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LT1086CM-3.3 Datasheet(PDF) 9 Page - Linear Technology |
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LT1086CM-3.3 Datasheet(HTML) 9 Page - Linear Technology |
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9 / 16 page  9 LT1086 Series plugging and unplugging in the system will not generate current large enough to do any damage. The adjustment pin can be driven on a transient basis ±25V, with respect to the output without any device degradation. Of course as with any IC regulator, exceeding the maximum input-to-output voltage differential causes the internal transistors to break down and none of the protection circuitry is functional. the power supply may need to be cycled down to zero and brought up again to make the output recover. Ripple Rejection For the LT1086 the typical curves for ripple rejection reflect values for a bypassed adjust pin. This curve will be true for all values of output voltage. For proper bypassing and ripple rejection approaching the values shown, the impedance of the adjust pin capacitor at the ripple fre- quency should equal the value of R1, (normally 100 Ω to 120 Ω). The size of the required adjust pin capacitor is a function of the input ripple frequency. At 120Hz the adjust pin capacitor should be 13 µF if R1 = 100Ω. At 10kHz only 0.16 µF is needed. For circuits without an adjust pin bypass capacitor the ripple rejection will be a function of output voltage. The output ripple will increase directly as a ratio of the output voltage to the reference voltage (VOUT/VREF). For ex- ample, with the output voltage equal to 5V and no adjust pin capacitor, the output ripple will be higher by the ratio of 5V/1.25V or four times larger. Ripple rejection will be degraded by 12dB from the value shown on the LT1086 curve. Typical curves are provided for the 5V and 12V devices since the adjust pin is not available. Output Voltage The LT1086 develops a 1.25V reference voltage between the output and the adjust terminal (see Figure 1). By placing a resistor R1 between these two terminals, a constant current is caused to flow through R1 and down through R2 to set the overall output voltage. Normally this current is chosen to be the specified minimum load current of 10mA. Because IADJ is very small and constant when compared with the current through R1, it repre- sents a small error and can usually be ignored. For fixed voltage devices R1 and R2 are included in the device. APPLICATIONS INFORMATION VIN VOUT LT1086 ADJ IN OUT LT1086 • AI01 CADJ 10 µF COUT 150 µF R1 D1 1N4002 (OPTIONAL) R2 + + Overload Recovery Like any of the IC power regulators, the LT1086 has safe area protection. The safe area protection decreases the current limit as input-to-output voltage increases and keeps the power transistor inside a safe operating region for all values of input-to-output voltage. The LT1086 protection is designed to provide some output current at all values of input-to-output voltage up to the device breakdown. When power is first turned on, as the input voltage rises, the output follows the input, allowing the regulator to start up into very heavy loads. During the start-up, as the input voltage is rising, the input-to-output voltage differential remains small, allowing the regulator to supply large output currents. With high input voltage, a problem can occur wherein removal of an output short will not allow the output voltage to recover. Older regulators such as the 7800 series also exhibited this phenomenon, so it is not unique to the LT1086. The problem occurs with a heavy output load when the input voltage is high and the output voltage is low, such as immediately after a removal of a short. The load line for such a load may intersect the output current curve at two points. If this happens there are two stable output operat- ing points for the regulator. With this double intersection Figure 1. Basic Adjustable Regulator R1 10 µF TANTALUM R2 IN OUT IADJ 50 µA ADJ VOUT LT1086 VIN 1086 • F01 VREF VOUT = VREF 1 + + IADJ R2 R2 R1 ( ) + |
Similar Part No. - LT1086CM-3.3 |
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Similar Description - LT1086CM-3.3 |
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