IMAX/10 IMAX/20 IMAX/30 IMAX/40 CNFG2031 1N4148 1N5400 1N5231 1N5821 2N3904 - Datasheet Archive

 

to Charge Lead-Acid Batteries pended, charging current is kept off by MOD being held low, and the state is annunciated by LED3

 

Using the bq2031 to Charge Lead-Acid Batteries pended, charging current is kept off by MOD being held low, and the state is annunciated by LED3 alternating high and low at approximately 1/6th second intervals. Description of Operation The bq2031 has two primary functions: lead-acid battery charge control and switch-mode power conversion control. Figure 1 is a block diagram of the bq2031. The charge control circuitry is capable of a variety of fullcharge detection techniques and supports three different charging algorithms. The Pulse-Width Modulator (PWM) provides control for high-efficiency current and voltage regulation. Temperature checks remain active throughout the charge cycle. They are masked only when the bq2031 is in the Fault state (see below). When the temperature returns to the allowed charging range, timers are restarted (not reset) and the bq2031 returns to the state it was in when the temperature fault occurred. When the thermistor is present and the temperature is within the allowed range, the bq2031 then checks for the presence of a battery. If the voltage between the BAT and SNS pins (VCELL) is between the Low-Voltage CutOff threshold (VLCO) and the High-Voltage Cut-Off (VHCO), the bq2031 perceives a battery to be present and begins pre-charge battery qualification after a 500ms (typical) delay. If any new temperature or voltage faults occur during this time, the bq2031 immediately transitions to the appropriate state. Starting a Charge Cycle and Battery Qualification When VCC becomes valid (rises past its minimum value), the first activates battery temperature monitoring. Temperature is indicated by the voltage between the pins TS and SNS (VTEMP). If the bq2031 finds the temperature out of range (or the thermistor is absent), it enters the Charge Pending State. In this state, all timers are sus- Figure 1. Block Diagram of the bq2031 Dec. 1995 1 Using the bq2031 to Charge Lead-Acid Batteries If VCELL is less than VLCO or above VHCO, the bq2031 believes no battery is present and enters the Fault state; MOD is held low and LED3 is turned on. This light gives the customer an indication that the charger is on, even though no battery is present. The bq2031 leaves the Fault state only if it sees VBAT rise past VLCO or fall past VHCO, indicating a new battery insertion. If temperature is within bounds, there will again be a 500ms delay before battery qualification tests start. Battery Qualification Tests In test 1, the bq2031 attempts to regulate a voltage = VFLT + 0.25V across the battery pack. The bq2031 monitors the time required for ISNS, the charging current, to rise to ICOND = IMAX/5. If the current fails to rise to this level before the time-out period tQT1 expires (e.g. the battery has failed open), the bq2031 enters the Fault state, indicated by the LED3 pin going high. Charging current is removed from the battery by driving the MOD pin low, and the bq2031 remains in this state until it detects the conditions to start a new charge cycle; the battery is replaced or VCC is cycled off and then back on. If test 1 passes, the bq2031 will start test 2 by attempting to regulate a charging current of ICOND into the battery pack. It will monitor the time required for the pack voltage to rise above VMIN (the voltage may already be over this limit). If the voltage fails to rise to this level before the time out period tQT2 expires (e.g., the battery has failed short), the bq2031 again enters the Fault state as described above. If test 2 passes, the bq2031 then begins fast (bulk) charging. Figure 2. Cycle Start/Battery Qualification State Diagram "Second Difference" of cell voltage drops below -8mV while VBAT is over 2.0V. Second