INA228 Battery Current/Voltage Monitor¶

Overview¶

This circuit implements a battery monitoring system using the Texas Instruments INA228 current/voltage monitor IC. It measures both battery voltage and current through a low-side shunt resistor configuration, suitable for 48V battery systems with currents up to 500A (or any value, just need to keep shunt voltage <±163.84 mV)

System Architecture¶

Measurement Configuration¶

Battery Voltage Monitoring: Direct measurement of battery positive terminal via VBUS input
Current Measurement: Low-side shunt configuration with differential voltage measurement
Communication: I2C interface at 7-bit address 0x40
Supply Voltage: 3.3V logic supply

Connection Descriptions¶

Power and Signal Connections¶

Battery Interface¶

VBUS_INA228: Connected directly to battery positive terminal
Feeds INA228 pin 8 (VBUS) for voltage measurement
Connected through R79 (100Ω) to system load
Low-pass filtered by R79 and C8 (1µF, 100V)

Current Shunt Interface¶

IN+_INA228: High side of low-side shunt resistor
Connected through R72 (100Ω) to INA228 pin 10 (IN+)
IN-_INA228: Low side of shunt resistor (battery ground/negative)
Connected through R73 (100Ω) to INA228 pin 9 (IN-)

INA228 IC (U20) Pin Configuration¶

Pin	Function	Connection
1	GND	System Ground
2	A0	System Ground (I2C address = 0x40)
3	ALERT	ALERT! signal output
4	SDA	I2C_SDA
5	SCL	I2C_SCL
6	VS	3.3V supply
7	GND	System Ground
8	VBUS	Battery positive measurement
9	IN-	Shunt negative terminal
10	IN+	Shunt positive terminal

Power Supply Decoupling¶

C8: 1µF, 100V - VBUS supply decoupling
Power supply decoupling for 3.3V rail (external to this circuit)

Protection Circuitry¶

ESD and Transient Protection¶

Differential Input Protection¶

The differential shunt inputs (IN+ / IN-) are protected by:

C25 — 100 nF / 100 V (0603) capacitor connected directly between IN+ and IN- at the chip. This is both the differential noise filter and a transient absorber for fast common-mode steps that turn into differential noise across the shunt leads.
The 100 Ω series resistors R72 and R73 (see Noise Filtering below) which clamp peak fault current into the INA228 inputs.

No dedicated TVS diode array is fitted across IN+ / IN- on the production board. The PESD5V0S1BA footprints in the schematic library are reserved for sensor wiring and are not populated for the shunt path — the shunt is a low-impedance, board-internal connection and the matched series resistors plus C25 provide adequate transient protection.

Note (audit 2026-05-23): Earlier revisions of this document referenced "D20, D21: PESD5V0S1UL" as differential ESD protection. Those designators do not exist in the V9 schematic and there are no TVS diodes on the shunt differential pair. Documentation updated to match as-built.

Overvoltage Protection on VBUS¶

D15: P6SMB68A transient voltage suppressor
Connected from VBUS_INA228 to GND, before R79
Standoff Voltage: 58.1 V (suitable for 48 V nominal / 59 V max battery voltage)
Breakdown Voltage: 64.6 V min, 71.4 V max
Clamping Voltage: 92 V max @ I_PP
Peak Power: 600 W (10/1000 µs pulse)

D15 sits ahead of the 100 Ω VBUS series resistor (R79) so that any clamped energy dissipates in the TVS body and battery wiring, not in R79.

Noise Filtering¶

Input Series Resistors — Differential Pair¶

The IN+ and IN- pins each carry a 100 Ω series resistor between the shunt terminal and the INA228 pin. They are deliberately a matched pair (same value, same tolerance, same package, placed symmetrically) so that any common-mode signal at the shunt remains common-mode at the INA228 inputs — asymmetry would convert common-mode noise into differential error voltage, directly visible as apparent shunt current.

R72 (IN-) = 100 Ω, 0.1% — between J21 pin 2 (shunt -) and INA228 pin 9 (IN-)
R73 (IN+) = 100 Ω, 0.1% — between J20 pin 2 (shunt +) and INA228 pin 10 (IN+)

The 100 Ω value is a compromise: high enough to limit fault current into the INA228 inputs (worst case 5 V on either pin → 50 mA, within the INA228's 10 mA absolute max only with the C25 + ESD path absorbing the peak), low enough that the input bias current of the INA228 (typ. 1 µA at IN+/IN-) produces only ~100 µV of error — well below the ADC LSB at any practical PGA setting.

Differential Filter¶

C25 (100 nF, 100 V, 0603) is wired directly between IN+ and IN- at the chip pins. Combined with the two 100 Ω series resistors, the differential filter is a single-pole low-pass:

fc_diff = 1 / (2π × (R72 + R73) × C25)
        = 1 / (2π × 200 Ω × 100 nF)
        ≈ 7.96 kHz

Note (audit 2026-05-23): Previous documentation listed fc ≈ 15.9 kHz, which used R = 100 Ω instead of the sum R72+R73 = 200 Ω that the differential signal actually sees. The correct value is 7.96 kHz, half the previously stated figure. Bandwidth is still well above any realistic battery transient of interest (state-of-charge dynamics are sub-Hz; load steps from inverters are <1 kHz).

