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LP3891ES-1.8 Datasheet(PDF) 8 Page - National Semiconductor (TI) |
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LP3891ES-1.8 Datasheet(HTML) 8 Page - National Semiconductor (TI) |
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8 / 12 page  Application Hints V BIAS RESTRICTIONS FOR PROPER START-UP To prevent misoperation, ensure that V BIAS is below 50mV before start-up is initiated. This scenario can occur in sys- tems with a backup battery using reverse-biased "blocking" diodes which may allow enough leakage current to flow into the V BIAS node to raise it’s voltage slightly above ground when the main power is removed. Using low leakage diodes or a resistive pull down can prevent the voltage at V BIAS from rising above 50mV. Large bulk capacitors connected to V BIAS may also cause a start-up problem if they do not discharge fully before re-start is initiated (but only if V BIAS is allowed to fall below 1V). A resistor connected across the capacitor will allow it to discharge more quickly. It should be noted that the probability of a "false start" caused by incor- rect logic states is extremely low. EXTERNAL CAPACITORS To assure regulator stability, input and output capacitors are required as shown in the Typical Application Circuit. OUTPUT CAPACITOR At least 10µF of output capacitance is required for stability (the amount of capacitance can be increased without limit). The output capacitor must be located less than 1cm from the output pin of the IC and returned to a clean analog ground. The ESR (equivalent series resistance) of the output capaci- tor must be within the "stable" range as shown in the graph below over the full operating temperature range for stable operation. 20069531 Minimum ESR vs Output Load Current Tantalum capacitors are recommended for the output as their ESR is ideally suited to the part’s requirements and the ESR is very stable over temperature. Aluminum electrolytics are not recommended because their ESR increases very rapidly at temperatures below 10˚C. Aluminum caps can only be used in applications where lower temperature operation is not required. A second problem with Al caps is that many have ESR’s which are only specified at low frequencies. The typical loop bandwidth of a linear regulator is a few hundred kHz to several MHz. If an Al cap is used for the output cap, it must be one whose ESR is specified at a frequency of 100 kHz or more. Because the ESR of ceramic capacitors is only a few milli Ohms, they are not suitable for use as output capacitors on LP389X devices. The regulator output can tolerate ceramic capacitance totaling up to 15% of the amount of Tantalum capacitance connected from the output to ground. INPUT CAPACITOR The input capacitor must be at least 10 µF, but can be increased without limit. It’s purpose is to provide a low source impedance for the regulator input. Ceramic capaci- tors work best for this, but Tantalums are also very good. There is no ESR limitation on the input capacitor (the lower, the better). Aluminum electrolytics can be used, but their ESR increase very quickly at cold temperatures. They are not recommended for any application where temperatures go below about 10˚C. BIAS CAPACITOR The 0.1µF capacitor on the bias line can be any good quality capacitor (ceramic is recommended). BIAS VOLTAGE The bias voltage is an external voltage rail required to get gate drive for the N-FET pass transistor. Bias voltage must be in the range of 4.5 - 6V to assure proper operation of the part. UNDER VOLTAGE LOCKOUT The bias voltage is monitored by a circuit which prevents the regulator output from turning on if the bias voltage is below approximately 4V. SHUTDOWN OPERATION Pulling down the shutdown (S/D) pin will turn-off the regula- tor. Pin S/D must be actively terminated through a pull-up resistor (10 k Ω to 100 kΩ) for a proper operation. If this pin is driven from a source that actively pulls high and low (such as a CMOS rail to rail comparator), the pull-up resistor is not required. This pin must be tied to Vin if not used. POWER DISSIPATION/HEATSINKING A heatsink may be required depending on the maximum power dissipation and maximum ambient temperature of the application. Under all possible conditions, the junction tem- perature must be within the range specified under operating conditions. The total power dissipation of the device is given by: P D =(VIN−VOUT)IOUT+(VIN)IGND where I GND is the operating ground current of the device. The maximum allowable temperature rise (T Rmax) depends on the maximum ambient temperature (T Amax) of the appli- cation, and the maximum allowable junction temperature (T Jmax): T Rmax =TJmax−TAmax The maximum allowable value for junction to ambient Ther- mal Resistance, θ JA, can be calculated using the formula: θ JA =TRmax /PD These parts are available in TO-220 and TO-263 packages. The thermal resistance depends on amount of copper area or heat sink, and on air flow. If the maximum allowable value of θ JA calculated above is ≥ 60 ˚C/W for TO-220 package and ≥ 60 ˚C/W for TO-263 package no heatsink is needed since the package can dissipate enough heat to satisfy these requirements. If the value for allowable θ JA falls below these limits, a heat sink is required. www.national.com 8 |
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