TS1108 Data Sheet
TS1108 Coulomb Counter: Bidirectional Current Sense Amplifier
with Integrator + Comparator
The TS1108 coulomb counter accurately measures battery depletion while also indicat-
ing the battery charging polarity. The battery discharge current is monitored by a current-
sense amplifier through an external sense resistor. Utilizing an Integrator and a Compa-
rator plus a Monoshot, the TS1108 voltage-to-frequency converter provides a series of
90 µs output pulses at COUT which represents an accumulation of coulombs flowing out
of the battery. The charge count frequency is adjustable by the integration resistor and
capacitor.
Applications
• Power Management Systems
• Portable/Battery-Powered Systems
• Smart Chargers
KEY FEATURES
• Coulomb Counting plus Charge Polarity
• Adjustable Charge Count Frequency
• External Crystal Oscillator Not Required
• Low Supply Current
• Current Sense Amplifier: 0.68 µA
• I
VDD
: 1.93 µA
• High Side Bidirectional Current Sense
Amplifier
• Wide CSA Input Common Mode Range: +2
V to +27 V
• Low CSA Input Offset Voltage: 150
µV(max)
• Low Gain Error: 1%(max)
• Two Gain Options Available:
• Gain = 20 V/V : TS1108-20
• Gain = 200 V/V : TS1108-200
• 16-Pin TQFN Packaging (3 mm x 3 mm)
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TS1108 Data Sheet
Ordering Information
1. Ordering Information
Table 1.1. Ordering Part Numbers
Ordering Part Number
TS1108-20IQT163
TS1108-200 IQT1633
Description
Coulomb counter: Bidirectional current sense amplifier with integrator and comparator
Coulomb counter: Bidirectional current sense amplifier with integrator and comparator
Gain V/V
20
200
Note:
Adding the suffix “T” to the part number (e.g. TS1108-200IQT1633T) denotes tape and reel.
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TS1108 Data Sheet
System Overview
2. System Overview
2.1 Functional Block Diagram
Figure 2.1. TS1108 Coulomb Counter Block Diagram
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TS1108 Data Sheet
System Overview
2.2 Current Sense Amplifier + Output Buffer
The internal configuration of the TS1108 bidirectional current-sense amplifier is a variation of the TS1101 bidirectional current-sense
amplifier. The TS1108 current-sense amplifier is configured for fully differential input/output operation.
Referring to the block diagram, the inputs of the TS1108’s differential input/output amplifier are connected to RS+ and RS– across an
external R
SENSE
resistor that is used to measure current. At the non-inverting input of the current-sense amplifier, the applied voltage
difference in voltage between RS+ and RS– is I
LOAD
x R
SENSE
. Since the RS– terminal is the non-inverting input of the internal op-amp,
the current-sense op-amp action drives PMOS[1/2] to drive current across R
GAIN[A/B]
to equalize voltage at its inputs.
Thus, since the M1 PMOS source is connected to the inverting input of the internal op-amp and since the voltage drop across R
GAINA
is
the same as the external V
SENSE
, the M1 PMOS drain-source current is equal to:
I
DS
(M 1)
=
V
SENSE
R
GAINA
I
LOAD
×
R
SENSE
R
GAINA
I
DS
(M 1)
=
The drain terminal of the M1 PMOS is connected to the transimpedance amplifier’s gain resistor, ROUT, via the inverting terminal. The
non-inverting terminal of the transimpedance amplifier is internally connected to VBIAS, therefore the output voltage of the TS1108 at
the OUT terminal is:
V
OUT
=
V
BIAS
−
I
LOAD
×
R
SENSE
×
R
GAINA
When the voltage at the RS– terminal is greater than the voltage at the RS+ terminal, the external VSENSE voltage drop is impressed
upon R
GAINB
. The voltage drop across R
GAINB
is then converted into a current by the M2 PMOS. The M2 PMOS’ drain-source current
is the input current for the NMOS current mirror which is matched with a 1-to-1 ratio. The transimpedance amplifier sources the M2
PMOS drain-source current for the NMOS current mirror. Therefore the output voltage of the TS1108 at the OUT terminal is:
V
OUT
=
V
BIAS
+
I
LOAD
×
R
SENSE
×
R
GAINB
When M1 is conducting current (V
RS+
> V
RS–
), the TS1108’s internal amplifier holds M2 OFF. When M2 is conducting current (V
RS–
>
V
RS+
), the internal amplifier holds M1 OFF. In either case, the disabled PMOS does not contribute to the resultant output voltage.
