LTC3226 2-Cell Supercapacitor Charger with Backup PowerPath Controller DESCRIPTION FEATURES 1x/2x Multimode Charge Pump Supercapacitor Charger n Automatic Cell Balancing n Ideal Diode Main PowerPath™ Controller (VIN to VOUT) n Internal 2A LDO Backup Supply (CPO to V OUT) n Automatic Main/Backup Switchover n Input Voltage Range: 2.5V to 5.5V n Programmable SCAP Charge Voltage n Programmable Input Current Limit (315mA Max) n No Load I VIN = 55μA (Typical) n Low Profile, 16-Lead 3mm × 3mm QFN Package The LTC®3226 is a 2-cell series supercapacitor charger with a backup PowerPath controller. It includes a charge pump supercapacitor charger with programmable output voltage, a low dropout regulator, and a power-fail comparator for switching between normal and backup modes. n The constant input current supercapacitor charger is designed to charge two supercapacitors in series to a resistor-programmable output voltage from a 2.5V to 5.3V input supply. The charger input current limit is programmable by an external resistor at up to 315mA. The internal backup LDO is powered from the supercapacitors and provides up to 2A peak output current with an adjustable output voltage. When the input supply falls below the power-fail threshold, the LTC3226 automatically enters a backup state in which the supercapacitors power the output through the LDO. The input supply power-fail voltage level is programmed by an external resistor divider. APPLICATIONS n n n n n n Smart Power Meters Battery-Powered Industrial/Medical Equipment 3.3V Solid-State Drives Industrial Alarms Data Backup Supplies Battery Hold-Up Supplies Low input noise, low quiescent current and a compact footprint make the LTC3226 ideally suited for small, batterypowered applications. Internal current limit and thermal shutdown circuitry allow the device to survive a continuous short-circuit from the PROG or CPO pins to ground. L, LT, LTC, LTM, Burst Mode, Linear Technology and the Linear logo are registered trademarks and PowerPath is a trademark of Linear Technology Corporation. All other trademarks are the property of their respective owners. TYPICAL APPLICATION 3.3V Backup Supply MPEXT LTC3226 VIN LDO 2.2µF 1.96M C 2.2µF 255k RST_FB + – + C– 5V VMID CSC 1.2F CHARGE PUMP CSC = 1.2F COUT = 47µF ILOAD = 2A CPO 4 VOUT 3 VIN 2 BACKUP BACKUP MODE MODE (LDO IN (LDO IN REGULATION) DROPOUT) 0 –1 1.21M GND 5 COUT 47µF 1 3.83M CPO_FB EN_CHG PFO RST CAPGOOD 80.6k CPO VIN PROG 33.2k 6 VOUT LDO_FB 1.2V PFI 1.21M GATE VOLTAGE (V) VIN 3.3V Automatic Normal-to-Backup Mode Switchover TO LOAD (2A) PFO (2V/DIV) 0 0.4 0.8 1.2 TIME (SECONDS) 1.6 2.0 3226 TA01b 3226 TA01a 3226f 1 LTC3226 PIN CONFIGURATION VIN, VOUT, VMID, CPO, RST, PFO, CAPGOOD, LDO_FB...................................... –0.3V to 6V EN_CHG, PFI, RST_FB, CPO_FB Voltage.............–0.3V to Max (VIN, CPO) + 0.3V Operating Junction Temperature................. –40 to 125°C Storage Temperature Range.......................–65 to 150°C VIN VMID C+ CPO TOP VIEW 16 15 14 13 12 C– VOUT 1 PFO 2 11 CAPGOOD 17 GND PFI 3 10 CPO_FB LDO_FB 4 7 8 RST 6 EN_CHG 9 5 GATE (Note 1) RST_FB ABSOLUTE MAXIMUM RATINGS PROG UD PACKAGE 16-LEAD (3mm × 3mm) PLASTIC QFN TJMAX = 125°C, θJA = 58.7°C/W (NOTE 2) EXPOSED PAD (PIN 17) IS GND, MUST BE SOLDERED TO PCB ORDER INFORMATION LEAD FREE FINISH TAPE AND REEL PART MARKING* PACKAGE DESCRIPTION TEMPERATURE RANGE LTC3226EUD#PBF LTC3226EUD#TRPBF LFZV 16-Lead (3mm × 3mm) Plastic QFN –40°C to 125°C LTC3226IUD#PBF LTC3226IUD#TRPBF LFZV 16-Lead (3mm × 3mm) Plastic QFN –40°C to 125°C Consult LTC Marketing for parts specified with wider operating temperature ranges. *The temperature grade is identified by a label on the shipping container. Consult LTC Marketing for information on non-standard lead based finish parts. For more information on lead free part marking, go to: http://www.linear.com/leadfree/ For more information on tape and reel specifications, go to: http://www.linear.com/tapeandreel/ 3226f 2 LTC3226 ELECTRICAL CHARACTERISTICS The l denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25°C (Note 3). VIN = 3.3V, VCPO = 5V, VOUT = 3.3V, VMID = 1/2 VCPO unless otherwise noted. SYMBOL PARAMETER VIN Input Supply Range CONDITIONS MIN l TYP 2.5 MAX 5.5 UNITS V IVIN(ST) VIN Quiescent Current in Normal Mode VPFI > 1.2V, VCPO_FB > 1.2V, VIN < VCPO 10 µA ICPO(ST) CPO Quiescent Current in Normal Mode VPFI > 1.2V, VCPO_FB > 1.2V, VIN < VCPO 20 µA IVOUT(ST) VOUT Quiescent Current in Normal Mode VOUT = VIN, VPFI > 1.2V, VCPO_FB > 1.2V, VIN < VCPO 5 µA ICPO(BU) CPO Quiescent Current in Backup Mode VPFI < 1.2V, VLDO_FB > 0.8V, VCPO > VOUT 24 µA IVOUT(BU) VOUT