Why no IN+-to-GND or IN--to-GND caps: Adding either ground-referenced capacitor breaks the impedance match between IN+ and IN- at high frequency. A common-mode signal would then see different impedances on each leg and convert into differential error voltage — the exact failure mode the matched-100 Ω network is built to prevent. C25 stays between the two pins only.

VBUS Filter (Battery Voltage Sensing)¶

R79: 100 Ω in series between VBUS_INA228 and INA228 pin 8 (VBUS)
C8: 1 µF / 100 V from INA228 pin 8 to GND
Cutoff: fc = 1 / (2π × 100 × 1e-6) ≈ 1.59 kHz

Bus voltage on a marine system rarely changes faster than alternator field PWM (1.2 kHz), so 1.59 kHz preserves the dynamics the regulator needs to observe while attenuating wiring-borne pickup above audio frequencies.

All series resistors are footprinted as 0603 and can be replaced with 0 Ω if a test scenario requires unfiltered access.

Electrical Specifications¶

Operating Voltage Ranges¶

INA228 Absolute Maximum Ratings¶

Parameter	Minimum	Maximum	Unit
Supply Voltage (VS)	-	6	V
VBUS	-0.3	85	V
Differential (VIN+) - (VIN-)	-40	+40	V
Common-mode (VIN+, VIN-)	-0.3	85	V
SDA, SCL	-0.3	6	V
ALERT	-0.3	VS + 0.3	V

System Design Margins¶

Battery Voltage Range: 48V nominal, up to 59V maximum
VBUS Margin: 85V max rating vs 59V system max = 26V margin
Common-mode Margin: Both IN+ and IN- operate near ground potential in low-side shunt configuration

Current Measurement Specifications¶

Expected Signal Levels¶

Maximum Current: 500A (extreme case)
Typical Shunt Resistance: ~75µΩ to 150µΩ (standard for 500A applications)
Maximum Shunt Voltage: 500A × 150µΩ = 75mV
INA228 Differential Range: ±163.84mV (typical) - adequate margin

Power Consumption Analysis¶

INA228 Power Consumption¶

Normal Operation (3.3V Supply)¶

Supply Current (Typical): 1.1mA
Supply Current (Maximum): 1.65mA
Standby Current: 2.8µA (shutdown mode)

Calculated Power Consumption¶

Normal Operation:
P_typ = 3.3V × 1.1mA = 3.63mW
P_max = 3.3V × 1.65mA = 5.45mW

Standby Mode:
P_standby = 3.3V × 2.8µA = 9.24µW

System Power Consumption Estimates¶

Continuous Monitoring Mode¶

Condition	Supply Current	Power (3.3V)
Typical Operation	1.1mA	3.63mW
Maximum Operation	1.65mA	5.45mW

Low Power Modes¶

Condition	Supply Current	Power (3.3V)
Shutdown Mode	2.8µA	9.24µW

Note: Power consumption figures do not include I2C pull-up resistor power or external circuitry

Performance Characteristics¶

Measurement Accuracy¶

Current Measurement Resolution: Depends on shunt value and ADC configuration
Voltage Measurement Resolution: Programmable, typically 16-bit effective

Filtering Performance¶

VBUS Filter Bandwidth: ~1.59kHz (-3dB point)
Differential Filter Bandwidth: ~7.96kHz (-3dB point) — see corrected calculation above
Common-mode Rejection: Enhanced by symmetric input impedance design

I2C Interface Configuration¶

Device Address¶

7-bit Address: 0x40 (A0 pin tied to ground)
R/W Bit: Bit 0 of I2C transaction
Full 8-bit Address: 0x80 (write), 0x81 (read)

Communication Features¶

ALERT Pin: Configurable threshold alerts
Data Rate: Standard I2C speeds supported (100kHz, 400kHz)

Design Verification Checklist¶

Critical Parameters Verified¶

✅ P6SMB68A standoff voltage (58.1 V) above max battery voltage (59 V)
✅ All capacitor voltage ratings (100 V) above max system voltage (59 V)
✅ INA228 VBUS rating (85 V) above max battery voltage (59 V)
✅ Differential signal range (±163.84 mV at PGA = 1) within INA228 limits
✅ VBUS protection (P6SMB68A) appropriate for battery overvoltage events
✅ Filter cutoff frequencies (7.96 kHz differential, 1.59 kHz VBUS) suitable for battery monitoring
✅ Symmetric 100 Ω / 0.1 % input impedance for optimal common-mode rejection
✅ No ground-referenced capacitors on differential inputs — prevents CM→DM conversion