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¶
- D20, D21: PESD5V0S1UL bidirectional ESD protection diodes
- Connected between IN+ and IN- inputs
- Protects against differential transients
- 5V standoff voltage will have no affect on mV-level shunt signals
Overvoltage Protection¶
- D15: P6SMB68A transient voltage suppressor
- Connected from VBUS to GND
- Standoff Voltage: 58.1V (~suitable for 59V max battery voltage)
- Breakdown Voltage: 64.6V minimum
- Clamping Voltage: 92V maximum @ Ipp
- Protects against battery overvoltage events
Noise Filtering¶
Input Series Resistance¶
Both differential measurement inputs feature matched 100Ω series resistors:
- IN+ Series Resistance: R72 (100Ω, 0.1% tolerance for matching)
- IN- Series Resistance: R73 (100Ω, 0.1% tolerance for matching)
These resistors provide: - Input protection against transients - Symmetric impedance for optimal common-mode rejection - EMI filtering when combined with differential capacitance
VBUS Filtering¶
- VBUS Filter: R79 (100Ω) + C8 (1µF)
- Cutoff Frequency: fc ≈ 1.59 kHz
Differential Noise Reduction¶
- C25: 100nF, 100V capacitor connected directly between IN+ and IN-
- Provides high-frequency noise filtering for differential measurement
- Critical Design Note: No capacitors are connected from IN+ or IN- to ground, which prevents common-mode to differential-mode noise conversion through asymmetric coupling
Differential Filter Characteristics:
fc = 1 / (2π × R × C)
fc = 1 / (2π × 100Ω × 100nF)
fc ≈ 15.9 kHz
All filters can be bypassed with jumpers for testing or different filtering requirements.
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: ~15.9kHz (-3dB point)
- 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 (58.1V) > max battery voltage (59V)
- ✅ All capacitor voltage ratings (100V) > max system voltage (59V)
- ✅ INA228 VBUS rating (85V) > max battery voltage (59V)
- ✅ Differential signal range within INA228 limits
- ✅ ESD protection appropriate for expected signal levels
- ✅ Filter cutoff frequencies suitable for battery monitoring application
- ✅ Symmetric input impedance for optimal common-mode rejection
- ✅ No ground-referenced capacitors on differential inputs to prevent CM→DM conversion