Quiescent Current in Backup Mode VIN = 0V, VPFI < 1.2V, VLDO_FB > 0.8V, VCPO > VOUT 3 µA Ideal Diode Controller VFWD(EDA) External Ideal Diode Forward Voltage (VIN-VOUT) IVOUT = 2mA 15 mV VRTO Fast Turn-Off Voltage (VIN-VOUT) VIN Falling –45 mV Fast Turn-On Voltage (VIN-VOUT) VOUT Falling 45 mV Charge Pump Supercapacitor Charger VCPO_FB CPO_FB Pin Threshold for Entering Sleep Mode l 1.18 CPO_FB Pin Hysteresis for Exiting Sleep Mode ICPO_FB Charge Pump FB Pin Input Leakage fOSC CLK Frequency ROL 1.21 1.24 20 VCPO_FB = 1.3V l –50 0.75 50 0.9 V mV 1.05 6 nA MHz VPROG Effective Open-Loop Output Impedance (Note 4) VCPO = 4.5V, CFLY = 1µF PROG Pin Servo Voltage VCPO_FB < 1.2V IVIN(ILIM) Input Current Limit RPROG = 33.3k, VCPO = 0V 360 mA hPROG Ratio of VIN Input Current Limit to PROG Pin Current RPROG = 33.3k, VCPO = 0V 10,500 A/A ICHRG(1x) CPO Pin Charging Current (1x Mode) VIN = 3.8V, RPROG = 33.3k, VCPO = 3V 315 mA 157.5 mA 600 mA l 0.98 1.0 Ω 1.02 V ICHRG(2x) CPO Pin Charging Current (2x Mode) RPROG = 33.3k ISC Short-Circuit Charge Current PROG Pin Grounded, VCPO = 0V VMODE VIN to CPO Voltage Differential for Switching Mode from 1x to 2x 200 mV 1x/2x Mode Comparator Hysteresis 120 mV Maximum Voltage Across Either Supercapacitor After Charging l VSTACK Maximum Supercapacitor Stack Voltage l VMID VMID Output Voltage VCLAMP VMID Current Sourcing Capability VMID < VCPO/2, VCPO_FB > 1.24V VMID Current Sinking Capability VMID > VCPO/2, VCPO_FB > 1.24V CPO_FB Pin Threshold Voltage (Rising) for CAPGOOD CPO_FB Pin Hysteresis for CAPGOOD ISINK = 5mA CAPGOOD High Impedance Leakage Current VCAPGOOD = 5V 2.75 V 5.3 5.5 V 2.5 V 4.5 mA 5.5 l CAPGOOD Output Low Voltage 2.65 l 1.09 1.11 mA 1.13 V 20 mV 65 mV 1 µA 3226f 3 LTC3226 ELECTRICAL CHARACTERISTICS The l denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25°C (Note 3). VIN = 3.3V, VCPO = 5V, VOUT = 3.3V, VMID = 1/2 VCPO unless otherwise noted. SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS 0.8 0.82 V LDO Minimum CPO Voltage for LDO Operation VLDO_FB IVOUT = 1mA Load Regulation ΔVLDO_FB/ΔIOUT 1mA < IVOUT < 2A LDO FET RDS(ON) ILDO_FB(LEAK) LDO_FB Input Leakage Current ILIM 2.4 LDO FB Servo Voltage l 0.76 V 2.7 VCPO = 3.6V mV/A 200 VLDO_FB = 0.9V l LDO Current Limit –60 mΩ 60 2 4 l 0.72 0.74 l –50 nA A RST_FB, RST VRST_FB(TH) RST_FB Threshold (Falling Edge) VRST_FB(HYS) RST_FB Hysteresis IRST_FB(LEAK) RST_FB Input Leakage Current 0.76 20 VRST_FB = 0.9V RST Output Low Voltage ISINK = 5mA RST High Impedance Leakage Current VRST = 5V mV 50 65 RST Delay (RST_FB Rising) nA mV 1 l V 290 µA ms Power-Fail Comparator VPFI(TH) PFI Input Threshold (Falling Edge) VPFI(HYS) PFI input Hysteresis IPFI(LEAK) PFI Input Leakage Current VPFI = 0.5V PFO Output Low Voltage ISINK = 5mA PFO High Impedance Leakage Current VPFO = 5V IPFO(LEAK) l 1.175 1.2 1.225 20 l –50 mV 50 65 nA mV 1 l PFI Delay to PFO (PFI Falling) V 0.5 µA µs EN_CHG VIH Input High Voltage l VIL Input Low Voltage l IIH Input High Current l IIL Input Low Current l Note 1: Stresses beyond those listed under Absolute Maximum Ratings may cause permanent damage to the device. Exposure to any Absolute Maximum Rating condition for extended periods may affect device reliability and lifetime. Note 2: Failure to solder the exposed backside of the package to the PC board ground plane will result in a thermal resistance much greater than 58.7°C/W. Note 3: The LTC3226E is guaranteed to meet specifications from 0°C to 85°C junction temperature. Specifications over the –40°C to 125°C operating junction temperature range are assured by design, characterization and correlation with statistical process controls. The LTC3226I is guaranteed over the full –40°C to 125°C operating junction temperature range. The junction temperature, TJ, is calculated from the ambient temperature, TA, and power dissipation, PD, according to the formula: TJ = TA + (PD • 58.7°C/W) 1.3 V 0.4 V –1 1 µA –1 1 µA Note that the maximum ambient temperature, consistent with these specifications, is determined by specific operating conditions in conjunction with board layout, the rated thermal package thermal resistance and the environmental factor. Note 4: Output not in regulation; ROL = (2 • VIN – VCPO)/ICPO. 3226f 4 LTC3226 TYPICAL PERFORMANCE CHARACTERISTICS LDO Load Regulation 3.310 CPO = 4V VOUT SET TO 3.3V VOUT (V) 3.290 3.285 3.280 3.325 IOUT = 10µA VOUT SET TO 3.3V 3.305 LDO OUTPUT VOLTAGE (V) 3.295 LDO Regulation Voltage vs Temperature LDO Supply Regulation 3.315 3.300 3.295 3.270 0.001 0.01 0.1 IOUT (A) 1 3.290 3226 G01 3.285 –45 –25 15 35 55 75 TEMPERATURE (°C) LDO Output Transient Step Response Waveform 