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+)
-
Low-pass filtered by R72 and C29 (1µF, 100V)
-
IN-_INA228: Low side of shunt resistor (battery ground/negative)
- Connected through R73 (100Ω) to INA228 pin 9 (IN-)
- Low-pass filtered by R73 and C41 (1µF, 100V)
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¶
RC Low-Pass Filters¶
All three measurement inputs feature identical RC low-pass filtering:
- VBUS Filter: R79 (100Ω) + C8 (1µF)
- IN+ Filter: R72 (100Ω) + C29 (1µF)
- IN- Filter: R73 (100Ω) + C41 (1µF)
Cutoff Frequency Calculation:
fc = 1 / (2π × R × C)
fc = 1 / (2π × 100Ω × 1µF)
fc ≈ 1.59 kHz
All filters can be bypassed with Jumpers, though in some future iteration we may want to change where the jumpers are located to keep some level of protection.
Differential Noise Reduction¶
- C25: 100nF, 100V capacitor connected directly between IN+ and IN-
- Reduces common-mode noise on differential measurement
- Higher frequency filtering compared to individual RC filters
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¶
- Input Filter Bandwidth: ~1.59kHz (-3dB point)
- Differential Noise Filtering: Enhanced by 100nF direct coupling
- Common-mode Rejection: Enhanced by matched RC filters on all inputs
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 frequency suitable for battery monitoring application