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LP2954 Datasheet(PDF) 9 Page - National Semiconductor (TI) |
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LP2954 Datasheet(HTML) 9 Page - National Semiconductor (TI) |
9 / 13 page Application Hints (Continued) OUTPUT ISOLATION The regulator output can be left connected to an active volt- age source (such as a battery) with the regulator input power turned off, as long as the regulator ground pin is con- nected to ground . If the ground pin is left floating, damage to the regulator can occur if the output is pulled up by an external voltage source. REDUCING OUTPUT NOISE In reference applications it may be advantageous to reduce the AC noise present on the output. One method is to reduce regulator bandwidth by increasing output capacitance. This is relatively inefficient, since large increases in capacitance are required to get significant improvement. Noise can be reduced more effectively by a bypass capacitor placed across R1 (refer to Figure 2). The formula for select- ing the capacitor to be used is: This gives a value of about 0.1 µF. When this is used, the output capacitor must be 6.8 µF (or greater) to maintain sta- bility. The 0.1 µF capacitor reduces the high frequency gain of the circuit to unity, lowering the output noise from 260 µV to 80 µV using a 10 Hz to 100 kHz bandwidth. Also, noise is no longer proportional to the output voltage, so improve- ments are more pronounced at high output voltages. SHUTDOWN INPUT A logic-level signal will shut off the regulator output when a “LOW” (<1.2V) is applied to the Shutdown input. To prevent possible mis-operation, the Shutdown input must be actively terminated. If the input is driven from open-collector logic, a pull-up resistor (20 k Ω to 100 kΩ rec- ommended) should be connected from the Shutdown input to the regulator input. If the Shutdown input is driven from a source that actively pulls high and low (like an op-amp), the pull-up resistor is not required, but may be used. If the shutdown function is not to be used, the cost of the pull-up resistor can be saved by simply tying the Shutdown input directly to the regulator input. IMPORTANT: Since the Absolute Maximum Ratings state that the Shutdown input can not go more than 0.3V below ground, the reverse-battery protection feature which protects the regulator input is sacrificed if the Shutdown input is tied directly to the regulator input. If reverse-battery protection is required in an application, the pull-up resistor between the Shutdown input and the regula- tor input must be used. Typical Applications DS011128-37 * In shutdown mode, ERROR will go high if it has been pulled up to an external supply. To avoid this invalid response, pull up to regulator output. ** Exact value depends on dropout voltage. (See Application Hints) FIGURE 3. ERROR Output Timing Typical Application Circuit DS011128-1 5V Regulator DS011128-6 www.national.com 9 |
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