980 LOAD = 500mA T = 25°C 200 150 VOUT 50mV/DIV AC-COUPLED T = –40°C 100 IOUT 50 0 2.2 4.2 3.7 3.2 CPO VOLTAGE (V) 2.7 FREQUENCY (kHz) 250 1000mA VIN = 3.3V T = –45°C 940 T = 25°C 920 900 880 T = 90°C 860 500mA 840 3226 G05 500µs/DIV 5.2 4.7 3.6 3.8 4.0 4.2 4.4 4.6 CPO VOLTAGE (V) 4.8 Charge Pump Input Current vs CPO Output Voltage 400 VIN = 3.8V 350 300 RPROG = 50k 250 200 RPROG = 100k 150 100 50 0 1x MODE 0 1 2 3 CPO VOLTAGE (V) RPROG = 33.3k 4 5 3226 G07 400 VIN = 3.8V 390 300 200 RPROG = 100k 150 370 360 350 100 0 RPROG = 33.2k VCPO = 0V 380 RPROG = 50k 250 50 2x MODE Charge Pump Input Current vs VIN with CPO Grounded IIN (mA) INPUT CURRENT LIMIT (mA) 350 Charge Pump Charging Current vs CPO Output Voltage CPO CHARGE CURRENT (mA) RPROG = 33.3k 5.0 3226 G06 3226 G04 400 115 Charge Pump Oscillator Frequency vs VCPO and Temperature 960 T = 90°C 95 3226 G03 300 RDS(ON) (mΩ) –5 3226 G02 LDO FET On-Resistance vs CPO Voltage and Temperature 350 3.305 3.295 3.280 3.4 3.6 3.8 4.0 4.2 4.4 4.6 4.8 5.0 5.2 CPO VOLTAGE (V) 10 3.310 3.300 3.290 3.285 3.275 VCPO = 5V IOUT = 1mA 3.320 VOUT (V) 3.300 TA = 25°C, unless otherwise noted. 1x MODE 0 1 2 3 CPO VOLTAGE (V) 4 90°C 25°C –45°C 340 2x MODE 5 3226 G08 330 3.3 3.6 3.9 4.2 VIN (V) 4.5 4.8 5.1 3226 G09 3226f 5 LTC3226 TYPICAL PERFORMANCE CHARACTERISTICS VMID Shunt Regulator Voltage vs Current and Temperature 2.70 12 VIN = 3.3V, VCPO = 5V 10 VMID SHUNT REG OFF 2.65 VMID SOURCING CURRENT 6 4 VMID VOLTAGE (V) VMID CURRENT (mA) 8 90°C 25°C –45°C 2 0 –2 VMID SINKING CURRENT –4 –6 –8 0 0.5 1 1.5 2 2.5 3 3.5 VMID VOLTAGE (V) 4 5 ONLY LEAKAGE BALANCER ACTIVE 2.60 VIN = 3.3V VCPO = 5V 5 1 10 100 VMID CURRENT (mA) Charge Profile When Top Capacitor = Bottom Capacitor VOLTAGE (V) VOLTAGE (V) 4 VIN VMID 2 1 VIN 3 –50 0 50 VIN – VOUT (mV) 100 3226 G12 1x MODE 2x MODE VMID TIME (5 SECONDS/DIV) 1x MODE 0 RPROG = 33.2k CSC = 1.2F –1 3226 G13 CPO 2 Charge Profile When Top Capacitor < Bottom Capacitor VMID CLAMPED 2x MODE (SHUNT ACTIVE) RPROG = 33.2k, CSC = 1.2F EXTRA 1.5F BETWEEN CPO AND VMID 3226 G14 TIME (5 SECONDS/DIV) Normal-to-Back-Up Mode Switching Transient Waveform 4 6 VOUT 5 CPO VIN 3 VMID 2 1 1x MODE 2x MODE VIN 3 TOP CAP CLAMPED RPROG = 33.2k, CSC = 1.2F, EXTRA 1.5F BETWEEN VMID AND GND TIME (5 SECONDS/DIV) 3226 G15 VOLTAGE (V) VOLTAGE (V) –4 –5 –100 1000 1 –1 –1 –3 5 CPO 3 0 FAST ON 6 5 4 FORWARD VOLTAGE Charge Profile When Top Capacitor > Bottom Capacitor 6 0 0 –1 3226 G11 3226 G10 4 1 –2 90°C 25°C –45°C 2.45 4.5 LINEAR REGION 3 2.50 2.40 0.1 FAST OFF 2 SHUNT REGULATOR ACTIVE 2.55 Ideal Diode Gate Current vs (VIN-VOUT) 4 IGATE (µA) Leakage Balancer Source and Sink Capability TA = 25°C, unless otherwise noted. 2 PFO (1V/DIV) 1 0 CSC = 1.2F COUT = 47µF ILOAD = 2A 0 100 200 300 TIME (µs) 400 500 3226 G16 3226f 6 LTC3226 PIN FUNCTIONS VOUT (Pin 1): Voltage Output. This pin is used to provide power to an external load from either the primary input supply (VIN) or the supercapacitor (CPO) if the primary input supply is not available. VOUT should be bypassed with a low ESR ceramic capacitor of at least 47µF capacitance to GND. PFO (Pin 2): Open-Drain Power-Fail Status Output. This pin is pulled to ground by an internal N-channel MOSFET when the PFI input is below 1.2V. Once the PFI input recovers, this pin becomes high impedance. PFI (Pin 3): Power-Fail Input. High impedance input to an accurate comparator with a 1.2V falling threshold and 20mV hysteresis. This pin controls the state of the PFO output pin and the operating mode of the LTC3226. LDO_FB (Pin 4): Internal LDO Feedback Pin. The voltage on this pin is compared to the internal reference voltage (0.8V) by the error amplifier to keep the output in regulation. An external resistor divider is required between VOUT, LDO_FB and GND to program the LDO output voltage. See the Applications Information section. GATE (Pin 5): External FET Gate Pin. This pin is driven by an internal ideal diode controller to regulate VOUT to 15mV below VIN. RST_FB (Pin 6): Reset Comparator Input. High impedance input to an accurate comparator with a 0.74V falling threshold and 20mV hysteresis. This pin controls the state of the RST output pin. An external resistor divider is required between VOUT, RST_FB and GND. It can be the same resistor divider as the LDO_FB divider. See the Applications Information section. RST (Pin 7): Open-Drain Status Output of the RESET Comparator. This pin is pulled to ground by an internal N-channel MOSFET whenever the RST_FB pin voltage falls below 0.74V. Once the RST_FB pin voltage recovers, the pin becomes high impedance after a 290ms delay indicating that VOUT is within 