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LTC1558CS-5 Datasheet(PDF) 8 Page - Linear Technology |
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LTC1558CS-5 Datasheet(HTML) 8 Page - Linear Technology |
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8 / 20 page  8 LTC1558-3.3/LTC1558-5 APPLICATIONS INFORMATION Overview The LTC1558 is a versatile backup battery control system designed to provide all the functions necessary to imple- ment a complete, highly integrated backup system within a single chip. It allows the system to maintain its rated supply voltage during backup, offering maximum system design flexibility. The LTC1558 allows the use of a low cost rechargeable NiCd cell for backup, eliminating the need for expensive, replaceable 4.5V lithium backup cells. The LTC1558 includes an onboard boost converter de- signed to generate an adjustable voltage (3V to 10V) from a single 1.2V NiCd cell. This voltage is connected to the system’s DC/DC converter input, enabling the system to continue operation when the main battery fails. A “smart” recharging circuit uses an accumulating gas gauge to measure the charge extracted from the backup battery during a backup cycle. This measured charge is then replaced in a fast recharge cycle, without wasting excess power or overcharging the backup cell. An externally adjustable trickle charge circuit maintains the cell charge after the fast charge cycle has completed, minimizing drain from the main battery during standby. Included in the LTC1558 is a complete backup circuit that monitors the main system power and automatically switches in the backup circuit as the primary power supply falls away (due to a weak or disconnected main battery). The LTC1558 also performs VCC supervisory functions during normal system operations. An LTC1558-3.3 moni- tors a 3.3V supply voltage at its VCC pin whereas an LTC1558-5 monitors a 5V supply at its VCC pin. In both cases, the LTC1558 derives power for the majority of the internal circuitry (except for the boost converter) from the VCC pin. Table 1 shows the signal conditions for the LTC1558’s various operating modes. Note that VCC in Table 1 refers to the rated VCC voltage, 3.3V or 5V. Boost Converter Operation The LTC1558 uses an onboard synchronous boost con- verter with a fixed peak current architecture that provides a simple and flexible system solution while eliminating the need for conventional frequency compensation. The boost converter’s output, set by the external divider connected to the FB pin, supports the main system regulator during Figure 1. Typical LTC1558 Connection + BOOST CONVERTER TO SYSTEM REGULATOR INPUT LTC1588 R1 FB MAIN BATTERY CIN R2 1558 F01 BACKUP LOGIC VREF VBAK BACKUP backup. It can supply a minimum backup power of 100mW. The boost converter operates in a modified pulse skipping mode; each switch cycle transfers a known amount of charge from the backup cell to the regulated output. This prevents uncontrolled discharge of the backup cell and allows the LTC1558 to accurately measure the charge removed from the backup cell by counting the charge pulses. The LTC1558 enters backup mode when the main battery voltage drops. As shown in Figure 1, the main battery voltage is scaled down by an external resistor divider and fed to the LTC1558’s backup comparators. These com- pare the scaled voltage with an internal trimmed VREF (1.272V), switching the LTC1558 into backup mode when VFB drops 7.5% below VREF. Upon entering backup mode, the BACKUP pin is asserted and the internal boost converter turns on. The BACKUP signal is used to turn off the external P-channel MOSFET, isolating the main bat- tery from the LTC1558 and the system regulator’s input. The LTC1558’s boost converter will charge the input capacitor CIN of the system regulator until VFB rises above (VREF – 7.5%). Table 1 OPERATING MODES CONDITIONS UVLO Reset 1V < VCC < VCC (Rated Value) – 9% or VBAT < 0.9V Pushbutton Reset VCTL < 250mV UVLO Reset Recovery VCC > VCC (Rated Value) – 6% Backup Mode Activation VFB < (VREF – 7.5%) Backup Mode Exit VFB > (VREF – 6%) Boost Converter Activation VFB < (VREF – 7.5%) Boost Converter Deactivation VFB > (VREF – 7.5%) |
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