The BQ26501 is the first product in the bqJUNIOR series for manual advanced gas manometer device applications and is a highly accurate self-contained single-cell Li-Ion and Li-Polymer battery capacity, targeting space monitoring and reporting equipment for limited portable applications The device monitors the voltage drop induced by small currents. A resistor is connected in series with the battery to determine the charging and discharging activity of the battery. Compensation for battery temperature, self-discharge and discharge rate provides usable capacity to the charging counter for a variety of operating conditions. The battery capacity is automatically recalibrated, or learned, emptied during a full discharge cycle. Internal registers include capacity, battery temperature and voltage, state of charge, status and control registers.

The BQ26501 can run Li-Ion and Li-polymer batteries and communication systems directly from a single cell through a simple one-wire bidirectional serial interface. 5 kbits/s HDQ bus interface reduces communication overhead to external microcontrollers.

feature

Reports accurate Li-Ion state of charge and Li-Pol batteries, no system processor required Gauge communicates directly with the integrated HDQ engine in the TI OMAP processor, reports battery temperature and voltage with high accuracy Coulomb charge and discharge current with automatic offset cancellation , No need for offset calibration, programmable input/output port internal time base to eliminate external time, four automatic low-power operation modes of crystal oscillator.

Bulge Active: < 100 A A.

Bulge sleep: <2.5 A A.

Bulge Ship: <1.7 AA

Bulwark dormancy: <1.5 A A.

Small 8-Pin TSSOP Package

application

Pocket PC

smart phone

MP3 player

digital camera

Internet equipment

Handheld devices

Simplified Application Diagram

HDQ Bit Timing Diagram

Typical Application Circuit Diagram

Functional block diagram

Function Description

The BQ26501 determines the battery capacity or Li-Pol battery by monitoring Li-ion input or the amount of charge removed from the Li-ion battery. The BQ26501 measures discharge and charge current, monitors the battery's low voltage threshold, and compensates for temperature and self-discharge rate. The current measurement is small by setting a series resistance between the negative of the battery and the ground of the battery pack (see RS in Figure 2). Reports available capacity with a resolution of 0.003/RS (mAh). Idle Time (TTE) reporting in minutes at the host-supplied rate current allows the requirement for host-based computations to be greatly reduced or eliminated; reading a single register pair can provide useful and meaningful information to the application user.

Figure 2 shows a typical application circuit. Voltage differential sensing across the current sense resistor, RS, improves device performance, thereby improving reported idle time accuracy. In typical applications, GPIO pins can be used as general-purpose programmable I/O ports. Internal pull-down of the HDQ line ensures that the device detects a logic '0' on the HDQ line and automatically enters a low-power state in sleep mode when the system is powered off or the battery pack is removed. A 100kΩ pull-up to VCC can be added to the HDQ line to disable this feature. The BQ26501 can directly use a single Li-ion battery or a Li-ion battery cell.

Measurement

As shown in Figure 3, the BQ26501 employs a fully differential, dynamically balanced voltage-to-frequency conversion (VFC) for charge and discharge counts and an analog-to-digital converter (ADC) for battery voltage and temperature measurement. Both the VFC and ADC will automatically compensate for the offset. No user calibration or compensation is required. Charge and discharge counting The BQ26501 uses a voltage-to-frequency converter (VFC) to perform continuous integration of the voltage waveform across a small value sense resistor, on the negative lead of the battery, as shown in Figure 2. The voltage integration across the sense resistor is derived from the battery charge added or removed. Since the VFC integrates the waveform directly, the shape of the current waveform through the sense resistor has no effect on the measurement accuracy. The low-pass filter feeds the sense resistor voltage into the BQ26501SRP and SRN inputs to filter out system noise, so it does not affect the measurement accuracy. The low-pass filter does not change the integral value of the waveform.

offset calibration

The offset voltage for VFC measurements must be very low to be able to measure small signal levels accurately. The BQ26501 provides automatic compensation to eliminate internal voltage offset errors for maximum charge measurement accuracy through SRP and SRN.