7.5% of its programmed value. EN_CHG (Pin 8): Enable Pin for the Charge Pump Supercapacitor Charger with an Internal Pull-Up. Tie this pin to a voltage below 0.4V to disable the internal charge pump. PROG (Pin 9): Charger Input Current Limit Programming Pin. A resistor connected between this pin and GND sets the input current limit for the charger. See the Applications Information section. CPO_FB (Pin 10): Feedback Pin for the Charge Pump. The voltage on this pin is compared to the internal reference voltage (1.2V) to keep the charge pump output CPO in regulation. An external resistor divider is required between CPO, CPO_FB and GND to program the CPO output voltage. See the Applications Information section. CAPGOOD (Pin 11): Open-Drain Status Output of the CPO Voltage. This pin is pulled to ground by an internal N-channel MOSFET until CPO_FB pin reaches 1.11V. Once the CPO_FB pin exceeds 1.11V, this pin becomes high impedance indicating that the CPO voltage is within 7.5% of its target value. C– (Pin 12): Internal Charge Pump Flying Capacitor Negative Terminal. VIN (Pin 13): Primary Input Supply. This pin supplies power to the VOUT pin through an external P-channel MOSFET and also to the supercapacitors attached to the CPO and VMID pins. VIN should be bypassed to GND with a low ESR ceramic capacitor of at least 2.2μF depending on the load transient. VMID (Pin 14): Midpoint of Two Series Supercapacitors. C+ (Pin 15): Internal Charge Pump Flying Capacitor Positive Terminal. A 1μF to 10μF X5R or X7R ceramic capacitor should be connected from C+ to C–. CPO (Pin 16): Backup Supply Pin. Connect CPO to the top plate of the top supercapacitor. This pin receives power from VIN through an internal charge pump doubler and supplies power to VOUT through an internal LDO when the primary input supply has failed. GND (Exposed Pad Pin 17): Ground. The exposed pad should be connected to a continuous ground plane on the second layer of the printed circuit board by several vias directly under the part to achieve optimal thermal performance. 3226f 7 LTC3226 BLOCK DIAGRAM EXTERNAL PFET 5 GATE VIN 2.5V TO 5.5V 13 2 RPF1 3 15mV VIN +– RPF2 1.2V VOUT COUT IDEAL DIODE CONTROLLER PFO PFI – + – LDO_FB LDO + + – – RST_FB 4 RFB1 6 RFB2 0.8V + 0.74V RST 7 DELAY CPO 8 EN_CHG VMID 1x/2x MODE CHARGE PUMP + CPO_FB – 9 – 2.65V CLAMP/ BALANCER 1V 1.11V CAPGOOD 16 14 10 CSC RCP1 RCP2 11 + PROG RPROG GND 17 C+ 15 C– 12 3226 F01 2.2µF Figure 1. LTC3226 Block Diagram 3226f 8 LTC3226 OPERATION The LTC3226 is a 2-cell series supercapacitor charger designed to back up a Li-ion battery or any system rail in the range of 2.5V to 5.3V. It has four principal circuit components: a dual mode (1x/2x) charge pump with an integrated balancer and a voltage clamp, an LDO to supply the load current from the charge stored on the supercapacitor, an ideal diode controller to control the gate of the external FET between VIN and VOUT, and a PFI comparator to decide whether to activate the charge pump to charge the supercapacitor stack or to activate the LDO to supply the load when VIN falls below an externally programmed value. The LTC3226 has two modes of operation: normal and backup. If VIN is above an externally programmable PFI threshold voltage, the part is in normal mode in which power flows from VIN to VOUT through the external FET and the internal charge pump stays on to top off the supercapacitor stack. If VIN is below this PFI threshold, the part is in backup mode. In this mode, the internal charge pump is turned off, the external FET is turned off and the LDO is turned on to supply the load current from the stored charge. CHARGE PUMP One of the principal circuit components of the LTC3226 is a dual mode low noise constant frequency (0.9MHz) regulated charge pump which transfers charge from VIN and stores it onto the supercapacitor stack at the CPO pin. The target or termination voltage on the CPO pin is programmed by an external resistor divider using the CPO_FB pin. The input current limit to the charger is programmed by an external resistor between the PROG pin and ground. The charge pump turns on when VIN exceeds the externally programmable PFI threshold. At the beginning of the charge cycle when the CPO pin voltage is less than VIN, the charge pump is in 1x mode (linear mode) in which the charge pump acts as a pass element and charges the supercapacitor with a charge current that is limited by the programmed input current limit. As the CPO voltage rises to within 200mV of the input supply voltage, the charge pump switches to 2x mode (doubler mode) in which the average charge current is approximately equal