The current-sense amplifier’s gain accuracy is therefore the ratio match of R
OUT
to R
GAIN[A/B]
. For each of the two gain options availa-
ble, The following table lists the values for R
GAIN[A/B]
.
Table 2.1. Internal Gain Setting Resistors (Typical Values)
GAIN (V/V)
20
200
R
GAIN[A/B]
(Ω)
2k
200
R
OUT
(Ω)
40 k
40 k
Part Number
TS1108-20
TS1108-200
R
OUT
R
OUT
The TS1108 allows access to the inverting terminal of the transimpedance amplifier by the FILT pin, whereby a series RC filter may be
connected to reduce noise at the OUT terminal. The recommended RC filter is 4 kΩ and 0.47 µF connected in series from FILT to GND
to suppress the noise. Any capacitance at the OUT terminal should be minimized for stable operation of the buffer.
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System Overview
2.3 Sign Output
The TS1108 SIGN output indicates the load current’s direction. The SIGN output is a logic HIGH when M1 is conducting current (V
RS+
> V
RS–
). Alternatively, the SIGN output is a logic LOW when M2 is conducting current (V
RS–
> V
RS+
). The SIGN comparator’s transfer
characteristic is illustrated in Figure 1. Unlike other current-sense amplifiers that implement an OUT/SIGN arrangement, the TS1108
exhibits no “dead zone” at I
LOAD
switchover.
Figure 2.2. TS1108 Sign Output Transfer Characteristic
2.4 Integrator + Comparator
The TS1108 Coulomb Counter function utilizes an Integrator and a Comparator plus a 90 µs Monoshot. The CSA’s buffered output is
applied to the integrator’s input. This signal is integrated by the comparator until it reaches a level that trips the comparator. The compa-
rator’s trip level is determined by the voltage applied to the comparator’s non-inverting terminal, CIN+. The Monoshot produces a 90 µs
output pulse at COUT and the integrator is reset. Therefore, each COUT 90 µs pulse represents an accumulation of coulombs (Please
refer to the equations in
2.6 Coulomb Counter).
The TS1108 Integrator works best when the 90 μs Monoshot represents less than 2%
of the total integration period. Therefore, the minimum integration time for a full-scale V
SENSE
should be limited to 4.7 ms. To guarantee
stable operation of the OUT buffer, an integration capacitance of 0.1 µF should be used for integration capacitor, C
INT
. The maximum
integration period can be very long, limited by the leakage current and offset.
A reset switch is configured internally to discharge the external integration capacitor, C
INT
. To enable the Coulomb Counting feature,
SW_RST should be tied to either GND or COUT, allowing the 90 µs Monoshot Pulse to control the discharge of C
INT
. To close the reset
switch and short out C
INT
, SW_RST may be tied high.
TS1108’s Coulomb Counting interrupt is provided by the internal comparator with a push-pull output configuration. As shown in the
block diagram, the integrator’s output is applied internally to the non-inverting terminal of the comparator, CIN+. Therefore the compara-
tor’s output will latch high for 90 µs once the integrator’s output is charged to the voltage supplied to the comparator’s inverting terminal,
CIN–. The inverting terminal of the comparator, CIN–, must be at a higher potential than the voltage supplied to VBIAS for proper oper-
ation. The capacitive load at COUT should be minimized for minimal output delays.
2.5 VREF Divider
The TS1108 provides an internal voltage divider network to set VBIAS and CIN–, eliminating the need for externally setting the required
voltages. The VREF Divider is activated once the voltage applied to VREF is 0.9 V or greater. The VREF divider connects to VBIAS
and CIN–, where the VBIAS voltage is equal to 50% of VREF while the CIN– voltage is equal to 90% of VREF . The VREF Divider
exhibits a typical total series resistance of 4.6 MΩ from VREF to GND when activated.
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