Difference is the accumulated differences between successive samples of VBAT. The Second Difference technique looks for a negative change in battery voltage as the battery begins overcharging (see Figure 6). Fast Charging The user configures the bq2031 for one of three fast charge and maintenance algorithms. Two-Step Voltage (Figure 3) Maintenance phase: Fixed-width pulses of charging current = ICOND are modulated in frequency to achieve an average value of IMIN. See Appendix A for implementation details. This algorithm consists of three phases: Fast Charge phase 1: The charging current is limited at IMAX until the cell voltage rises to VBLK. Pulsed Current (Figure 5) Fast Charge phase 2: The charging voltage is regulated at VBLK until the charging current drops below IMIN. Maintenance phase: lated at VFLT. This algorithm consists of two phases: Fast Charge phase: The charging current is regulated at IMAX until the cell voltage rises to VBLK. The charging voltage is regu- Maintenance phase: Charging current is removed until the battery voltage falls to VFLT; charging current is then restored and regulated at IMAX until the battery voltage once again rises to VBLK. This cycle is repeated indefinitely. Two-Step Current (Figure 4) This algorithm consists of two phases: Fast Charge phase: The charging current is regulated at IMAX until the cell voltage rises to VBLK or the Dec. 1995 2 V BLK voltage V FLT fast charge phase 1 maintenance V MIN phase 2 ICOND voltage current I MAX qualification Using the bq2031 to Charge Lead-Acid Batteries current IMIN IFLT time V BLK current V FLT voltage V MIN maintenance fast charge ICOND voltage current I MAX qualification Figure 3. Two-Step Voltage Algorithm time ICOND maintenance current V BLK V FLT voltage V MIN voltage current I MAX qualification Figure 4. Two-Step Current Algorithm fast charge time Figure 5. Pulsed Current Algorithm Dec. 1995 3 Using the bq2031 to Charge Lead-Acid Batteries Safety Time-Out Hold-off Periods A safety timer limits the time the charger can spend in any phase of the charging cycle except maintenance. This Maximum Time-Out (MTO) timer is reset at the end of successful pre-charge qualification when the bq2031 begins fast charging1. If MTO times out before a fast charge termination criterion is met, the charging current is turned off (MOD driven low) and the bq2031 enters the Fault state exactly as if it had failed a pre-charge qualification test. Old age and/or abuse can create conditions in lead-acid batteries that may generate a large transient voltage spike when current-regulated charging is first applied. This spike could cause early termination in the fast charge algorithms by mimicking their voltage-based termination criteria. To prevent this, the bq2031 uses a "hold-off" period at the beginning of the fast charge phase. During this time, all voltage criteria are ignored except cutoff voltages. (Straying outside the range between VHCO and VLCO still causes the bq2031 to believe the battery has been removed, and the bq2031 enters the Fault state and shuts off charging current.) A hold-off period is also enforced during test 2 of pre-charge qualification for the same reason. There is one exception. In the Two-Step Voltage algorithm, MTO is reset when the bq2031 transitions from the current-limited phase 1 to the voltage-regulated phase 2 of fast charging. If MTO expires while the bq2031 is still in phase 1, it does not enter the Fault state but instead transitions to phase 2 (and MTO is reset). If MTO expires while the bq2031 is still in state 2, the bq2031 enters the Fault state. Configuration Instructions Selecting Charge Algorithm and Display Mode During maintenance, the MTO timer is reset at the beginning of each new pulse in the Two-Step Current and Pulsed Current algorithms. It expires (and puts the bq2031 in the Fault state) only if the bq2031 became "jammed" with a pulse stuck on. The MTO timer is not active during the maintenance phase of the Two-Step Voltage algorithm. 