to half the input current limit. As the CPO voltage exceeds the target value by approximately 1%, the charge pump switches turn off and the charge pump enters the sleep mode. In sleep mode, most of the charge pump control circuitry is turned off to minimize quiescent current. As the supercapacitor discharges due to leakage and internal quiescent current load, the CPO pin voltage slowly drops. When the CPO pin voltage drops 1% below the programmed voltage, the charge pump turns on to replenish charge on the supercapacitor and the cycle continues. The charge pump can be turned off by pulling the EN_CHG pin below 0.4V. However, by default, the charge pump is always enabled via an internal low current pull-up circuit if the EN_CHG pin is left floating. Voltage Clamp The LTC3226 charge pump is equipped with circuitry to limit the voltage across any supercapacitor in the stack to a maximum allowable preset voltage of 2.65V. If the voltage across the top capacitor (VMID-VCPO) ever gets to 2.65V before the CPO pin reaches the target voltage, the charge pump stops charging the top of the stack via the CPO pin, switches to 1x mode and delivers charge directly to the bottom capacitor via the VMID pin until the stack voltage reaches its programmed value. If the voltage across the bottom capacitor reaches 2.65V before the stack gets to its target value, the charge pump continues to deliver charge to the top of the stack via the CPO pin and a shunt regulator turns on to bleed charge off of the bottom capacitor and prevents the VMID pin voltage from rising any further. The shunt regulator is able to shunt the maximum allowable charge current which is approximately 315mA (in 1x mode). In the event both capacitors exceed 2.65V, the charge pump enters sleep mode by turning off most of its circuitry. Leakage Balancer The LTC3226 is equipped with an internal leakage balancing amplifier which servos the VMID pin voltage to exactly half of the CPO pin voltage. However, it has limited source (~4.5mA) and sink (~5.5mA) capability. It is designed to handle slight mismatch of the supercapacitors due to leakage currents; not to correct any gross mismatch due to defects. The balancer is only active as long as the input supply voltage is above the PFI threshold. The internal balancer eliminates the need for external balancing resistors. 3226f 9 LTC3226 OPERATION CAPGOOD Status Output The LTC3226 charge pump includes a comparator to report the status of the supercapacitors via an open-drain NMOS transistor on the CAPGOOD pin. This pin is pulled to ground until the CPO pin voltage rises to within 7.5% of the programmed value. Once the CPO pin is above this threshold, the CAPGOOD pin goes high impedance. PROG Pin Short-Circuit Protection Typically the maximum current that the LTC3226 charge pump can deliver is set by the PROG resistor. However, if for any reason, the PROG pin gets shorted to GND, or the user chooses a PROG resistor value which is far smaller than recommended, the charge pump input current is limited to an internally set value of approximately 600mA. Also the maximum current that can be sourced from the PROG pin is limited by an internal resistor to less than 1mA. LOW DROPOUT REGULATOR (LDO) Another principal circuit component of the LTC3226 is the low dropout regulator (LDO) which transfers power from the supercapacitor stack to VOUT through a pass element with an RDS(ON) of approximately 200mΩ. This LDO has a current limit internally set to 4A. In the event that the input supply voltage falls below the PFI threshold, the PFI comparator promptly turns on the LDO to supply the necessary load without letting the VOUT rail droop too much. However, to prevent unrestricted current flow from the input to the supercapacitors through the ideal diode, the LDO is turned off until the CPO voltage is greater than VIN by 100mV typical. The LDO output voltage is programmed through an external resistor divider via the LDO_FB pin. POWER-FAIL (PFI) COMPARATOR The LTC3226 contains a fast comparator which switches the part from normal to backup mode in the event the input voltage, VIN, falls below an externally programmed threshold voltage. In backup mode, the charge pump shuts off and the LDO powers the load as long as there is enough charge stored on the supercapacitors. The PFI threshold voltage is programmed by an external resistor divider via the PFI pin. The output of the PFI comparator also drives the gate of an open-drain NMOS to report the status via the PFO pin. In normal mode, the PFO pin is high Impedance but in backup mode, the pin is pulled down to ground. IDEAL DIODE CONTROLLER