AN3395 Application note Sensing resistor selection and usage in STC310x battery monitoring applications Introduction Voltage measurement and coulomb counting are the two most common methods used to implement battery monitoring for gas gauge applications. Although the use of voltage measurement has been a popular method, it does not produce the most accurate results. The STC310x series battery monitor ICs developed by STMicroelectronics combine the two methods into one integrated solution. It updates the battery State-of-Charge (SOC) at light load (relaxation/standby period) with the real battery Open-Circuit-Voltage (OCV) while using coulomb counting to track the battery capacity under heavy load to provide the most accurate SOC value under all application conditions. In coulomb counting, the sensing resistor is used to measure the battery current. The specified maximum voltage drop on the sensing resistor is only 80 mV, thus it plays an important role in the gas gauge accuracy and merits careful attention. This document describes: ■ the sensing resistor (Rcg) selection ■ the Rcg power considerations ■ the Rcg layout recommendations December 2011 Doc ID 018779 Rev 1 1/11 www.st.com Contents AN3395 Contents 1 STC310x external components . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 2 Rcg resistance selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 2.1 Maximum peak current in the application . . . . . . . . . . . . . . . . . . . . . . . . . . 4 2.2 Power rating of the resistor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 2.3 ADC code usage efficiency . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 2.4 Selection of Rcg (example) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 3 Rcg power loss consideration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 4 Rcg layout considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 5 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 6 Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 2/11 Doc ID 018779 Rev 1 AN3395 1 STC310x external components STC310x external components Figure 1 illustrates the typical connections for a gas gauge application using the STC3105. The SDA, SCL and ALM (I/O0 in the STC3100) pins are open drain and require external pull-up resistors to either system I/O voltage or VCC (pull up to battery voltage). The components shown in Figure 1 connected to the VCC and VIN pins are used to provide additional ESD protection and input filtering, please refer to AN3064 for more information. The resistor (Rcg) connected between the CG and GND pins is the sensing resistor. In order to obtain higher accuracy, refer to the following application guidelines. Figure 1. STC3105 typical connections System I/O supply Rpu1 Rpu2 Rpu3 System supply STC3105 C1 D1 ALM VCC SDA VIN R1 R2 MCU Battery C2 SCL CG Rcg GND Gnd AM045291v1 Doc ID 018779 Rev 1 3/11 Rcg resistance selection 2 AN3395 Rcg resistance selection The Rcg resistor is used to sense the current flowing "into" or "out of" the battery. The voltage drop on Rcg is input to the current measurement ADC through the CG pin. There are three common rules for the selection of the Rcg resistance: 2.1 1. Maximum peak current 2. Power rating 3. ADC code usage Maximum peak current in the application As specified in the datasheet (refer to the STC3105 or STC3100 datasheet), the voltage drop across the Rcg resistor (input voltage range on CG pin) must not exceed ±80 mV. That is Rcg x IPEAK must be ≤ 80 mV. This gives a maximum limit for the Rcg resistor value in Equation 1. Equation 1 80 ( mv ) Rcg ( mΩ ) ≤ ----------------------I PEAK ( A ) 2.2 Power rating of the resistor The second step is to consider the power dissipation limit of the resistor as given in Equation 2. The power dissipation in the resistor must be kept within the power rating of the resistor calculated by: Equation 2 Power dissipation = Rcg x IRMS2 Note: Must be less than the power rating of the resistor However, it may be better to choose a smaller resistance value with a smaller power rating to: 2.3 ● have a smaller PCB footprint and ● reduce the power loss in the resistor. ADC code usage efficiency The full scale voltage range of the ADC is designed for the input on the CG pin to reach ±80 mV (max). To make better use of the ADC performance, Rcg must not be too small: Rcg x IPEAK must be > 40 mV for a reasonable ADC code usage. 