2.7 QSEL/LED3, DSEL/LED2, and TSEL/LED1 are bi-directional pins with two functions: they are LED driver pins as outputs and programming pins for the bq2031 as inputs. The selection of pull-up, pull-down, or no pull resistor for these pins programs the charging algorithm on QSEL and TSEL per Table 1 and the display mode on DSEL per Table 2. The bq2031 forces the output driver on these bi-directional pins to their high-impedance state (as well as their common return output pin, COM) and latches the programming data sensed on the inputs when any one of the following three events occurs: Voltage Profile for Lead-Acid Charging with Constant Current Regulation 2.6 1. VCC rises to a valid level. 2. The bq2031 leaves the Fault state. 3. The bq2031 detects battery insertion. Voltage per Cell, V 2.5 2.4 The LEDs go blank for approximately 0.75s. (typical) while new programming data is latched. 2.3 Figure 7 shows the bq2031 configured for the Pulsed Current algorithm and display mode 2. 2.2 Table 1. Programming Charge Algorithms 2.1 Charge Algorithms QSEL 1.9 0 25 50 75 100 125 Programmable Thresholds Two-Step Voltage L H/L* IMAX, VBLK, VFLT Two-Step Current H L IMAX, VBLK, IMIN Pulsed Current H H IMAX, VBLK, VFLT Note: * Set either high or low; do not float pin. Charge Rate = C/20 2.0 TSEL 150 Previous Dicharge Capacity Returned, % 1 The MTO timer also resets at the beginning of the pre-charge qualification period. However, tQT1 or tQT2 (the qualification test time limits) expire and put the bq2031 in the Fault state before the MTO limit can be reached. The Figure 6. Voltage Roll-Off In Constant MTO timer is suspended while the bq2031 is in the Fault state, and is reset by the conditions that allow the bq2031 to exit that state. Current Charging Profile Dec. 1995 4 Using the bq2031 to Charge Lead-Acid Batteries Table 2. bq2031 Display Output Summary Mode Charge State LED2 LED3 Low Low High Pre-charge qualification Flash* Low Low Fast charging High Low Low Maintenance charging DSEL = 0 (Mode 1) LED1 Battery absent Low High Low Charge pending (temperature out of range) X Flash* X X High Battery absent Low Low High Pre-charge qualification DSEL = 1 (Mode 2) X Fault High High Low Fast charge Low High Low Maintenance charging High Low Low Charge pending (temperature out of range) X X Flash* Fault X X High Pre-charge qualification Flash* Flash* Low Battery absent Low Low High Fast charge: current regulation Low High Low Fast charge: voltage regulation High High Low Maintenance charging DSEL = Float (Mode 3) High Low Low Charge pending (temperature out of range) Note: X X Flash* Fault X X High 1 = VCC, 0 = VSS, X = LED state when fault occurred. * Flash = 1/6 sec. low, 1/6 sec. high Figure 7. Configuring 10K Two-Step Charging Algorithm and Display Mode Selection Dec. 1995 5 Using the bq2031 to Charge Lead-Acid Batteries Setting Voltage and Current Thresholds BAT+ V CC Fixed Thresholds RB1 The bq2031 uses the following fixed thresholds: FLOAT VHCO-High-Cutoff Voltage: VBAT rising above this level is interpreted as battery removal, cutting off charging current. VHCO = 0.6 * VCC. BAT 13 VLCO-Low-Cutoff Voltage: VBAT dropping below this level is interpreted as battery removal, cutting off charging current. VLCO = 0.8V. V SS Figure 8. Configuring the Battery Divider and Current Sense Circuit VBLK-Upper voltage limit during fast charge, typically specified by the battery manufacturers to be 2.45V­2.5V per cell @ 25°C. Equation 3 IMAX = VFLT-Minimum charge voltage required to compensate for the battery's self-discharge rate and maintain full charge on the battery. A value is usually recommended by the battery manufacturer. N = Number of series cells in the battery pack VFLT = Value recommended by manufacturer VBLK = Value recommended by manufacturer at 25°C. If you have selected the Two-Step Current