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Technical content
DESCRIPTION
The DS2715 is well suited for cost-sensitive charger applications where the battery pack is either internal or external to the application. It has been optimized for safe and reliable charging of 1 to 10 NiMH cells in series. The internal gain block can be selected as either a comparator or transconductance amplifier for charge current regulation. This makes the DS2715 configurable as a switched DC charger, a linear current regulator, or a switchmode current source. The DS2715 pre-conditions severely depleted cells before entering full charge mode. It terminates full charge using the dT/dt technique, which requires an external sensing thermistor. Over-temperature, under- temperature, and over-v oltage detection prevents charging under unsafe conditions. A user selectable charge timer allows charge rates from 0.167C to 2C. FAST-CHARGE, TOPOFF and DONE modes are included for highly reliable, safe charging of NiMH cells. Discharge mode allows the DS2715 to enter a low power sleep state while the cell pack is being discharged.
FEATURES
Charges 1 to 10 NiMH Cells FAST-CHARGE at up to a 2C Rate PRECHARGE and TOPOFF Charge Modes Help Cell Conditioning Load Detection Allows the DS2715 to Enter Low Power Sleep Mode (Less than 10µA) while the Cell Pack is Discharged dT/dt Charge Termination Eliminates Cell Charge Stress Monitors Voltage, Temperature, and Time for Safety and Secondary Termination Regulates Current through Either Linear Control or Switch-Mode Control LED Output Displays Charge State Small 16-Pin SO Package
APPLICATIONS
Table 1. ORDERING INFORMATION + Denotes a lead(Pb)-free/RoHS-compliant package. See Table 1 for Ordering Information. may be simultaneously available through various sales channels. For information about device errata, click here: www.maxim-ic.com/errata.
DS2715: NiMH Battery Pack Charge Controller 2 of 17 ABSOLUTE MAXIMUM RATINGS* Voltage on VDD and VCH Pins with Respect to VSS -0.3V to +18V Voltage on LED1 Pin -0.3V to +18V Voltage on SNS- -0.3V to +0.3V Voltage on C BIAS -0.3V to 6V Voltage on all Other Pins -0.3V to V Cbias Continuous Sink Current VCH and LED1 28mA Operating Temperature Range -20°C to +85°C Storage Temperature Range -55°C to +125°C Soldering Temperature See IPC/JEDEC J-STD-020 Human Body Model (HBM) ESD Limit of V CH Pin 500V HBM ESD Limit of all Other Pins 2KV *This is a stress rating only and functional operation of the device at these or any other conditions above those indicated in the operation sections of this specification is not implied. Exposure to absolute maximum rating conditions for extended periods of time may affect reliability. RECOMMENDED DC OPERATING CONDITIONS (4.5V VDD 16.5V; TA = 0C to +70C.) PARAMETER SYMBOL CONDITIONS MIN TYP MAX UNITS Supply Voltage V DD (Note 1) 4.5 16.5 V LED1 Voltage V LED (Note 1) 0.0 16.5 V Mode Voltage V MODE (Note 1) 0.0 V Cbias V VCH Voltage V VCH (Note 1) 0.0 16.5 V CBIAS Capacitor Range C Cbias .02 .15 μF RT Resistor Range R Rt 20 240 K Ω DC ELECTRICAL CHARACTERISTICS (4.5V VDD 16.5V, TA = 0C to +70C, unless otherwise noted.) PARAMETER SYMBOL CONDITIONS MIN TYP MAX UNITS Linear Mode, VDD = 16.5V 1.3 1.6 mA Operating Current (Note 2) IDDA Comparator Mode, VDD = 16.5V 150 250 μA Idle Current I DDS V DD < VUVLO 10 μA Discharge Current I DDD Discharge latch set (Note 2) 200 μA UVLO Threshold V UVLO V DD Rising (Note 3) 3.8 3.9 4.0 V UVLO Hysteresis V UVLO-HYS V DD Falling 35 mV VCH Sink Current I OL-Vch V OL = 1.5V 20 mA LED1 Sink Current I OL-LED V OL = 1.0V 20 mA Leakage Current, VCH, LED1 ILKG Pin Inactive or Device Idle -1 +1 μA THM Pin Leakage Current ILKG-THM -1 +1 μA VBATT Pin Leakage Current ILKG-Vbatt -50 +50 nA CBIAS Voltage V Cbias 0 < I Cbias < 0.4mA 3.9 4.0 4.3 V DIV Pin Load Current I Div 500 uA Current Sense Amplifier Gain GERR 100 μA < I Vch < 20mA 5 6.25 7.5 Ω-1