The LTC3226 contains an ideal diode controller which controls the gate of an external PFET connected between the input, VIN, and the output, VOUT, through the GATE pin. Under normal operating conditions, this external FET constitutes the main power path from input to output. For very light loads, the controller maintains a 15mV delta across the FET between the input and output voltage. In the event VIN suddenly drops below VOUT, the controller quickly turns the FET completely off to prevent any reverse conduction from VOUT back to the input supply. RESET COMPARATOR The LTC3226 contains a RESET comparator which monitors VOUT under all operating modes via the RST_FB pin and reports the status via an open-drain NMOS transistor on the RST pin. At any time, if VOUT falls 7.5% from its programmed value, it pulls the RST pin low almost instantaneously. However, on the rising edge the comparator waits 290ms after VOUT crosses the threshold before making the RST pin high impedance. GLOBAL THERMAL SHUTDOWN The LTC3226 includes a global thermal shutdown which shuts down the entire part in the event the die temperature exceeds 152°C. It resumes normal operation once the temperature drops by about 15°C to approximately 137°C. 3226f 10 LTC3226 APPLICATIONS INFORMATION Programming the Supercapacitor Termination Voltage (CPO) Programming the Input Voltage Threshold for the Power-Fail Comparator The termination voltage of the supercapacitor stack on the CPO pin can be programmed for any voltage between 2.5V to 5.3V by using a resistor divider from the CPO pin to GND via the CPO_FB pin such that: The input voltage threshold below which the power-fail status pin PFO indicates a power-fail condition and the LTC3226 switches the internal LDO on can be programmed by using a resistor divider from the VIN pin to GND via the PFI pin such that: R VCPO = VCPO _ FB • 1+ CP1 RCP2 where VCPO_FB is 1.2V. See the block diagram in Figure 1. Typical values for RCP1 and RCP2 are in the range of 40k to 5M. Programming the Input Current Limit for the Charger The input current limit for the LTC3226 charge pump is programmed by using a single resistor from the PROG pin to ground. The input current limit is typically 10,500 times the current out of the PROG pin. The PROG pin voltage always servos to 1V as long as the part is not in sleep mode. The program resistor and the input current limit are calculated using the following equations: RPROG = 10,500 • 1V IVIN(ILIM) , IVIN(ILIM) = 10,500 • 1V RPROG where IVIN(ILIM) is the input current limit for the charge pump charger. The maximum allowable input current limit of 315mA can be achieved by using a PROG resistor of 33.2k. To maximize the charge transfer rate, the charge pump operates in 1x mode when the supercapacitor voltage is less than the input voltage and the charge current out of CPO pin is only limited by the programmed input current limit. However, in 2x mode, the average charge current is approximately half the input current limit. R VIN(PFO _ HI_ LO) = VPFI • 1+ PF1 RPF2 where VPFI is 1.2V. See Figure 1. Typical values for RPF1 and RPF2 are in the range of 40k to 5M. For a smooth transition from normal to back-up mode, the PFI threshold should be set 50mV to 100mV above the programmed LDO output voltage, VOUT. The input voltage above which the power-fail status pin PFO is high impedance and the supercapacitor charger and the ideal diode are enabled is: R VIN(PFO _ LO _ HI) = VPFI + VPFI(HYS) • 1+ PF1 RPF2 ( ) where VPFI(HYS) is the hysteresis of the PFI comparator. It is internally set to a typical value of 20mV. The hysteresis can be increased externally by adding a resistor, RH, in series with a diode, D1, between the PFO and PFI pins as shown in Figure 2. This network will increase the low-to-high VIN threshold for PFO while keeping the high-to-low threshold intact. The increase in hysteresis at the input can be calculated as shown: ( ) ∆VIN(HYS) = VPFI + VPFI(HYS) – VF • RPF1 RH 3226f 11 LTC3226 APPLICATIONS INFORMATION where VF is the forward voltage of the diode. As an example, if RPF1 = 200k, RPF2 = 120k, RH = 2M, and VF = 0.4V, the additional hysteresis provided by this network can be calculated using the above equation as follows: ∆VIN(HYS) = (1.2 + 0.02 – 0.4) V • 200kΩ = 82mV 2MΩ The threshold for the reset comparator can be programmed by using a resistor divider from the VOUT pin to GND via the RST_FB pin such that: VOUT VIN RPF1 RH 470k LTC3226 PFI RPF2 PFO D1 3226 F02 Figure 2. Increasing PFI Comparator Hysteresis Externally Programming the LDO Output Voltage (VOUT) The LDO output voltage in backup mode can be programmed for