APPLICATION NOTE 105: Current Sense Circuit Collection Batteries The science of battery chemistries and the charging and discharging characteristics is a book of its own. This chapter is intended to provide a few examples of monitoring current flow into and out of batteries of any chemistry. Charge/Discharge Current Monitor on Single Supply with Shifted VBIAS RSENSE TO CHARGER/ LOAD 1 FIL– To see other chapters in this Application Note, return to the Introduction. 2 VS– Input Remains Hi-Z when LT6100 is Powered Down 4 8 + BATTERY 4.1V TO 48V DNC ROUT VEE – + VCC FIL VOUT VEE A2 A4 6100 F08 This is the typical configuration for an LT6100, monitoring the load current of a battery. The circuit is powered from a low-voltage supply rail rather than the battery being monitored. A unique benefit of this configuration is that when the LT6100 is powered down, its battery sense inputs remain high impedance, drawing less than 1uA of current. This is due to an implementation of Linear Technology’s Over-The-Top® input technique at its front end. C2 1µF 5 VOUT LT1634-1.25 C3* 1000pF OUTPUT 1787 F04 Here the LT1787 is used in a single supply mode with the VBIAS pin shifted positive using an external LT1634 voltage reference. The VOUT output signal can swing above and below VBIAS to allow monitoring of positive or negative currents through the sense resistor. The choice of reference voltage is not critical except for the precaution that adequate headroom must be provided for VOUT to swing without saturating the internal circuitry. The component values shown allow operation with VS supplies as low as 3.1V. Battery Current Monitor IL CHARGE RSENSE 0.1Ω DISCHARGE A2 1/2 LT1495 + V S+ – LT6100 VS – 20k 5% VBIAS 6 *OPTIONAL TO LOAD 3.3V VS+ 7 ISENSE RSENSE POWER DOWN OK VCC 3V 0V INPUTS REMAIN Hi-Z 3 FIL+ LT1787HV 3.3V TO 60V C1 1µF RA RA RA 2N3904 DISCHARGE OUT RB 12V 5V RA – A1 1/2 LT1495 + 2N3904 CHARGE OUT VO = IL () RB RSENSE RA RB FOR RA = 1k, RB = 10k VO = 1V/A IL 1495 TA05 One LT1495 dual op-amp package can be used to establish separate charge and discharge current monitoring outputs. The LT1495 features Over-the-Top operation allowing the battery potential to be as high as 36V with only a 5V amplifier supply voltage. Batteries-1 APPLICATION NOTE 105: Current Sense Circuit Collection Input Current Sensing Application 5V + 1 RP1 3k 1% 8 AVG 7 PROG LT1620MS8 6 3 VCC GND 2 4 SENSE IOUT IN LOAD + C2 1µF 4.7µF RL RP2 12k 1% CF+ 4.7µF INT LTC4150 CF– CLR CHG GND TO SYSTEM LOAD SHDN 4150 TA01a 22µF L1B 10µH MBRS340 7 VSW VIN 4.7µF LT1513 RUN 6 4 VFB S/S GND GND TAB 8 IFB VBATT = 12.3V 5 L1A 10µH 57k + 2 3 22µF ×2 Li-ION 24Ω 6.4k VC 0.22µF 1 µP DISCHG POL R1 0.033Ω + RL SENSE – SENSE + VDD 5 IN+ – CHARGER RSENSE C1 1µF 22µF Coulomb Counter RSENSE 0.1Ω 0.1µF X7R 1620/21 • F04 The LT1620 is coupled with an LT1513 SEPIC battery charger IC to create an input over current protected charger circuit. The programming voltage (VCC – VPROG) is set to 1.0V through a resistor divider (RP1 and RP2) from the 5V input supply to ground. In this configuration, if the input current drawn by the battery charger combined with the system load requirements exceeds a current limit threshold of 3A, the battery charger current will be reduced by the LT1620 such that the total input supply current is limited to 3A. The LTC4150 is a micropower high-side sense circuit that includes a V/F function. Voltage across the