NOTE: No calibration required. For more information, see the Layout Considerations section Minimizing PCB-Induced SRP and SRN Pin Skew.

digital amplitude filter

A Digital Magnitude Filter (DMF) threshold can be set in the EEPROM to indicate that the threshold below which charge or discharge accumulations are ignored. This allows the threshold to be set higher than the maximum VFC offset expected from the device and PCB combination. This ensures that the measured capacity change of the BQ26501 is now zero when there is no charge or discharge current. Note that even small PCB offsets can increase large errors over a long period of time. In addition to setting the threshold higher than the expected maximum offset (DMF) should be set below the minimum signal level to be measured. Since the measurement signal can only be measured as accurately as the PCB-induced VFC offset, the sense resistor value should be large enough to allow for a minimum current level that provides a signal level well above the maximum offset voltage. Instead, the sense resistor must be small enough to meet the system's insertion loss requirements to keep the maximum voltage across the sense resistor below the ±100-mV accurate measurement that the VFC can achieve. DMF thresholds are programmed in EEPROM in 6µV increments. Programming zero in the DMF value disables the DMF function and does not ignore any VFC counts.

Voltage

The BQ26501 monitors the battery voltage through the BAT pin and reports the offset correction value in an internal register. The BQ26501 also monitors the voltage at the end-of-discharge voltage (EDV) threshold. The EDV threshold level is used to determine when the battery reaches an empty state.

temperature

The BQ26501 uses an integrated temperature sensor to monitor the battery pack temperature. Air temperature measurements reported through internal registers are used to adjust charge and discharge rate compensation and self-discharge capacity loss estimates.

RBI input

The register backup input pin, RBI, is used with an external capacitor to provide a backup potential for the internal register when VCC falls below the power-on reset voltage V(POR). When VCC is higher than VCC, VCC outputs V(POR) on RBI to charge the capacitor. Figure 2 shows an optional 1MΩ resistor from the RBI pin to VCC. This allows the device to retain RAM register data when the battery voltage is below V(POR) and above 1.3 V. The BQ26501 checks for RAM corruption by storing redundant copies of the high bytes of NAC and LMD. After reset, the BQ26501 compares the redundant NAC and LMD values and verifies the accuracy of the 2 check byte values. If the redundant copies match and the check character is correct, NAC and LMD are preserved, and the CI bits are preserved in FLAGS. If these checks are incorrect, the NAC is cleared, the LMD is initialized from the EEPROM, and the CI bit in FLAGS is set to '1'. All other RAMs are initialized on all resets.

Layout Considerations

The automatic compensation VFC method effectively cancels the internal offset voltage inside the bq26501, but any external offset caused by the PCB layout is not canceled. This makes it crucial to pay special attention to PCB layout. For best performance, the decoupling capacitors from VCC to VSS and the filter capacitors from SRP and SRN to VSS should be as close as possible to the BQ26501, both have brief tracking runs

signal and VSS pins. All low current VSS connections should be separated from the high current discharge path from the battery should be connected to the high current trace at a point next to the sense resistor. This should be a trace connection to the edge or inside of the sense resistor connection so that no part of the VSS interconnect carries any load current and no part of the high current PCB trace is included in the rms sense resistor (i.e. Kelvin connection).

Application infographic

The BQ26501 measures the capacity of the battery under actual usage conditions and updates the Last Measured Battery Discharge (LMD) register with the latest measured value. The BQ26501 retains the learned LMD value unless a full reset occurs. The bq26501 learns the capacity of the battery by measuring the threshold at which the battery goes from fully discharged to EDV1 without any out-of-spec events. The condition during normal use is that the BQ26501 can only learn a new capacity after it is fully discharged. There are several exceptions to disqualification for a study cycle (see the list at the end of this section). If a significant reduction in learning capacity occurred during the learning cycle, the new LMD value was limited to any single learning discharge of LMD/8 during the maximum LMD reduction. The capacity inaccuracy (CI) bit in FLAGS is cleared after the first learning cycle. This bit remains clear unless a full reset occurs. Battery full state is defined as Nominal Available Capacity (NAC) = LMD. Valid Discharge Flag (VDQ) When this occurs, the FLAGS register in the register is set and remains set until the learning discharge cycle is complete or an event that does not fit the learning cycle occurs.

When the battery is discharged to the state of VOLT≤EDV1, the discharge cycle completion threshold is learned. The EDV1 threshold should be set to a voltage below that threshold that guarantees at least 6.25% of the battery capacity. The EDVF threshold should be set to the battery voltage that the system considers zero capacity.

The BQ26501 does not know the capacity between the EDV1 and EDVF thresholds, but assumes that the capacity is 6.25% of the LMD, so care should be taken to set EDV1 according to the characteristics of the battery. The measured LMD value is determined by measuring the battery output capacity from NAC = LMD to the battery

VOLT ≤ EDV1, plus LMD/16, represents 6.25% capacity still below the EDV1 threshold.