DS2715: NiMH Battery Pack Charge Controller 3 of 17 PARAMETER SYMBOL CONDITIONS MIN TYP MAX UNITS Current Sense Comparator Gain GCOMP (Note 7) 10 Ω-1 AC ELECTRICAL CHARACTERISTICS (4.5V VDD 16.5V, TA = 0C to +70C, unless otherwise noted.) PARAMETER SYMBOL CONDITIONS MIN TYP MAX UNITS UVLO Debounce Time t UVLO 10 μs Current Sense Comparator Propagation Delay t COMP (Note 7) 250 ns Discharge Detect Propagation Delay tDD From detection of current reversal 1 μs Return To Normal Function (Op-Amp or Comparator Mode) tRNF Time from reset of discharge latch 1 μs RT Timing Accuracy t Rt (Note 4) -10 +10 % Internal Clock Accuracy t BASE -10 +10 % ELECTRICAL CHARACTERISTICS: CHARGING (4.5V VDD 16.5V, TA = 0C to +70C, unless otherwise noted.) PARAMETER SYMBOL CONDITIONS MIN TYP MAX UNITS FAST-CHARGE Comparator Threshold VFC FAST-CHARGE -127 -121 -115 mV FAST-CHARGE Comparator Hysteresis VHYS-FC FAST-CHARGE 31 28 25 mV TOPOFF and PRECHARGE Comparator Threshold V TO TOPOFF and PRECHARGE -38 -33 -28 mV TOPOFF and PRECHARGE Comparator Hysteresis VHYS-TO TOPOFF and PRECHARGE 10 8 6 mV Discharge Latch Reset Threshold VDCHG-RST -15 -10 -5 mV Discharge Latch Set Threshold VDCHG-SET Reverse current through sense resistor 5 10 15 mV Low Battery Detect Threshold VLB From presence detect into PRECHARGE 0.95 1.0 1.05 V Cell Detect Threshold V DET 1.50 1.55 1.60 V No Cell Detect Threshold V OPEN 1.60 1.65 1.70 V Presence Detect Threshold Hysteresis VHYS-PD 90 100 110 mV 2.88 2.92 2.96 V Minimum Charge Temp V THM-MIN (Note 5, 6) 0 ºC 1.28 1.32 1.36 V Maximum Charge Temp V THM-MAX (Note 5, 6) 45 ºC 1.12 1.16 1.20 V Over Temp V THM-STOP (Note 5, 6) 50 ºC dT/dt Detect T TERM 0.425 0.5 0.575 ºC/min
DS2715: NiMH Battery Pack Charge Controller 4 of 17 PARAMETER SYMBOL CONDITIONS MIN TYP MAX UNITS dT/dt Blanking Time t BLANK 3.85 4.3 4.75 Min PRECHARGE Timeout t PC-TO 30 Min FAST-CHARGE Timer Range tFC 0.5 6 Hours DS2715 1:2 TOPOFF to FAST- CHARGE Duration Ratio DS2715B 1:4 Note 1: Voltages relative to VSS. Note 2: Does not include current through VCH, RT, and DIV pins. Note 3: Below this voltage no I/O pins are active. Note 4: Does not include tolerance of RT resistor. Note 5: VBIAS and resistor tolerances must be added to determine actual threshold. Note 6: Specified temperature thresholds are only valid if recommended thermistor types are used. Note 7: Specification is guaranteed by design. DETAILED PIN DESCRIPTION PIN NAME DESCRIPTION
1 C BIAS Bypass for Internal Voltage Regulator
2 V CH Cell Stack Charge Control Output
3 V SS Chip Supply Return and Local Ground Reference
4 LED1 Charging Indicator Output
5 DNC Do Not Connect
6 V SS Chip Supply Return and Local Ground Reference
7 CTG Connect to Local Ground (V SS)
8 CTG Connect to Local Ground (V SS)