algorithm and want Second Difference detection to be your primary fast charge termination criterion, use VBLK = 2.75V. VFLT, VBLK, and IMAX are configured by the user when selecting resistor values for the battery voltage divider network (see Figure 8). VFLT is set by RB1 and RB2 by: IMAX = Desired maximum charge current Equation 1 The bq2031 internal band-gap reference voltage at 25°C is 2.2V. This reference shifts with temperature at 3.9mV/°C to compensate for the negative temperature coefficient of lead-acid chemistry. (N VFLT) -1 2.2V The total resistance presented by the divider between BAT+ and BAT- (RB1 + RB2) should be between 150k and 1M . The minimum value ensures that the divider network does not drain the battery excessively when the power source is disconnected. Exceeding the maximum value increases the noise susceptibility of the BAT pin. VBLK is determined by: Equation 2 + RB1 RB3 = ( N VBLK 2.2 ) 0.275V RSNS where: IMAX-Fast charge current specified as a function of "C," the capacity of the battery in Ampere-hours (e.g., a charge rate of 1C for a 5Ah battery is 5A). Typical values range from C/10 to C, although some battery vendors may approve higher charge rates. RB1 BATR SNS The bq2031 uses the following configurable thresholds: RB2 7 bq2031 Configurable Thresholds = RB2 V SS SNS ICOND-Conditioning Current: Used in the maintenance phase of the Two-Step Current algorithm and pre-charge qualification tests 1 and 2. ICOND = IMAX/5. IMAX is set by Equation 3. RB2 3 V CC 12 VMIN-Minimum Voltage: Used in pre-charge qualification test 2. VMIN = 0.34 * VCC. RB1 2 RB3 -1 An empirical procedure for setting the values in the resistor network is as follows: IMAX is determined by: 1. Set RB2 to 49.9 k (for 3 to 18 series cells) Dec. 1995 6 Using the bq2031 to Charge Lead-Acid Batteries 2. Determine RB1 from equation 1 given VFLT 3. Determine RB3 from equation 2 given VBLK 4. Determine RSNS from equation 3 given IMAX. Table 4. Programming IMIN Two-Step Voltage Two-Step Current IGSEL Number of Cells RB1 (k) RB2 (k) 102.0 49.9 383.0 6 261.0 49.9 475.0 12 562.0 49.9 511.0 18 866.0 49.9 536.0 IMAX/10 IMAX/10 IMAX/20 IMAX/20 IMAX/30 IMAX/30 Z IMAX/40 IMAX/40 The bq2031 senses temperature by monitoring the voltage between the TS and SNS pins. The bq2031 assumes a Negative Temperature Coefficient (NTC) thermistor, so the voltage on the TS pin is inversely proportional to the temperature (see Figure 9). The temperature thresholds used by the bq2031 and their corresponding TS pin voltage are: TCO-Temperature Cut-Off: Higher limit of the temperature range in which charging is allowed. VTCO = 0.4 * VCC. IMIN-In the Two-Step Voltage algorithm, IMIN is the level to which charging current must drop to terminate fast charge. In the Two-Step Current algorithm, it is the average value of pulsed current in the maintenance phase. IMIN is a fraction of IMAX programmed by the state of the pin IGSEL and the charging algorithm selected, per Table 4. V CC L H Setting Temperature Thresholds RB3 (k) 3 IMIN IMAX/10 IMAX/10 IMAX/20 IMAX/20 Z Table 3. Example Resistor Values by Number of Cells IGSEL L H Table 3 shows the results of these calculations at several example cell counts for VFLT = 2.25V and VBLK = 2.45V. 1% resistors are recommended. IMIN HTF-High-Temperature Fault: Threshold to which temperature must drop after Temperature Cut-Off is exceeded before charging can begin again. VHTF = 0.44 * VCC LTF-Low-Temperature Fault: Lower limit of the temperature range in which charging is allowed. VLTF = 0.6 * VCC. Colder V CC V LTF = .6V CC V HTF = .44V CC V TCO = .4V CC LTF HTF TCO Temperature Voltage RT1 13 12 NTC thermistor V CC V SS RT2 RT °t SNS TS 7 BAT- 8 R SNS bq2031 V SS V SS Hotter Figure 10. Configuring Figure 9. Voltage Equivalent of Current Thresholds Temperature Sensing Dec. 1995 7 Using the bq2031 to Charge Lead-Acid Batteries A resistor-divider network must be implemented that presents the defined voltage levels to the TS pin at the desired temperatures (see Figure 10). V CC R The equations for determining RT1 and RT2 are: 1 Equation 4 0.6 VCC = TMTO C (VCC - 0.275) RT1 (RT2 + RLTF) 1+ (RT2 RLTF) V CC V SS Equation 5 0.44 = 13 12 1 RT1 (RT2 + RHTF) 1+ (RT2 RHTF) where: bq2031 V SS RLTF = Thermistor resistance at LTF RHTF = Thermistor resistance at HTF Figure 11. RC Network TCO is determined by the values of RT1 and RT2. 