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LP38690-ADJ Datasheet(PDF) 10 Page - National Semiconductor (TI) |
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LP38690-ADJ Datasheet(HTML) 10 Page - National Semiconductor (TI) |
10 / 13 page Application Hints EXTERNAL CAPACITORS Like any low-dropout regulator, external capacitors are re- quired to assure stability. These capacitors must be correctly selected for proper performance. INPUT CAPACITOR: An input capacitor of at least 1µF is required (ceramic recommended). The capacitor must be located not more than one centimeter from the input pin and returned to a clean analog ground. OUTPUT CAPACITOR: An output capacitor is required for loop stability. It must be located less than 1 centimeter from the device and connected directly to the output and ground pins using traces which have no other currents flowing through them. The minimum amount of output capacitance that can be used for stable operation is 1µF. Ceramic capacitors are recommended (the LP38690/2-ADJ was designed for use with ultra low ESR capacitors). The LP38690/2-ADJ is stable with any output capacitor ESR between zero and 100 Ohms. SETTING THE OUTPUT VOLTAGE: The output voltage is set using the external resistors R1 and R2 (see Typical Application Circuit). The output voltage will be given by the equation: V OUT =VADJ X (1 + R1/R2) Because the part has a minimum load current requirement of 100 µA, it is recommended that R2 always be 12k Ohms or less to provide adequate loading. Even if a minimum load is always provided by other means, it is not recommended that very high value resistors be used for R1 and R2 because it can make the ADJ node susceptible to noise pickup. A maximum value of 100k is recommended for R2 to prevent this from occurring. ENABLE PIN (LP38692-ADJ only): The LP38692-ADJ has an enable pin which turns the regulator output on and off. Pulling the enable pin down to a logic low will turn the part off. The voltage the pin has to be pulled up to in order to assure the part is on depends on input voltage (refer to Electrical Characteristics section). This pin should be tied to V IN if the enable function is not used. FOLDBACK CURRENT LIMITING: Foldback current limit- ing is built into the LP38690/2-ADJ which reduces the amount of output current the part can deliver as the output voltage is reduced. The amount of load current is dependent on the differential voltage between V IN and VOUT. Typically, when this differential voltage exceeds 5V, the load current will limit at about 450 mA. When the V IN -VOUT differential is reduced below 4V, load current is limited to about 1500 mA. SELECTING A CAPACITOR It is important to note that capacitance tolerance and varia- tion with temperature must be taken into consideration when selecting a capacitor so that the minimum required amount of capacitance is provided over the full operating tempera- ture range. Capacitor Characteristics CERAMIC: For values of capacitance in the 10 to 100 µF range, ceramics are usually larger and more costly than tantalums but give superior AC performance for bypassing high frequency noise because of very low ESR (typically less than 10 m Ω). However, some dielectric types do not have good capacitance characteristics as a function of voltage and temperature. Z5U and Y5V dielectric ceramics have capacitance that drops severely with applied voltage. A typical Z5U or Y5V capacitor can lose 60% of its rated capacitance with half of the rated voltage applied to it. The Z5U and Y5V also exhibit a severe temperature effect, losing more than 50% of nomi- nal capacitance at high and low limits of the temperature range. X7R and X5R dielectric ceramic capacitors are strongly rec- ommended if ceramics are used, as they typically maintain a capacitance range within ±20% of nominal over full operat- ing ratings of temperature and voltage. Of course, they are typically larger and more costly than Z5U/Y5U types for a given voltage and capacitance. TANTALUM: Solid Tantalum capacitors have good tempera- ture stability: a high quality Tantalum will typically show a capacitance value that varies less than 10-15% across the full temperature range of -40˚C to 125˚C. ESR will vary only about 2X going from the high to low temperature limits. The increasing ESR at lower temperatures can cause oscil- lations when marginal quality capacitors are used (if the ESR of the capacitor is near the upper limit of the stability range at room temperature). PCB LAYOUT Good PC layout practices must be used or instability can be induced because of ground loops and voltage drops. The input and output capacitors must be directly connected to the input, output, and ground pins of the regulator using traces which do not have other currents flowing in them (Kelvin connect). The best way to do this is to lay out C IN and COUT near the device with short traces to the V IN,VOUT, and ground pins. The regulator ground pin should be connected to the exter- nal circuit ground so that the regulator and its capacitors have a "single point ground". It should be noted that stability problems have been seen in applications where "vias" to an internal ground plane were used at the ground points of the IC and the input and output capacitors. This was caused by varying ground potentials at these nodes resulting from current flowing through the ground plane. Using a single point ground technique for the regulator and it’s capacitors fixed the problem. Since high current flows through the traces going into V IN and coming from V OUT, Kelvin connect the capacitor leads to these pins so there is no voltage drop in series with the input and output capacitors. RFI/EMI SUSCEPTIBILITY RFI (radio frequency interference) and EMI (electromagnetic interference) can degrade any integrated circuit’s perfor- mance because of the small dimensions of the geometries inside the device. In applications where circuit sources are present which generate signals with significant high fre- quency energy content (> 1 MHz), care must be taken to ensure that this does not affect the IC regulator. If RFI/EMI noise is present on the input side of the regulator (such as applications where the input source comes from the output of a switching regulator), good ceramic bypass ca- pacitors must be used at the input pin of the IC. If a load is connected to the IC output which switches at high speed (such as a clock), the high-frequency current pulses required by the load must be supplied by the capacitors on the IC output. Since the bandwidth of the regulator loop is less than 100 kHz, the control circuitry cannot respond to www.national.com 10 |
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