9 MODE Mode Select. Connect to VSS for linear mode of operation or CBIAS for comparator mode of operation. 10 DIV Thermistor Divider. Stable output to form a resistor divider for measuring temperature on THM 11 SNS+ Positive Current Sense. Connect to the charge source side of the sense resistor 12 SNS- Negative Current Sense. Connect to the cell stack side of the sense resistor 13 V DD Chip Supply Input: +4.5V to +16.5V range 14 R T Failsafe Timeout. Timeout is selected by an external resistor from R T to VSS 15 THM Thermistor Input. Connect to a thermistor located in the cell pack and a divider resistor from the Div pin
16 V BATT
Battery Voltage Sense Input. Connect to a divider from the positive terminal of the cell stack to measure the voltage of a single cell
Figure 1. BLOCK DIAGRAM
Figure 2. STATE DIAGRAM
DS2715: NiMH Battery Pack Charge Controller 7 of 17 DETAILED DESCRIPTION Charge Cycle Overview The DS2715 regulates the charge of up to 10 NiMH cells in a series configuration. With the mode select pin, the DS2715 can be configured to regulate either as an error am plifier in linear mode or as a comparator in switched mode. A charge cycle begins in one of two ways: with the application of power to the DS2715 while the cell pack is already inserted, or with the detection of cell insertion after power-up. After cell presence confirmation, PRECHARGE qualificat ion occurs to prevent fast charging of deeply depleted cells or charging under extreme temperature conditions . Pre-charging is performed at a reduced rate of approximately ¼ the FAST-CHARGE current until each cell reaches 1V. The algorithm then proceeds to the FAST- CHARGE phase. Fast charging continues as long as the cell pack temperature is less than 50 C based on the THM voltage, and the cell voltage as measured at the V BATT pin in the middle of the resistor divider remains below 1.65V, indicating the cell pack is still pr esent. Fast charging terminates no rmally by measuring the cell pack’s thermal rate of change, dT/dt. When the cell pack’s thermal rate of change exceeds 0.5°C per minute, the DS2715 enters TOPOFF. The DS2715 has an internal charge timer as secondary overcharge protection if the charge is not terminated properly by the dT/dt method. The charge termination timer duration is user selectable from 30 minutes up to 6 hours by an external resistor on the RT pin. Following a normally terminated or timed-out FAST-CHARGE phase, the DS2715 enters TOPOFF. It remains in this state for one-half (one-quarter for DS2715B) of the peri od of the FAST-CHARGE timeout as selected by the external resistor on R T. After the TOPOFF charge timer expires, the DONE phase continues indefinitely until the cell pack is removed from the charger or DISCHARGE mode is entered. For the standard application circuit configuration, when a load drawing at least Vdchg-set acro ss the sense resistor is attached to the cell pack, the DS2715 switches to DISCHARGE mode. All charge function s are disabled and the regulation FET is driven on to allow the cell pack to discharge. Throughout the char ging process, the open-drain LED1 output indicates the charge status to the user. Undervoltage Lockout (Reset) The UVLO circuit serves as a power-up and brownout detector by monitoring V DD to prevent charging until V DD rises above VUVLO, or when V DD drops below V UVLO - V UVLO-HYS. If UVLO is active, charging is prevented, the state machine is forced to the RESET state, and all charge timers are reset. A 10 s deglitch circuit provides noise immunity. Once VDD reaches an acceptable operating voltage, the DS2715 enters the PRESENCE state.