sense resistor is cyclically integrated and reset to provide digital transitions that represent charge flow to or from the battery. A polarity bit indicates the direction of the current. Supply potential for the LTC4150 is 2.7V to 8.5V. In the freerunning mode (as shown, with CLR & INT connected together) the pulses are approximately 1µs wide and around 1Hz full-scale. Li-Ion Gas Gauge POWER-DOWN SWITCH LOAD 2.5V RSENSE 0.1Ω 2-CELL Li-Ion 6V ~ 8.4V 1 SENSE + 2 SENSE – VDD C F+ GND INT LTC4150 CLR 3 + CF 4.7µF 4 5 C F– SHDN POL 10 RL 3k CL 47µF RL 3k 9 8 7 C2 4.7µF µP 6 SHUTDOWN This is the same as the Coulomb Counter circuit, except that the microprocessor clears the integration cycle complete condition with software, so that a relatively slow polling routine may be used. Batteries-2 APPLICATION NOTE 105: Current Sense Circuit Collection NiMH Charger Q3 INPUT SWITCH DCIN 0V TO 20V C1 0.1µF R8 147k 0.25% VLOGIC R11 100k R12 100k ICL ACP BATMON DCIN VFB INFET ICL LTC4008 CLP FLAG R10 32.4k 1% FAULT TGATE FLAG BGATE NTC PGND THERMISTOR 10k NTC RT 150k D1 GND NiMH BATTERY PACK C3 20µF R4 3.01k 1% BAT ITH R7 6.04k 1% Q2 RSENSE 0.025Ω 1% L1 10µH Q1 CSP RT R9 C7 13.3k 0.47µF 0.25% SYSTEM LOAD C2 20µF CLN ACP/SHDN FAULT RCL 0.02Ω 1% C4 0.1µF R1 5.1k 1% R5 3.01k 1% PROG CHARGING CURRENT MONITOR C5 0.0047µF R6 26.7k 1% C6 0.12µF D1: MBRS130T3 Q1: Si4431ADY Q2: FDC645N 4008 TA02 The LTC4008 is a complete NiMH battery pack controller. It provides automatic switchover to battery power when the external DC power source is removed. When power is connected the battery pack is always kept charged and ready for duty. Single Cell Li-Ion Charger Li-Ion Charger VIN 5V TO 22V USB PORT 0.1µF 10µF VCC BAT 1µF GATE 2k 800mA (WALL) 500mA (USB) LTC4076 WALL ADAPTER BAT DCIN USBIN 1µF HPWR + IUSB 2k IDC 1% 1.24k 1% ITERM GND 4.2V SINGLE CELL Li-Ion BATTERY 1k 1% LTC4002ES8-4.2 CHARGE STATUS 6.8µH CHRG 4076 TA01 SENSE Just a few external components are required for this single Li-Ion cell charger. Power for the charger can come from a wall adapter or a computer’s USB port. 68mΩ 0.47µF COMP NTC BAT GND 22µF 2.2k T + 10k NTC Li-Ion BATTERY 4002 TA01 NTC: DALE NTHS-1206N02 Controlling the current flow in Lithium-Ion battery chargers is essential for safety and extending useful battery life. Intelligent battery charger ICs can be used in fairly simple circuits to monitor and control current, voltage and even battery pack temperature for fast and safe charging. Batteries-3 APPLICATION NOTE 105: Current Sense Circuit Collection Battery Monitor CHARGER VOLTAGE RS 0.2Ω RA 2k IBATT RA' 2k + A 1/4 LT1491 Q1 2N3904 – C 1/4 LT1491 – LOGIC + RB 2k RB' 2k LOAD + Q2 2N3904 – + + RG 10k VBATT = 12V S1 IBATT = VOUT V = OUT AMPS (RS)(RG /RA)(GAIN) GAIN Op-amp sections A & B form classical high-side sense circuits in conjunction with Q1 & Q2 respectively. Each section handles a different polarity of battery current flow and delivers metered current to load resistor RG. Section C operates as a comparator to provide a logic signal indi- Batteries-4 LOGIC HIGH (5V) = CHARGING LOGIC LOW (0V) = DISCHARGING B 1/4 LT1491 D 1/4 LT1491 VOUT – 10k 90.9k S1 = OPEN, GAIN = 1 S1 = CLOSED, GAIN = 10 RA = R B VS = 5V, 0V 1490/91 TA01 cating whether the current is a charge or discharge flow. S1 sets the section D buffer op-amp gain to +1 or +10. Rail-to-Rail op-amps are required in this circuit, such as the LT1491 quad in the example.