ω to 3k
3.0V
GS NanoTech
N/A
500Ah
40Ω
3.0V
APS-Autec
LIR2025
20mAh
0.75Ω
3.0V
APS-Autec
LIR1025
6mAh
30Ω
2.75V
IPS
MEC120
300Ah
150Ω to 180Ω
2.1V
IPS
MEC101
0.7mAh
50Ω to 60Ω
2.1V
IPS
MEC102
1.2mAh
22Ω to 24Ω
2.1V
GM Battery
GMB031009
8mAh
10Ω to 20Ω
2.75V
For example, given a 0.1% duty cycle 5ms load pulse of
20mAanda0.7mAhIPSthin-filmbatterywithanequivalent
series resistance of 50, the voltage drop at VCC can be
as high as 1V while the load is on. However once the load
pulse ends, the battery voltage recovers, as the capacity
LTC4071
BAT
NTCBIAS
NTC
LBSEL
ADJ
RIN
VIN
GND
Li-Ion
VCC
+
CBYPASS
4071 F04
FLOAT
10k
NTHS0402N02N1002F
T
PULSED
ILOAD
SYSTEM LOAD