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PDF LTC2942 Data sheet ( Hoja de datos )
| Número de pieza | LTC2942 | |
| Descripción | Battery Gas Gauge | |
| Fabricantes | Linear Technology | |
| Logotipo |  |
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Features
n Indicates Accumulated Battery Charge and
Discharge
n High Accuracy Analog Integration
n ADC Measures Battery Voltage and Temperature
n Integrated Temperature Sensor
n High Side Sense
n 1% Voltage and Charge Accuracy
n ±50mV Sense Voltage Range
n SMBus/I2C Interface
n Configurable Alert Output/Charge Complete Input
n 2.7V to 5.5V Operating Range
n Quiescent Current Less than 100µA
n Small 6-Pin 2mm × 3mm DFN package
Applications
n Low Power Handheld Products
n Cellular Phones
n MP3 Players
n Cameras
n GPS
LTC2942
Battery Gas Gauge
with Temperature,
Voltage Measurement
Description
The LTC®2942 measures battery charge state, battery
voltage and chip temperature in handheld PC and portable
product applications. Its operating range is perfectly suited
for single-cell Li-Ion batteries. A precision coulomb counter
integrates current through a sense resistor between the
battery’s positive terminal and the load or charger. Battery
voltage and on-chip temperature are measured with an
internal 14-bit No Latency ∆∑™ ADC. The three measured
quantities (charge, voltage and temperature) are stored in
internal registers accessible via the onboard SMBus/I2C
interface.
The LTC2942 features programmable high and low thresh-
olds for all three measured quantities. If a programmed
threshold is exceeded, the device communicates an alert
using either the SMBus alert protocol or by setting a flag
in the internal status register.
The LTC2942 requires only a single low value sense resis-
tor to set the measured current range.
L, LT, LTC, LTM, Linear Technology and the Linear logo are registered trademarks and
No Latency ∆Σ, ThinSOT and Bat-Track are trademarks of Linear Technology Corporation.
All other trademarks are the property of their respective owners.
Typical Application
CHARGER
I2C/SMBus
TO HOST
SENSE+
LTC2942
AL/CC
SDA SENSE–
SCL
GND
RSENSE
100mΩ
+ 1-CELL
Li-Ion
2942 TA01a
LOAD
0.1µF
Total Charge Error vs
Differential Sense Voltage
2.0
VSENSE+ = 3.6V
1.5
1.0
0.5
0
–0.5
–1.0
–1.5
–2.0
0.1
1 10
VSENSE (mV)
100
2942 TA01b
2942fa
1
1 page  
LTC2942
Typical Performance Characteristics
Total Charge Error vs Differential
Sense Voltage
3
Total Charge Error
vs Supply Voltage
1.00
Total Charge Error vs Temperature
1.00
2 0.75
0.50
1 0.25
00
0.75
0.50
0.25
0
–1
–2
–3
0.1
VSENSE+ = 2.7V
VSENSE+ = 4.2V
1 10
VSENSE (mV)
100
2942 G01
–0.25
–0.50
–0.75
–1.00
2.5 3.0
VSENSE = –50mV
VSENSE = –10mV
3.5 4.0 4.5 5.0 5.5 6.0
VSENSE+ (V)
2942 G02
–0.25
–0.50
–0.75
–1.00
–50 –25
VSENSE = –50mV
VSENSE = –10mV
0 25 50 75
TEMPERATURE (°C)
100
2942 G03
Supply Current vs Supply Voltage
100
TA = 25°C
90
TA = –40°C
TA = 85°C
80
70
60
50
40
2.5 3.0
3.5 4.0 4.5 5.0
VSENSE+ (V)
5.5 6.0
2942 G04
Shutdown Supply Current
vs Supply Voltage
2.0
1.5
1.0
0.5
TA = 25°C
TA = –40°C
TA = 85°C
0
2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0
VSENSE+ (V)
2942 G05
Voltage Measurement ADC
Total Unadjusted Error
10
8
6
4 TA = 85°C
2
0
–2 TA = –45°C
–4
–6 TA = 25°C
–8
–10
2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0
VSENSE– (V)
2942 G06
Voltage Measurement ADC
Integral Nonlinearity
1.0
TA = 85°C
0.5
Temperature Error vs Temperature
3
2
1
0
TA = –40°C
–0.5 TA = 25°C
0
–1
–2
–1.0
2.5 3.0
3.5 4.0 4.5 5.0
VSENSE– (V)
5.5 6.0
2942 G07
–3
–50 –25
0
25 50
TEMPERATURE (°C)
75 100
2942 G08
2942fa
5
5 Page 
LTC2942
Applications Information
Choosing Coulomb Counter Prescaler M B[5:3]
If the battery capacity (QBAT) is very small compared to
the maximum current (IMAX) (QBAT < IMAX • 0.1 Hours)
the prescaler value M should be changed from its default
value (128).