1% resistors are recommended. As an example, the resistor values for several temperature windows computed for a Philips 2333-640-63103 thermistor are shown in Table 5. for Setting MTO Disabling Temperature Sensing Temperature sensing may be disabled by removing the thermistor and RT1, and using a value of 100k for RT1 and RT2. Table 5. RT1 and RT2 Values for Temperature Thresholds LTF (°C) HTF (°C) TCO (°C) RT1 (k) RT2 (k) 0 45 47 3.57 7.50 5 45 47 3.65 8.66 ­5 50 52 2.74 5.36 Setting Timers The user sets the Maximum Time-Out (MTO) value. All other timing periods used in the bq2031 are fixed as fractions of MTO (see Table 6). MTO is set by an R-C network on the TMTO pin as shown in Figure 11. Table 6. Timing Parameters Symbol Parameter Minimum Typical Maximum Unit tMTO Maximum Time Out range 1 - 24 hours tQT1 Qualification time-out test 1 - 0.02tMTO - - tQT2 Qualification time-out test 2 - 0.16tMTO - - 2 tDV - V termination sample frequency - 0.008tMTO - - tHO1 Qualification test 2 hold-off period - 0.002tMTO - - tHO2 Bulk-charge hold-off period - 0.015tMTO - - Dec. 1995 8 Using the bq2031 to Charge Lead-Acid Batteries Each PWM loop starts with a comparator whose positive terminal is driven by VS. The negative terminal is driven by the output of an Operational Transconductance Amplifier (OTA) which, with the compensation network connected via VCOMP or ICOMP, generates the control signal VC. The OTA characteristics are: RO = 250k; GM = 0.42m-mho; gain bandwidth = 80MHz. The output of each comparator, along with the ramp waveform (VS), is used to generate a pulse-width modulated waveform at a constant frequency on the MOD output. Figure 14 shows the relationship of MOD with VC and VS. The equation for MTO is: Equation 6 MTO (in hours) = 0.5 R C where R is in k and C is in µF. The value for C must not exceed 0.1µF. Example: An MTO of 5 hours is set by R = 100k and C = 0.1µF The MOD output swings rail-to-rail and can source and sink 10mA. It is used to control the drive circuitry of the switching transistor. Switch-Mode Power Conversion The bq2031 incorporates the necessary PWM control circuitry to support switch-mode voltage and current regulation. The pulse-width modulated square-wave signal on the MOD pin is synchronized to the internal sawtooth ramp signal. The ramp-down time (TD) is fixed at approximately 20% of the ramp time-period (TP). This limits the maximum duty-cycle achievable to approximately 80%. See Figure 14. Figure 12 shows a functional block diagram of a switchmode buck topology converter using the bq2031. The battery voltage is divided down to a per-cell equivalent value at the BAT pin. During voltage regulation, the voltage on the BAT pin (VBAT) is regulated to the internal band-gap reference of 2.2V at 25°C (with a temperature drift of -3.9 mV/°C). The charge current through the inductor L is sensed across the resistor RSNS. During current regulation, the bq2031 regulates the voltage on the SNS pin (VSNS) to a temperature-compensated reference of 0.275V. Example: At a switching frequency of FS = 100kHz, TD = 2µs. Inductor Selection The inductor selection criteria for a DC-DC buck converter vary depending on the charging algorithm used. For the Two-Step Current and Pulsed Current charge algorithms, the inductor equation is: The passive components CI on the ICOMP pin, RV and CV on the VCOMP pin, and CF across the high side of the battery voltage divider form the phase compensation network for