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LT1175IS8-5 Datasheet(PDF) 10 Page - Linear Technology |
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LT1175IS8-5 Datasheet(HTML) 10 Page - Linear Technology |
10 / 12 page 10 LT1175 APPLICATIONS INFORMATION TA = Maximum ambient temperature TMAX = Maximum LT1175 die temperature (125°C for commercial and industrial grades) θJA = LT1175 thermal resistance, junction to ambient VIN = Maximum continuous input voltage at maximum load current ILOAD = Maximum load current Example: LT1175S8 with ILOAD = 200mA, VOUT = 5V, VIN = 7V, TA = 60°C. Maximum die temperature for the LT1175S8 is 125 °C. Thermal resistance from Table 2 is found to be 80 °C/W. Die Temperature = 60 + 80 (0.2A)(8 – 5) = 108 °C Maximum W V Power Dissipation = 125 – 60 80 125 – 60 = = () += 081 80 0 2 59 . . Output Voltage Reversal The LT1175 is designed to tolerate an output voltage reversal of up to 2V. Reversal might occur, for instance, if the output was shorted to a positive 5V supply. This would almost surely destroy IC devices connected to the negative output. Reversal could also occur during start-up if the positive supply came up first and loads were connected between the positive and negative supplies. For these reasons, it is always good design practice to add a reverse biased diode from each regulator output to ground to limit output voltage reversal. The diode should be rated to handle full negative load current for start-up situations, or the short-circuit current of the positive supply if supply-to- supply shorts must be tolerated. Input Voltage Lower Than Output Linear Technology’s positive low dropout regulators LT1121 and LT1129, will not draw large currents if the input voltage is less than the output. These devices use a lateral PNP power transistor structure that has 40V emitter base breakdown voltage. The LT1175, however, uses an NPN power transistor structure that has a parasitic diode between the input and output of the regulator. Reverse voltages between input and output above 1V will damage the regulator if large currents are allowed to flow. Simply disconnecting the input source with the output held up will not cause damage even though the input-to-output volt- age will become slightly reversed. High Frequency Ripple Rejection The LT1175 will sometimes be powered from switching regulators that generate the unregulated or quasi-regu- lated input voltage. This voltage will contain high fre- quency ripple that must be rejected by the linear regulator. Special care was taken with the LT1175 to maximize high frequency ripple rejection, but as with any micropower design, rejection is strongly affected by ripple frequency. The graph in the Typical Performance Characteristics section shows 60dB rejection at 1kHz, but only 15dB rejection at 100kHz for the 5V part. Photographs in Figures 4a and 4b show actual output ripple waveforms with square wave and triwave input ripple. Maximum Continuous Input Voltage (for Thermal Considerations) COUT = 4.7µF TANT COUT = 1µF TANT 5 µs/DIV 1175 F04 Figure 4a. COUT = 4.7µF TANT COUT = 1µF TANT 2 µs/DIV 1175 F04 Figure 4b. INPUT RIPPLE 100mV/DIV OUTPUT 100mV/DIV f = 100kHz f = 50kHz OUTPUT 20mV/DIV INPUT RIPPLE 100mV/DIV |
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