any voltage between 2.5V to 5.3V by using a resistor divider from the VOUT pin to GND via the LDO_FB pin such that: Programming the Reset Threshold R VOUT = VLDO _ FB • 1+ FB1 RFB2 where VLDO_FB is 0.8V. See the Block Diagram in Figure 1. Typical values for RFB1 and RFB2 are in the range of 40k to 500k. Too small a resistor will result in a large quiescent current whereas too large a resistor coupled with LDO_FB pin capacitance will create an additional pole and may cause loop instability. R VOUT = VRST _ FB • 1+ FB1 RFB2 where VRST_FB is 0.74V. See Figure 1. Typical values for RFB1 and RFB3 are in the range of 40k to 5M. In most applications, the LDO_FB and RST_FB pins can be shorted together and only one resistor divider between VOUT and GND is needed to set VOUT and the reset threshold 7.5% below the VOUT programmed voltage. However, the reset threshold can be set independent of VOUT by an additional resistor divider. Effective Open-Loop Output Resistance (ROL) of the Charge Pump The effective open-loop output resistance (ROL) of a charge pump determines the strength of a charge pump. The value of this parameter depends on many factors such as the oscillator frequency (fOSC), value of the flying capacitor (CFLY), the nonoverlap time, the internal switch resistances (RS), and the ESR of the external capacitors. A first order approximation of ROL is given below: ROL ≅ 2 Σ RS + S=1to 4 1 fOSC • CFLY 3226f 12 LTC3226 APPLICATIONS INFORMATION For the LTC3226 charge pump, the sum of the switch resistances is approximately 2.5Ω in a typical application where VIN = 3.3V and VCPO = 5V. For CFLY = 1µF and fOSC = 1MHz, the effective open-loop output resistance of the charge pump can be approximated from the above equation as follows: ROL ≅ 2 • 2.5Ω + 1 = 6Ω 1MHz • 1µF Maximum Available Charge Current In the absence of any internal current limit, the maximum available current out of a charge pump in 2x mode can be calculated from the charge pump input and output voltage and the effective open-loop output resistance ROL using the following equation: ICHRG = 2VIN – VCPO ROL For example, if the LTC3226 charge pump (ROL ≅ 6Ω) has to charge a supercapacitor to 5V from 2.5V input, the charge current available when VCPO = 4.8V can be calculated as follows: ICHRG = 2 • 2.5V – 4.8V = 33.3mA 6Ω So even if the charge pump input current limit is programmed for 315mA (RPROG = 33.2k), the actual charge current will be considerably less than 157.5mA (half of programmed limit) in 2x mode for very low input supply. For VIN = 2.5V, the CPO voltage above which the charge current will decrease from the programmed value of 157.5mA (RPROG = 33.2k) can be calculated from the previous equation as follows: VCPO = (2 • 2.5V – 157.5mA • 6Ω) = 4.055V Choosing the LDO Output Capacitor In the event VIN falls below the programmed PFI threshold, the PFI comparator turns off the charge pump and turns on the internal LDO to supply the load from the backedup supercapacitor storage. However, due to the delay associated with the PFI comparator and LDO circuitry, it could be up to 2μs before the LDO is capable of supplying the load demand at VOUT. In order to prevent VOUT from drooping too much during this transition, a 47μF ceramic capacitor is recommended at the VOUT terminal. For any output capacitance, COUT, delay, Δt, and load current, ILOAD, the drop in VOUT, ∆V, can be calculated using the following equation: ILOAD = COUT • ∆V ∆t For example, if VOUT can not tolerate more than 100mV drop under a maximum load of 2A during this transition, the minimum capacitance required at the LDO output can be calculated using the above equation as follows: COUT(MIN) = 2A • 2µs = 40µF 100mV 3226f 13 LTC3226 APPLICATIONS INFORMATION Charging a Single Supercapacitor The LTC3226 can also be used to charge a single supercapacitor by connecting two series-connected matched ceramic capacitors with a minimum capacitance of 100µF in parallel with the supercapacitor as shown in Figure 3. Supercapacitor Manufacturers Refer to the following table for supercapacitor manufacturers. Table 1. Supercapacitor Manufacturers CAP-XX www.cap-xx.com NESS CAP www.nesscap.com Maxwell www.maxwell.com Bussmann www.cooperbussmann.com AVX www.avx.com Illinois Capacitor www.illinoiscapacitor.com Tecate Group www.tecategroup.com CPO LTC3226 C1 CSUP VMID C2 GND 3226 F03 Figure 3. Charging a Single Supercapacitor Board Layout Considerations Due to high switching frequency and high transient currents produced by the LTC3226 charge pump, careful board layout is necessary for optimum performance. A true ground plane and short connections to all of the external capacitors will improve performance. Also, to be able to deliver maximum load current from the LDO under all conditions, it is critical that the exposed metal pad on the backside of the QFN package has a good thermal contact to the PC board ground plane. Lack of proper thermal contact can cause the junction temperature to exceed the threshold for thermal shutdown. 