4/11 Doc ID 018779 Rev 1 AN3395 2.4 Rcg resistance selection Selection of Rcg (example) Assume IPEAK = 2.2 A and IRMS = 1.5 A in a mobile phone. According to Equation 1, the maximum limit of Rcg is obtained, that is Rcg < 36 mΩ. Let's choose a 33 mΩ resistor. Power rating = 33 mΩ x 1.52 A = 74 mW Therefore, a 1/8 W (125 mW) resistor is sufficient, however, it is possible to use a 20 mΩ resistor that will only dissipate 45 mW instead of 74 mW. A 20 mΩ resistor is optimal because 20 mΩ x 2.2 A = 44 mV, which is acceptable in comparison with the full scale range of 80 mV. Doc ID 018779 Rev 1 5/11 Rcg power loss consideration 3 AN3395 Rcg power loss consideration The power loss of the Rcg should be considered. Let's look at the worst case when the mobile phone is drawing current during continuous talk time. Assuming a battery with capacity of 5.55 Wh (1500 mAh) and a 300 mA average current consumption (5 hours talk time), and assuming typical GSM load current waveform below in Figure 2: Figure 2. Typical GSM load profile Current (mA) ~2 A ~4.1 ms Avg ~70 mA 0.5 ms Time (ms) AM045292v1 For the power loss calculation we must consider RMS current, not average current: Equation 3 2 I RMS = 2 2 × 0.5 + 0.07 × 4.1 -------------------------------------------------------- = 0.66A 4.6 Equation 4 Power loss = Rcg x IRMS2 x talk time = 0.02 x 0.662 x 5 = 0.044 Wh 0.044 Wh equals 0.8% of battery capacity (5.55 Wh). This is the worst case condition. If the device is not used in continuous talking mode but in mixed mode usage (with lower RMS power consumption), the power loss will be less and will not significantly affect the accuracy of the battery capacity measurement. 6/11 Doc ID 018779 Rev 1 AN3395 4 Rcg layout considerations Rcg layout considerations Figure 3 shows the recommended layout of the PCB for Rcg. Figure 3. Rcg recommended layout with the STC3105 AM045293v1 In order to obtain the most accurate SOC estimation, follow these recommendations: 1. Place Rcg as close as possible to the CG pin. The STC3105 measures the battery current by sensing the voltage between the CG and GND pin. Between these two pins are two PCB traces and Rcg. Any voltage drop on the traces will directly add error to the measurement. The shorter the traces are, the better the accuracy of the measurement obtained. 2. The STC3105 GND pin should be connected directly to a terminal of Rcg (CG– in Figure 3), not through the ground plane. This Rcg terminal (CG– in Figure 3) should be connected to the ground plane on the PCB. This connection avoids the occurrence of high current through Rcg to the voltage potential on the STC3105 GND pin. Any ground disturbances will directly affect the accuracy of the current measurement and therefore the SOC estimation. 3. If the STC3105 is on a PCB near RF components, special care should be taken to avoid ground disturbance by the RF interference. The user can simply connect two high-frequency ceramic capacitors in parallel with Rcg to minimize the RF effect, if needed. The capacitance should be selected based on the frequencies which produce the highest RF disturbance. For example, 15 pF (ex. Murata 15 pF, part code: GQM2195C2A150GB01) and 68 pF (ex. Murata 68 pF, part code: GQM1885C1H680GB01) can be selected to avoid the disturbances from 1.8 GHz and 900 MHz (strongest RF power in a cell phone). This is illustrated in Figure 4 on page 8. Doc ID 018779 Rev 1 7/11 Rcg layout considerations Figure 4. AN3395 Capacitive filtering using ceramic capacitors Main PCB Battery Battery connectors Sensing resistor Capacitors STC3105 RF source AM045294v1 8/11 Doc ID 018779 Rev 1 AN3395 5 Conclusion Conclusion This document provides guidance to the user for the selection, the power consumption consideration and the layout of the sensing resistor used with the STC310x battery monitor for gas gauge applications. In most applications, 33 mΩ or 20 mΩ resistance is preferred to avoid power consumption concerns. With regards to PCB layout, the sensing resistor should be positioned as close to the STC310x as possible and the two terminals of Rcg directly connected to the CG and GND pins of the STC310x. The sensing resistor should be connected to the PCB ground plane. In the event of RF frequency exposure, capacitive filtering should be implemented by the parallel placement of high-frequency ceramic capacitors to avoid the disturbance of the GND pin of the STC310x for more accurate SOC measurement. Doc ID 018779 Rev 1 9/11 Revision history 6 AN3395 Revision history Table 1. 10/11 Document revision history Date Revision 05-Dec-2011 1 Changes Initial release. Doc ID 018779 Rev 1 AN3395 Please Read Carefully: Information in this document is provided solely in connection with ST products. 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