the current and voltage control loops, respectively. The diodes (Db1 and Db2) serve to prevent battery drain when VDC is absent, while the pull-up resistor (RP) is used to detect battery removal. The resistor RS, typically a few tens of m, is optional and depends on the battery impedance and the resistance of the battery leads to and from the charger board. Equation 8 L= (N VBLK 0.5) FI where: N = Number of cells Pulse-Width Modulator VBLK = Bulk voltage per cell, in volts The bq2031 incorporates two PWM circuits, one for each control loop (voltage and current, see Figure 13). Each PWM circuit runs off a common saw-tooth waveform (VS) whose time-base is controlled by a timing capacitor (CPWM) on the TPWM pin. FS = Switching frequency, in hertz I = Ripple current at IMAX, in amps The ripple current is usually set between 20­25% of IMAX. Example: A 6-cell SLA battery is to be charged at IMAX = 2.75A in a buck topology running at 100kHz. The VBLK threshold is set at 2.45V per cell and the charger is configured for Pulsed Current mode. Assuming a ripple = 25% of IMAX, the inductor value required is: The relationship between CPWM and the switching frequency (FS) is given by : Equation 7 FS = 0.1 kHz CPWM Equation 9 L= where CPWM is in µF. Dec. 1995 9 (6 2.45 0.5) = 107µH (100000 0.6875) Using the bq2031 to Charge Lead-Acid Batteries Figure 12. Functional Diagram of a Switch-Mode Buck Regulator Lead-Acid Charger Using the bq2031 Dec. 1995 10 Using the bq2031 to Charge Lead-Acid Batteries Figure 13. Block Diagram of the bq2031 PWM Control Circuitry Figure 14. Relationship of MOD output to Sawtooth Waveform VS and Control Signal VC Dec. 1995 11 Using the bq2031 to Charge Lead-Acid Batteries where: The inductor formula for the Two-Step Voltage charge algorithm is dictated by the inductor current, which must remain continuous down to IMIN during Fast Charge phase 2 (voltage regulation phase). N = Number of cells VBLK = Bulk voltage threshold per cell Equation 10 Logical Control of Charging N VBLK 0.5 L= FS 2 IMIN Charge Inhibit An inhibit input may be implemented by connecting the cathode of a small-signal diode to the TS pin. A CMOS logic-level "1" applied to the anode of the diode then functions as an inhibit input, by driving the temperature sense voltage out of its allowed range and simulating an under-temperature condition. The bq2031 enters the Charge Pending state, shutting off charging current (driving MOD low) and suspending all timers. When the Inhibit signal is allowed to float, the bq2031 returns to its previous state (as long as the temperature is still within the allowed range). The bq2031 restarts (but does not reset) its timers, and the suspended charge cycle resumes at the point where it stopped. Example: A 6-cell SLA battery is to be charged at IMAX = 2.75A in a buck topology running at 100 kHz. The VBLK threshold is set at 2.45V per cell and the charger is configured for Two-Step Voltage mode, with IMIN = IMAX/20 IMAX/20. The inductor value required is: Equation 11 L= 6 2.45 0.5 = 267µH (100000 2 0.1375) Phase Compensation For buck-mode switching applications, the suggested component values shown in Figure 12 are good starting points. Further assistance is available from the bq2031 configuration software program "CNFG2031 CNFG2031." More details on the calculations used in this program are available in the application note entitled "Compensating the bq2031 in BuckMode Switching Applications." For assistance with other power supply topologies, contact the factory. Miscellaneous Issues VCC Supply Reset A logical Reset signal for the bq2031 can be created in a manner similar to the Charge Inhibit input described above. Instead of being connected to the TS pin, however, the diode is connected to the BAT input. In this configuration, a logic "1" on the diode drives VBAT above VHCO, simulating battery removal. The bq2031 enters the Fault state and waits to