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LM2991S Scheda tecnica(PDF) 8 Page - Texas Instruments |
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LM2991S Scheda tecnica(HTML) 8 Page - Texas Instruments |
8 / 19 page LM2991 SNVS099H – MAY 1999 – REVISED JUNE 2013 www.ti.com Minimum Load A minimum load current of 500 μA is required for proper operation. The external resistor divider can provide the minimum 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 resistor divider using the following equation: VOUT = VREF x (1 + R2/R1) − (IADJ x R2) where • VREF = −1.21V (1) The output voltage can be programmed 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 error due to the adjust pin current negligible. For example, neglecting the adjust pin current, and setting R2 to 100 k Ω and VOUT 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. The impedance of the voltage source driving the ON/OFF pin should be low enough to source the ON/OFF pin input current to meet the OFF threshold voltage level, 100 µA maximum at 2.4V. If the ON/OFF function is not needed, the pin should be connected to Ground. The ON/OFF pin should not be left floating, as this is not an ensured operating condition. See the Adjustable Current Sink Application, Figure 21. 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 positive 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 diode to ground. Thermal Shutdown The LM2991 has an internally set thermal shutdown point of typically 160°C, with approximately 10°C of hysteresis. This thermal shutdown temperature point is outside the specified Operating Rating range, above the Absolute Maximum Ratings, and is intended as a safety feature for momentary fault conditions only. Continuous operation near the thermal shutdown temperature should be avoided as it may have a negative affect on the life of the device. 8 Submit Documentation Feedback Copyright © 1999–2013, Texas Instruments Incorporated Product Folder Links: LM2991 |
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