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LM2991T Arkusz danych(PDF) 6 Page - National Semiconductor (TI) |
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LM2991T Arkusz danych(HTML) 6 Page - National Semiconductor (TI) |
6 / 12 page Application Hints (Continued) MINIMUM LOAD A minimum load current of 500 µA is required for proper op- eration. The external resistor divider can provide the mini- mum load, with the resistor from the adjust pin to ground set to 2.4 k Ω. SETTING THE OUTPUT VOLTAGE The output voltage of the LM2991 is set externally by a resis- tor divider using the following equation: V OUT =VREF x(1+R2/R1)−(IADJ xR2) where V REF = −1.21V. The output voltage can be pro- grammed within the range of −3V to −24V, typically an even greater range of −2V to −25V. The adjust pin current is about 60 nA, causing a slight error in the output voltage. However, using resistors lower than 100 k Ω makes the adjust pin cur- rent negligible. For example, neglecting the adjust pin cur- rent, and setting R2 to 100 k Ω and V OUT to −5V, results in an output voltage error of only 0.16%. ON/OFF PIN The LM2991 regulator can be turned off by applying a TTL or CMOS level high signal to the ON/OFF pin (see Adjustable Current Sink Application). FORCING THE OUTPUT POSITIVE Due to an internal clamp circuit, the LM2991 can withstand positive voltages on its output. If the voltage source pulling the output positive is DC, the current must be limited to 1.5A. A current over 1.5A fed back into the LM2991 could damage the device. The LM2991 output can also withstand fast posi- tive voltage transients up to 26V, without any current limiting of the source. However, if the transients have a duration of over 1 ms, the output should be clamped with a Schottky di- ode to ground. Typical Applications Output Capacitor ESR Range DS011260-5 Fully Isolated Post-Switcher Regulator DS011260-6 www.national.com 6 |
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Podobny opis - LM2991T |
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