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LTC4040 Datasheet(PDF) 13 Page - Linear Technology |
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LTC4040 Datasheet(HTML) 13 Page - Linear Technology |
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13 / 26 page  LTC4040 13 4040fa For more information www.linear.com/LTC4040 operaTion TheLTC4040isacompletebatterybackupsystemmanager for a 3.5V to 5.5V supply rail. The system has three prin- cipalcircuitcomponents:afull-featuredstep-down(buck) battery charger, a step-up (boost) backup converter with automaticburstfeaturetodeliverpowertothesystemload when external input power is lost and a power-fail com- parator to decide which one to activate. The LTC4040 has several other auxiliary components: an input current limit (CLPROG)amplifier,anoptionalinputovervoltageprotec- tion (OVP) circuit and a reset comparator. The LTC4040 has three modes of operation: normal, backup and shutdown. If the input supply is above an externally programmable PFI threshold voltage, the part is considered to be in normal mode in which power flows from input to output (VSYS) while the step-down switch- ing regulator charges the battery to one of eight charge voltage settings programmed by the F0, F1, and F2 digital inputs. Please refer to the Block Diagram. The total system load is monitored by the CLPROG amplifier via an external series resistor, RS, connected between the VIN and CLN pins. This amplifier can reduce the charge current from its programmed value (set by the PROG pin external resis- tor RPROG) if the external load demand increases beyond a programmable level set by RS. When the input supply falls below the PFI threshold, backup mode disconnects the switches (MN1 and MN2) to isolate the system (VSYS) fromtheinputwhiletheboostconverterpowersthesystem load from the battery using the same external inductor, L1. THE BATTERY CHARGER The LTC4040 includes a full-featured constant-current (CC)/constant-voltage(CV)batterychargerwithautomatic recharge, automatic termination by safety timer, low volt- age trickle charging, bad-battery detection and thermis- tor sensor input for out-of-temperature charge pausing. The battery charger is a high efficiency buck switching converter used to transfer charge from VSYS to BAT via the SW pin. The charger can be disabled by pulling the CHGOFF pin above 1.2V. Buck Switching Charger The LTC4040 battery charger is a constant frequency (2.25MHz)synchronousbuckconvertercapableofdirectly chargingthebatterytoitschargevoltagewithanexternally programmable charge current up to 2.5A from an input supplyashighas5.5V.Azerocurrentcomparatormonitors the inductor current and shuts off the NMOS synchronous rectifieroncethecurrentreducestoapproximately250mA. This prevents the inductor current from reversing and improves efficiency for low charging current. Battery Preconditioning (Trickle Charge) and Bad-Battery Fault When a battery charge cycle begins, the battery charger first determines if the battery is deeply discharged. If the battery voltage is below VLOWBAT, typically 2.85V, an automatic trickle charge feature sets the charge current to 1/8th or 12.5% of the programmed value. To improve charge current accuracy at this low level, the buck switch- ing charger is turned off and a secondary linear charger is used to deliver charge to the battery. If the low voltage persists for more than half an hour, the battery charger automatically terminates and indicates, via the CHRG and FAULT pins, that the battery is in bad-battery fault. Constant-Current Mode Charging Once the battery voltage is above VLOWBAT, the charger begins charging in full power constant-current mode. The current delivered to the battery will try to reach 2000V/ RPROG. Depending on the external load condition, the battery charger may or may not be able to charge at the full programmed rate. The external load will always be prioritized over the battery charge current. The battery charger will charge at the full programmed rate only if the sum of the external load and the charger input current is less than or equal to the input current limit set by RS. |
Similar Part No. - LTC4040_15 |
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Similar Description - LTC4040_15 |
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