Design Article
Supercaps can be a good choice over batteries for backup applications
Steve Knoth, Linear Technology Corporation
6/5/2012 8:08 AM EDT
Power path control and the diode
The LTC3226 contains a diode controller which controls the gate of an external PFET connected between the input, VIN, and the output, VOUT, through the GATE pin. Refer to Figure 2 for details. 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.
Operating modes
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 super capacitor 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. See Figure 3 for details.
Voltage clamp circuitry
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 balancing circuitry
The LTC3226 is equipped with an internal leakage balancing amplifier which serves 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.
Table 2 shows a comparison of Linear Technology’s family of supercapacitor chargers.
Supercapacitors are now being used in applications where batteries were once the norm. Initial applications were low current, but technology has advanced and supercaps are now found in a variety of medium and high power applications in both consumer and non-consumer segments. Supercaps have many inherent advantages over batteries such as higher peak power delivery, longer cycle life and smaller form factor. However, product designers using supercaps are faced with problems such as cell balancing and potential over-voltage damage to supercap cells. Fortunately, Linear Technology has addressed these needs by continuously adding to its supercap charger IC family. The LTC3226 is a charge pump based supercap charger with seamless PowerPath control featuring automatic cell balancing, voltage clamping, reverse current protection, various operation modes, low current consumption, and up to 2A of backup current. The LTC3226 offers useful features in a small footprint, reducing overall solution size and in turn enabling more compact, simpler designs.
About the author:
Steve Knoth is senior product marketing engineer, Power Products Group, Linear Technology Corp.
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The LTC3226 contains a diode controller which controls the gate of an external PFET connected between the input, VIN, and the output, VOUT, through the GATE pin. Refer to Figure 2 for details. 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.
Figure 2. LTC3226 Block diagram
Operating modes
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 super capacitor 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. See Figure 3 for details.
Figure 3: LTC3226 Normal-to-backup mode switching transient waveform
Voltage clamp circuitry
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 balancing circuitry
The LTC3226 is equipped with an internal leakage balancing amplifier which serves 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.
Table 2 shows a comparison of Linear Technology’s family of supercapacitor chargers.
Table 2: Comparison of Linear Technology supercap chargers
(Click on image to enlarge)
ConclusionSupercapacitors are now being used in applications where batteries were once the norm. Initial applications were low current, but technology has advanced and supercaps are now found in a variety of medium and high power applications in both consumer and non-consumer segments. Supercaps have many inherent advantages over batteries such as higher peak power delivery, longer cycle life and smaller form factor. However, product designers using supercaps are faced with problems such as cell balancing and potential over-voltage damage to supercap cells. Fortunately, Linear Technology has addressed these needs by continuously adding to its supercap charger IC family. The LTC3226 is a charge pump based supercap charger with seamless PowerPath control featuring automatic cell balancing, voltage clamping, reverse current protection, various operation modes, low current consumption, and up to 2A of backup current. The LTC3226 offers useful features in a small footprint, reducing overall solution size and in turn enabling more compact, simpler designs.
About the author:
Steve Knoth is senior product marketing engineer, Power Products Group, Linear Technology Corp.
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SUPERCAPACITOR
6/6/2012 9:46 AM EDT
1)Regarding the supercap limitation of 2,5V and 2,75V maximum per cell mentioned in the article.
Vina Technology (korea) www.vina.co.kr launched a supercap with 3V from 3F to 600F.
2)Also a chinese company has a patent and is in the market a hybrid unit: Supercap Lithium battery (3,2V from 10Ah to 100Ah) can be check at http://www.cyliyuan.com/en/product3.asp
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William Miller
1/30/2013 10:52 AM EST
Every generation of capacitors naturally is better than the previous one. That's called evolution or progress. Using these supercapacitors in SSD will improve their characteristics significantly. I use SSD in my PC and I can't wait to replace it for even faster one.
_______________
William - http://www.carid.com/
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I_B_GREEN
6/8/2012 12:49 PM EDT
And how do they compare on,leakage current?
not a good choice for ultra low power sleep wake systems.
Leakage a function of capacity, so trying to replace batteries for long duration backup not a good choice from efficiency standpoint.
excellent choice for agmenting a battery for high pulse low duration help.
Can decrease battery capacity needed.
cross over of battery and super cap leakage rates needs to be graphed and desiminated.
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prabhakar_deosthali
6/10/2012 7:34 AM EDT
If Super caps can be combined with batteries in EVs , they may assist in quick energy recovery during braking and also help in quicker acceleration , in my opinion
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hm
6/10/2012 8:41 PM EDT
@I_B_Green: Yes, leakage current is parameter of concern for replacing battery in RTC application. Many application requires one year or more retention duration. Supercap leakage current can work only up to few weeks.
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DadOf3TeenieBoppers
6/21/2012 9:35 AM EDT
There are now 5000F capacitors out there with sub-milliohm ESR (IOXUS makes them). They are rated for 2.7V. Put a 2.4V zener in parallel across them to keep them from overcharging, and they can be ganged as far as the eye can see. It may take a while to trickle charge them (it will take a day and a half for a 100mA charger to do so; the current limit is to keep a 500mW 2% 2.4V zener from overheating if it carries all the current in a confined space), but they would make an ideal short term power storage.
For any facility that cannot have power shut down, they will keep power up long enough for backup generators to turn on and come on line.
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Abrahim
6/28/2012 2:58 AM EDT
the best use of SupCaps is to reduce the size of the battery which is often governed by the peak current requirements of a circuit. rather than using supcaps as a replacement for batteries (which is not advisable unless the outage is short) it should be used to supplement on certain parameters like peak/burst outputs of power.
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