see a battery insertion; VBAT rising past VLCO or falling past VHCO. Removing the logic "1" from the diode creates this transition (as long a battery is still present), and the bq2031 starts a new charge cycle. The VCC supply provides bq2031 power and serves as the reference voltage for all temperature sense thresholds (VLTF, VHTF, and VTCO) and the battery voltage thresholds VHCO and VMIN. The timer thresholds (MTO and its derivatives) are trimmed within 5% of the typical value with VCC = 5V. Caution: To avoid damage the bq2031, always keep the voltage applied to the anode of the diode below VCC for either the Charge Inhibit or Reset implementations. The VBLK and VFLT thresholds are set from an external divider network powered by the battery. These thresholds are referenced to an internal band-gap reference, and the accuracy of voltage regulation will not be adversely affected by variation in VCC. The current regulation threshold (IMAX) is referenced to a temperature compensated reference and is also unaffected by VCC. Printed circuit board layout must adhere to the following guidelines to minimize noise injection on the highimpedance pins (BAT, VCOMP, ICOMP, and SNS). Layout Guidelines 1. 2. The charging path components and traces must be isolated from the voltage and current feedback small signal paths. 3. DC Power Supply Use a single-point grounding technique such that the isolated small-signal ground path and the highcurrent power ground path return to the power supply ground. 0.1µF and 10µF decoupling capacitors must be placed close to the VCC pin. This also helps to prevent voltage dips while the bq2031 is driving the LEDs. The DC power supply voltage (VDC) for a switch-mode application must satisfy the following criterion: Equation 12 VDC = (N VBLK) + 3V Dec. 1995 12 Using the bq2031 to Charge Lead-Acid Batteries 4. A 100pF capacitor, if used for coupling the BAT and SNS pins, must be placed close to those pins. 5. The compensation network on ICOMP and VCOMP must be placed close to their respective pins. 6. current limit of 1.25 * IMAX. Similarly, during periods of fast charge current regulation, the bq2031 enforces a VBLK upper limit on voltage, and regulates to VBLK if the voltage tries to rise above this level. During the maintenance phase, the bq2031 regulates to VFLT and ICOND during periods of current or voltage regulation, respectively. Minimize loop area in paths with high pulsating currents. In the example in Figure 15, this loop is formed by D9, Q2, and C5. Applications Example: SingleEnded Buck Charger Battery Removal Detection Charger Specifications: The bq2031 interprets VBAT rising past VHCO or falling past VLCO as battery removal, and the bq2031 enters the Fault state until a new battery insertion is seen. The battery removal transitions are precluded during periods of voltage regulation unless circuitry (e.g., a pull-up to VDC) is provided to pull VBAT out of the "battery present" range. Charge Algorithm: Two-Step Voltage mode Charge Current: 2.75 A Battery Specifications: Yuasa 12V, 10Ah The following information is derived from the battery specifications: Voltage regulation occurs during phase 2 of the Two-Step Voltage fast charge algorithm and in battery qualification test 1 which precedes all three algorithms. The time-out period of this test (= 0.02 * MTO) is at least 1.2 minutes and may be as long as 28.8 minutes. Unless waiting through this period before detecting battery removal is acceptable, the pull-up is required in the purely current regulated algorithms as well. A diode should also be installed in the path of the pull-up to prevent the power supply from draining the battery when the supply is turned off. Refer to resistor R12 and diode D3 in the example design in Figure 15. Number of cells = 6 (divide battery voltage by 2, the SLA nominal V/cell) Capacity = 10 amp-hours VBLK threshold = 2.4V per cell (from the cyclic voltage