3226f 14 LTC3226 PACKAGE DESCRIPTION UD Package 16-Lead Plastic QFN (3mm × 3mm) (Reference LTC DWG # 05-08-1691) 0.70 ±0.05 3.50 ± 0.05 1.45 ± 0.05 2.10 ± 0.05 (4 SIDES) PACKAGE OUTLINE 0.25 ±0.05 0.50 BSC RECOMMENDED SOLDER PAD PITCH AND DIMENSIONS 3.00 ± 0.10 (4 SIDES) BOTTOM VIEW—EXPOSED PAD PIN 1 NOTCH R = 0.20 TYP OR 0.25 × 45° CHAMFER R = 0.115 TYP 0.75 ± 0.05 15 16 PIN 1 TOP MARK (NOTE 6) 0.40 ± 0.10 1 1.45 ± 0.10 (4-SIDES) 2 (UD16) QFN 0904 0.200 REF 0.00 – 0.05 NOTE: 1. DRAWING CONFORMS TO JEDEC PACKAGE OUTLINE MO-220 VARIATION (WEED-2) 2. DRAWING NOT TO SCALE 3. ALL DIMENSIONS ARE IN MILLIMETERS 4. DIMENSIONS OF EXPOSED PAD ON BOTTOM OF PACKAGE DO NOT INCLUDE MOLD FLASH. MOLD FLASH, IF PRESENT, SHALL NOT EXCEED 0.15mm ON ANY SIDE 5. EXPOSED PAD SHALL BE SOLDER PLATED 6. SHADED AREA IS ONLY A REFERENCE FOR PIN 1 LOCATION ON THE TOP AND BOTTOM OF PACKAGE 0.25 ± 0.05 0.50 BSC 3226f Information furnished by Linear Technology Corporation is believed to be accurate and reliable. However, no responsibility is assumed for its use. Linear Technology Corporation makes no representation that the interconnection of its circuits as described herein will not infringe on existing patent rights. 15 LTC3226 TYPICAL APPLICATION Li-Ion Backup Supply TO LOAD MPEXT Li-Ion + C1 4.7µF R1 2.21M VIN GATE VOUT CPO PFI R2 1.21M 5V VMID LTC3226 CFLY 2.2µF H/L C+ CPO_FB C– LDO_FB RST_FB RST PFO CAPGOOD EN_CHG RPROG 33.2k PROG GND CSC 1.2F R4 3.83M R6 1.21M R3 255k 47µF R4 80.6k 470k 470k 470k µP CSC: CAP-XX HS230 MPEXT: VISHAY Si2333 3226 TA02 RELATED PARTS PART NUMBER DESCRIPTION COMMENTS LTC3225/ LTC3225-1 150mA Supercapacitor Charger Low Noise, Constant Frequency Charging of Two Series Supercapacitors. Automatic Cell Balancing Prevents Capacitor Overvoltage During Charging. Programmable Charge Current (Up to 150mA). Selectable 2.4V or 2.65V Regulation per Cell (LTC3225), Selectable 2V or 2.25V Regulation per Cell (LTC3225-1). 2mm × 3mm DFN Package LT3485 Photoflash Capacitor Chargers with Output Voltage Monitor and Integrated IGBT Drive Integrated IGBT Driver; Voltage Output Monitor; Uses Small Transformers: 5.8mm × 5.8mm × 3mm. Operates from Two AA Batteries, Single Cell Li-Ion or Any Supply from 1.8V Up to 10V. No Output Voltage Divider Needed; No External Schottky Diode Required. Charges Any Size Photoflash Capacitor; 10-Lead (3mm × 3mm) DFN Package LTC3625/ LTC3625-1 1A High Efficiency 2-Cell Supercapacitor Charger with Automatic Cell Balancing High Efficiency Step-Up/Step-Down Charging of Two Series Supercapacitors. Automatic Cell Balancing Prevents Capacitor Overvoltage During Charging. Programmable Charging Current Up to 500mA (Single Inductor), 1A (Dual Inductor). VIN = 2.7V to 5.5V, Selectable 2.4V/2.65V Regulation per Cell (LTC3625). Selectable 2V/2.25V Regulation per Cell (LTC3625-1), Low No-Load Quiescent Current: 23µA. 12-lead 3mm × 4mm DFN Package LT3750 Capacitor Charger Controller Charges Any Size Capacitor; Easily Adjustable Output Voltage. Drives High Current NMOS FETs; Primary-Side Sense—No Output Voltage Divider Necessary. Wide Input Range: 3V to 24V; Drives Gate to VCC – 2V. 10-Lead MS Package LT3751 High Voltage Capacitor Charger Controller with Regulation Charges Any Size Capacitor; Low Noise Output in Voltage Regulation Mode. Stable Operation Under a No-Load Condition; Integrated 2A MOSFET Gate Driver with Rail-to-Rail Operation for VCC ≤ 8V. Selectable 5.6V or 10.5V Internal Gate Drive Voltage Clamp; User-Selectable Over/Undervoltage Detect. Easily Adjustable Output Voltage; Primary or Secondary Side Output Voltage Sense. Wide Input VCC Voltage Range (5V to 24V). 20-Pin QFN 4mm × 5mm and 20-Lead TSSOP Packages LTC4425 Supercapacitor Charger with Current Limited Ideal Diode Constant-Current/Constant-Voltage Linear Charger for 2-cell Series Supercapacitor Stack. VIN: Li-Ion/Polymer Battery, a USB Port, or a 2.7V to 5.5V Current-Limited Supply. 2A Charge Current, Auto Cell Balancing, 20µA Quiescent Current, Shutdown Current <2µA. Low Profile 12-Pin 3mm × 3mm DFN or a 12-Lead MSOP Package 3226f 16 Linear Technology Corporation LT 0711 • PRINTED IN USA 1630 McCarthy Blvd., Milpitas, CA 95035-7417 (408) 432-1900 ● FAX: (408) 434-0507 ● www.linear.com LINEAR TECHNOLOGY CORPORATION 2011