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LT1239CS Datasheet(PDF) 10 Page - Linear Technology |
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LT1239CS Datasheet(HTML) 10 Page - Linear Technology |
10 / 12 page 10 LT1239 APPLICATIONS INFORMATION system power and the backup batteries. The comparator output will be driven high if the output of the 5V system supply is greater than the 4.85V output of regulator 2. Regulator 2 will act as a diode to prevent current flow from the 5V system supply back into the backup battery. Cur- rent flow into the output of regulator 2, with the output pulled up to 5V, will be limited to approximately 6 µA and will flow to ground. If the main 5V system supply drops below the 4.85V output of regulator 2 the comparator will switch off and regulator 2 will provide power to the memory. The comparator combined with regulator 2 and the batteries provide an uninterruptable power source to the memory and power monitoring circuitry. Choosing Current Limiting Resistors Due to UL safety considerations, circuits used to charge lithium-ion batteries must have external resistors (passive components) to limit the available charge current in the event of a failure in the charging circuit. The LT1239 allows these resistors to be placed in series with the output transistor of the regulator 1 as shown in Figure 5. The current limiting resistor (R4) will be in series with the main charge current path but will be inside the feedback loop of regulator 1. Because the resistors are inside the feedback loop they will not affect output voltage regulation in normal operating conditions. The resistors should be selected so that they limit the charge current below the maximum level specified by the battery manufacturer. For a typical 3.4V, 50mA rechargeable backup cell (Panasonic VL2330) the maximum charge current is specified at 300mA. Most users will choose to limit the current well below the maximum charge current. It is important to note that these resistors can also limit the charge current during normal operation. Since the charge current for a typical lithium-ion button cell is normally less than 20mA, limited by the internal impedance of the cells during a constant voltage charge, the current limiting resistors do not significantly affect the charge times for the backup cells. The worst case would occur if the regulator failed as a short and the main battery is at its maximum charge voltage. The current limiting resistor (R4) must be chosen to limit the current to less than the manufacturers maxi- mum charging current with the difference between the main battery voltage and the backup battery voltage dropped across it. For example with a main battery voltage of 24V max, a backup battery voltage of 6.8V and a maximum charge current of 300mA, R4 must be greater than (24V – 6.8V) / 300mA, R4 > 57 Ω. R4 can also be used to limit the power dissipated by regulator 1 as shown in the following section. C1 is needed for stability in circuits with protection resistors (R4). The power dissipation in R4 during fault conditons can be significant. it will be equal to: (VINL – VBATTERY) 2 R4 Power resistors with ratings greater than 0.25W or fusable resistors may be required. Thermal Considerations The power dissipation of this device is made up of several components.They are the power dissipation of each regu- lator, the comparator and the error amplifier. The largest component will be due to the power in regulator 1, when the charge current for the batteries is the highest and the input voltage to regulator 1 is at the maximum. In most systems this condition only occurs for a short period after the backup battery has been completely discharged. Both regulators have thermal limiting circuitry which limits the power in the regulator when the junction temperature reaches about 100 °C. The thermal limit temperature is set low because the device is designed to work with batteries specified to run at ambient temperatures below 60 °C. The power in regulator 1 can be limited with external resistors placed in the feedback loop as shown in Figure 5. In lithium-ion systems these resistors are required for safety reasons. The power in regulator 1 will be equal to: [(VMAINBATTERY – VBACKUPBATTERY) × ICHG] – (ICHG ×R4) Note that for circuits with a current limiting resistor (R4) the worst-case power point occurs when ICHG is equal to the maximum charging current/2. Example: [(24V – 6.8V) × (71mA/2)] – [(71mA/2) × 240] = 300mW This is the only significant component of power dissipation in the device and this condition will only occur when the |
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