specification) VFLT threshold = 2.2V per cell (from the float voltage specification) The safety timer (MTO) is set equal to the amp-hour capacity divided by the charge current: This pull-up creates a background trickle charge current to the battery that can be minimized by minimizing the voltage overhead; that is, the voltage difference between the VDC supply and the battery stack. Equation 13 MTO = Load-Only Operation 10 = approximately 4 hours 2.75 The temperature window of operation from the Yuasa battery specifications is 0°C­45°C. This gives: LTF = 0°C and HTF = 45°C The bq2031 supports the case in which the charger must supply the load in the absence of a battery, provided the load can pass the two pre-charge qualifications tests (draw current of at least ICOND when regulated at VFLT + 0.25V and maintain voltage of at least VMIN when regulated at ICOND). Further, the load must not create conditions that cause fast charge termination or it must be able to tolerate the conditions of maintenance regulation for the charge algorithm selected. This is regulation at VFLT in the case of the Two-Step Voltage algorithm or constant or hysteretic pulsed current supply in the case of the Two-Step Current and Pulsed Current algorithms, respectively. This can be a problem for intermittent loads unless circuitry is provided to maintain these conditions during the low-load or no-load periods. Apply these parameters in equations 1 through 10 to determine values for the following: Battery divider network Sense resistor MTO time-out PWM frequency Temperature window network Back-up Supply Regulation To protect the system from damage during periods of fast charge voltage regulation, the bq2031 regulates to IMAX if the current tries to rise above that level, and has an absolute Dec. 1995 13 The final schematic (see Figure 15) uses a low VCE (sat), high-gain, high-bandwidth PNP transistor (Q2) as the switching device in a single-ended buck topology powered from a DC supply of 24 V. The power filter inductor (L2) is a 78-turn powdered-iron toroidal core inductor from Micrometals. The capacitor Using the bq2031 to Charge Lead-Acid Batteries (C4) is used to suppress the inductive effect of long lead wires from the board to the battery. The low Q inductor (L1) causes transistor Q2 to turn off faster during the commutation period. See Table 7 for the parts list for the design. Figure 15. Example Schematic of a Single-Ended Buck Topology Charger Using the bq2031 Dec. 1995 14 Using the bq2031 to Charge Lead-Acid Batteries Table 7. Parts List for a Single-Ended Buck Charger Item Quantity Reference Part 1 2 C1, C3 0.1µF 2 1 C2 10µF 3 1 C4 47µF 4 1 C5 5 6 7 8 9 10 11 12 2 2 3 1 1 3 1 1 C6, C7 C8, C9 D1, D2, D8 D3 D4 D5, D6, D7 D9 L1 100µF 10nF 1nF 1N4148 1N4148 1N5400 1N5400 1N5231 1N5231 LED 1N5821 1N5821 10µH 13 1 L2 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 2 1 2 2 4 1 1 1 1 1 1 1 1 1 1 1 1 1 2 Q1, Q3 Q2 R9, RT1 R1, R5 R2, R3, R4, R8 R6 R7 R10 R11 R12 R13 R14 R15 R16 R17 R18 R19 R20 R21, R22 33 1 U1 Dec. 1995 15 487µH 2N3904 2N3904 ZBD949 ZBD949 10K 50K 1K 6.81K 1% 3.74K 1% 460K 1% 82K 51K 120K 1K, 1W, 5% 330 1W 120 51 243K 1% 49.9K 1% 0.1 0.1 bq2031 Using the bq2031 to Charge Lead-Acid Batteries Table 8 gives the values of TP programmed by IGSEL. Appendix A: Implementation Details of Pulsed Maintenance Charging Table 8. Fixed Pulse Period by IGSEL Two-Step Current Algorithm IGSEL TP (sec.) L 0.4 H 0.8 Z Maintenance charging in the Two-Step Current Algorithm is implemented by varying the period (TP) of a fixed current (ICOND = IMAX/5) and duration (0.2 second) pulse to achieve the configured average maintenance current value. See Figure 16. 1.6 Maintenance current can be calculated by: Equation 14 Maintenance current = (0.2) ICOND) (0.04) IMAX) = TP TP where TP is the period of the waveform in seconds. Figure 16. Implementation of Fixed-Pulse Maintenance Charge Dec. 1995 16