DS2715: NiMH Battery Pack Charge Controller 8 of 17 PRESENCE The DS2715 enters the PRESENCE state whenever V DD > V UVLO and V BATT > V OPEN indicating that the charge source is present, but no cell is ava ilable to charge. The DS2715 will remain in the PRESENCE state until a cell is inserted into the circuit causing the voltage on V BATT to fall below 1.55V (V DET) and the cell temperature is inside a valid charging range between 0°C and 45°C (T THM-MIN and T THM-MAX when used with recommended thermistor and resistor values). If both of these co nditions are met, the DS2715 will enter PRE-CHARGE. If cells are inserted, but the temperature is outside the valid charging range, the DS271 5 will remain in the PRESENCE state until the cell temperature falls within the valid charging range. PRE-CHARGE The DS2715 enters the PRECHA RGE state when a valid cell voltage is applied to V BATT and the cell temperature as measured by the DS2715 thermistor circuit is within the valid charging range. PRE-CHARGE mode has a 4 second filter to suppress noise on VBATT caused by cell insertion that may cause a premature return to PRESENCE state. The DS2715 precharges the cell by regulating t he voltage drop across the sense resistor to -33mV (V TO) in linear mode or -29mV (VTO + 0.5 x V HYS-TO) in comparator mode. The polarity of the voltage drop across the sense resistor is referenced to the polarity relationship indicat ed by the SNS+ and SNS- pins on the device. Precharging will last until the cell voltage measured by V BATT exceeds 1.0V (V BATT > V LB), at which time the DS2715 will enter the FAST-CHARGE state. If the cell voltage does not exceed V LB within 30 minutes or if the cell temperature exceeds 50°C at any time during PRECHARGE, the DS 2715 enters the FAULT state. If at any time during PRECHARGE the voltage on V BATT exceeds 1.65V (V OPEN), the DS2715 determines that the cell pack has been removed and returns to the PRESENCE state. FAST-CHARGE In FAST-CHARGE mode, the DS2715 regulates the voltage across the sense resistor to -121mV (V FC) in linear mode or about -107mV (V FC + 0.5 x V HYS-FC) in comparator mode. LED1 indicates the cell pack is being charged. During FAST-CHARGE, the DS2715 constantly measures the rate of change of the cell temperature (dT/dt). When the cell pack’s dT/dt exceeds 0.5°C per minute (TTERM) the DS2715 enters the TOPOFF state. The DS2715 ignores changes in the cell temperature caused by charge initiation for the first 4.3 minutes (t BLANK). As secondary overcharge protection, the DS2715 will terminate F AST-CHARGE and enter TOPOFF based on a time delay set by the external resistor on the R T pin. This resistor value can set the secondary charge termination delay to anywhere from 30 minutes up to 6 hours. If the cell temper ature exceeds 50°C at any time during FAST-CHARGE, the DS2715 enters the DONE st ate. If at any time during FAST-CHARGE the voltage on V BATT exceeds 1.65V (VOPEN), the DS2715 determines that the cell pack has been removed and returns to the PRESENCE state. TOPOFF In TOPOFF mode, the DS2715 regulates the voltage across the sense resistor to -33mV (V TO) in linear mode or - 29mV (VTO + 0.5 x VHYS-TO) in comparator mode. LED1 indicates the cell pack is being charged. The charge timer is reset and restarted with a time-out period of one-half (one-quarter for DS2715B) the fast-charge duration. When the charge timer expires or if the measured temperature exceeds 50°C, the charger enters the DONE state. DONE/Maintenance The DS2715 enters the DONE st ate whenever the charge completes norma lly or if the measured cell temperature exceeds 50°C during the charge. While in the done state V CH is driven to high impedance to prevent further regulated charging of the cell pack and LED is driven high-i mpedance to indicate no charging is taking place. A maintenance charge can be applied to the cells by providing a one-way resistive path from the charge source to the cell pack bypassing the regulating transistor. See the example in the circuit of Figure 3. The DS2715 remains in the DONE state until a cell voltage greater than 1.65V (VOPEN) is detected on VBATT indicating cell pack removal, the Rt pin is floated (see SUSPEND function), or DISCHARGE mode is entered. FAULT The DS2715 enters FAULT if PRECHARGE is unable to charge the cell above 1.0V (V LB) before the 30 minute PRECHARGE timeout (tPC-TO) or if the cell temperature exceeds 50°C during PRECHARGE. In the fault state, V CH is high impedance and LED1 output pulses to indicate the fault condition. The DS2715 remains in FAULT until a cell voltage greater than 1.65V (V OPEN) is detected on V BATT indicating the cell pack has been removed. The DS2715 then enters the PRESENCE state and waits for the next cell insertion. SUSPEND
means to stop charging by the application circuit, such as with a microcontroller signal for example. through the parasitic diode of the PFET regulating tr ansistor until the DS2715 switches to DISCHARGE mode. events occur, the DS2715 enters the PRESENCE state to begin a new charge cycle. pattern to indicate cells are charging. LED1 blinks at 4H z, 50% duty cycle to signal a charging fault has occurred. summarizes the LED operation for each charge condition. Table 2. LED DISPLAY PATTERNS BASED ON CHARGE STATE Cbias pin configures the DS2715 as a comparator for switched mode of operation. loop circuit. During PRECHARGE and TOPOFF, the voltage across the sense resistor is maintained at VTO.