In these applications with a small battery but a high
maximum current, qLSB can get quite large with respect
to the battery capacity. For example, if the battery capacity
is 100mAh and the maximum current is 1A, the standard
equation leads to choosing a sense resistor value of
50mΩ, resulting in:
qLSB = 0.085mAh = 306mC
The battery capacity then corresponds to only 1176 qLSBs
and less than 2% of the accumulated charge register is
utilized.
To preserve digital resolution in this case, the LTC2942
includes a programmable prescaler. Lowering the pres-
caler factor M allows reducing qLSB to better match the
accumulated charge register to the capacity of the battery.
The prescaling factor M can be chosen between 1 and its
default value 128. The charge LSB then becomes:
qLSB
=
0.085mAh
•
50mΩ
RSENSE
•
M
128
To use as much of the range of the accumulated charge
register as possible the prescaler factor M should be
chosen for a given battery capacity QBAT and a sense
resistor RSENSE as:
M ≥ 128
•
216
QBAT
• 0.085mAh
•
RSENSE
50mΩ
M can be set to 1, 2, 4, 8, … 128 by programming B[5:3] of
the control register as M = 2(4•B[5]+2•B[4]+B[3]). The default
value after power up is M = 128 = 27 (B[5:3] = 111).
In the above example of a 100mAh battery and an RSENSE
of 50mΩ, the prescaler should be programmed to M = 4.
The qLSB then becomes 2.656µAh and the battery capacity
corresponds to roughly 37650 qLSBs.
Note that the internal digital resolution of the coulomb
counter is higher than indicated by qLSB. The digitized
charge qINTERNAL is M • 8 times smaller than qLSB. qINTERNAL
is typically 299µAs for a 50mΩ sense resistor.
ADC Mode B[7:6]
The LTC2942 features an ADC which measures either
voltage on SENSE– (battery voltage) or temperature via
an internal temperature sensor. The reference voltage and
clock for the ADC are generated internally.
The ADC has four different modes of operation, as shown
in Table 3. These modes are controlled by bits B[7:6] of
the control register. At power-up, bits B[7:6] are set to
[00] and the ADC is in sleep mode.
A single voltage conversion is initiated by setting the bits
B[7:6] to [10]. A single temperature conversion is started
by setting bits B[7:6] to [01]. After a single voltage or
temperature conversion, the ADC resets B[7:6] to [00]
and goes to sleep.
The LTC2942 also offers an automatic scan mode where
the ADC converts voltage, then temperature, then sleeps
for approximately two seconds before repeating the voltage
and temperature conversions. The LTC2942 is set to this
automatic mode by setting B[7:6] to [11] and stays in this
mode until B[7:6] are reprogrammed by the host.
Programming B[7:6] to [00] puts the ADC to sleep. If
control bits B[7:6] change within a conversion, the ADC
will complete the current conversion before entering the
newly selected mode.
A conversion of either voltage or temperature requires 10ms
conversion time (typical). At the end of each conversion,
the corresponding registers are updated. If the converted
quantity exceeds the values programmed in the threshold
registers, a flag is set in the status register and the AL/CC
pin is pulled low (if alert mode is enabled).
During a voltage conversion, the SENSE– pin is connected
through a small resistor to a sampling circuit with an
equivalent resistance of 2MΩ, leading to a mean input
current of I = VSENSE–/2MΩ.
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11
11 Page | ||
| Páginas | Total 18 Páginas | |
| PDF Descargar | [ Datasheet LTC2942.PDF ] | |
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