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LTC1558-3.3 Datasheet(PDF) 11 Page - Linear Technology |
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LTC1558-3.3 Datasheet(HTML) 11 Page - Linear Technology |
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11 / 20 page  11 LTC1558-3.3/LTC1558-5 internal logic to ensure that the low cell voltage reset is triggered only if the CTL pin is between 0.9V and 0.25V. This will prevent a pushbutton reset (which pulls CTL below 250mV) from being mistaken as a low cell voltage condition. Unusual situations where the NiCd cell voltage drops drastically below 0.25V will also trigger UVLO, since the LTC1558 will treat this as a “hard” reset after 2 seconds. An optional LOBAT output, available in the 16-pin GN or SO package, can be used to signal the system when the cell voltage falls below 1V, giving an early warning that the backup cell is heavily discharged. The LOBAT output is disabled when the LTC1558 is in trickle charge mode because the CTL pin is pulled to 0.5V by the LTC1558. Fault Protection and Thermal Limit The LTC1558’s boost converter incorporates two internal timers that turn off the switch transistors if the inductor charge or discharge time gets abnormally long. The inductor charge time may get abnormally long if the NiCd cell voltage drops below 0.25V without triggering the 0.25V < VBAT < 0.9V low cell voltage comparator. In this case, the NiCd cell is assumed to be damaged and the LTC1558’s priority is shutting down the system grace- fully. In this case, the timer will shut off the N-channel switch transistor after a maximum charging time (14 µs). The boost converter continues switching but delivers reduced output power, causing VCC to drop. The LTC1558 will enter UVLO either when VCC drops below (VCC(RATED VOLTAGE) – 9%) or after the LTC1558 detects CTL lower than 0.25V for 2 seconds, in which case “hard” reset occurs. The discharging time can also get abnormally long if a serious overload condition occurs during switching. The timer will shut off the P-channel pass transistor after 10 µs, protecting the boost converter. The LTC1558 will end up in UVLO as VCC drops below (VCC(RATED VOLTAGE) – 9%). In addition, the LTC1558 is protected for safe area opera- tion with an internal thermal shutdown circuit. If the device is overloaded for a long period of time, the thermal shutdown circuit forces the LTC1558 into UVLO. The threshold temperature for thermal shutdown is typically 155 °C. APPLICATIONS INFORMATION The LTC1558’s boost converter is designed so that no current drains from the battery to the load during output short circuit or VCC = 0V conditions. This assures that the system can be powered down for a long period of time. This eliminates the risk of finding a nonfunctioning backup system upon power-up. Backup Cell Fast Recharge The LTC1558 includes an onboard gas gauge circuit, consisting of a 23-bit divider and a 9-bit up/down counter. The gas gauge logic assumes that the boost converter uses a 22 µH inductor, allowing it to estimate battery charge by counting switch pulses. The gas gauge counts up from zero as charge is removed from the backup cell in backup mode. It takes roughly 8.4 million 165mA boost pulses (low current mode) to increment the up/down counter by one count. In high current mode, the 330mA pulses skip the first two bits of the divider because each 330mA pulse carries four times as much energy as a 165mA pulse. At maximum load and VCC = 5V, the gas gauge divider will increment by one count every 7.5 µs while the boost converter is running. Full count is reached after approximately 2.2 hours, equivalent to about 512mAhr of charge. Upon entering recharge mode (after the main battery is restored) the LTC1558 connects a 16mA fast recharge current source from VCC to the SW pin. At the same time, an internal free running oscillator counts down the gas gauge counter at a rate designed to replace about 160% of the charge previously removed from the backup cell. When the gas gauge counter reaches zero, the LTC1558 reduces the charging current at the SW pin to the user- programmed trickle charge current level. Under some circumstances, the LTC1558 can exit the backup mode with invalid gas gauge contents. This can occur under three possible conditions: a) The backup cell was completely exhausted during a backup cycle and the LTC1558 entered UVLO. b) The backup cell was replaced while the main supply was disabled. c) A backup cycle was terminated prematurely by a “hard” reset or an output overload. |
Similar Part No. - LTC1558-3.3 |
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Similar Description - LTC1558-3.3 |
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