DS2715: NiMH Battery Pack Charge Controller 10 of 17 as a discharge path to the load. Hysteresis on the compar ator input provides the difference between the ON and OFF state thresholds. In a closed-loop regulation circ uit, the comparator regulates voltage across the sense resistor as referenced to the SNS pins to a DC average of: VSNS = VFC + 0.5 x VHYS-FC = -0.107V during FAST-CHARGE VSNS = VTO + 0.5 x VHYS-TO = -0.029V during TOPOFF
Figure 3. Ideal Comparator Input and Charge Control Output Waveforms time-out period. When the timer expires in TOPOFF, the DS2715 enters the DONE state.
thermistors are shown in Table 3. the voltage change on THM exceeds the equivalent TTERM ºC per minute (dT/dt Detect specification). Table 3. THM THRESHOLDS Figure 4. RATIO OF THM PIN TO CBIAS PIN OVER TEMPERATURE
DS2715: NiMH Battery Pack Charge Controller 15 of 17 Cell Stack Size Adjustment R12 and R13 of the application circuits form a voltage divider such that the voltage of a single cell is present on the Vbatt pin. This is required for proper operation of the DS2715. Given a 100k Ω resistor for R13, adjust R12 as follows for the number of cells in the battery pack: R12 = (Number of Cells -1) * R13 To charge 3-cell stacks, a value of 200kΩ is used for R12 for a 100kΩ R13. It is important that the voltage seen by the Vbatt pin is rela tively error free compared to the actual cell voltages in the battery pack. Any parasitic resistances in the connections between the battery cells and the resistor divider will cause errors that will increase with increasing charge current. The error se en by the Vbatt pin is the overall parasitic error divided by the number of cells. So, for a given parasitic resistance, it is more of a concern for circuits with a smaller number of cells. If parasitic resistances of a problematic level cannot be avoided, the connection location of the resistor divider can be manipulated to se nse the true battery voltage, or the divider ratio can be adjusted to account for the sensed voltage error. Application PCB Layout Proper layout rules must be followed to ensure a successful application circuit. For all modes of operation, currents in excess of 1A may flow through the charge and dischar ge paths. All of these paths should be properly sized to handle the worst case current flow, whether that be from charging or from powering the load with the battery. The linear mode of operation in some cases must dissipat e large amounts of heat. This is typically accomplished with either an external heatsink on the regulating trans istor, or through the use of PCB copper to spread the thermal energy that must be removed. Typically, for the TO-220 package transistor used in the application example, a 1 inch square area of 1oz. copper with the transistor firmly attached will have about a 50°C/W temperature rise. Utilizing 2oz. or heav ier copper can improve this number by 20% or so. If better heatsinking is needed for the ergonomic or reliability aspects of the application, an add-on heatsink must be used. Switchmode operation presents its own unique challenges with fast voltage and current transients. Proper switchmode buck power supply layout should always be obse rved. Referring to the example circuit and layout of Figure 7, the loop labeled as Loop1 encompassing C1, C2 , Q1 and D1 should be kept as small as possible to minimize the change in inductance that occurs when Q1 switches to the on state. Loop2 should also be minimized as much as practical, although it contains DC current components for the most part. The returning ground currents should be allowed to follow a path on a layer directly under the outgoing path since the high frequency components will try to follow the path of least impedance. Low ESR and ESL capacitors should be used when possible and for all capacitors 10uF and smaller. Typical surface mount ceramic types with a X5R or better dielectric are recommended.
DS2715: NiMH Battery Pack Charge Controller 16 of 17 Figure 7. Switching Circuit with Example Layout recommended connection of the sense lines to the resistor footprint. Figure 8. Sense Resistor Connection Layout
Package Information
For the latest package outline information and land patterns, go to www.maxim-ic.com/packages. PACKAGE TYPE PA CKAGE CODE DOCUMENT NO.
16 SO — 21-0041
DS2715: NiMH Battery Pack Charge Controller 17 of 17
REVISION HISTORY
011408 Various comprehensive changes. 1, 3, 4, 6–15
031609 Added device versi on DS2715B 1,4,7,8,10
042809 Updated Ordering Information table. 1