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LM4140CCM-2.5 Datasheet(PDF) 10 Page - National Semiconductor (TI) |
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LM4140CCM-2.5 Datasheet(HTML) 10 Page - National Semiconductor (TI) |
10 / 15 page Application Hints (Continued) MULTILAYER CERAMIC CAPACITORS Surface-mountable multilayer ceramic capacitors may be an attractive choice because of their relatively small physical size and excellent RF characteristics. However, they sometimes have an ESR values lower than the minimum required by the LM4140, and relatively large capacitance change with temperature. The manufacturer’s datasheet for the capacitor should be consulted before se- lecting a value. Test results of LM4140 stability using multi- layer ceramic capacitors show that a minimum of 0.2µF is usually needed. Multilayer ceramic capacitors that have been verified as suitable for use with the LM4140 are shown in Table 2. TABLE 2. Surface-Mount Ceramic Capacitors Selection Guide 2.2µF Surface-Mount Ceramic Manufacturer Part Number Tokin 1E225ZY5U-C203 Murata GRM42-6Y5V225Z16 4.7µF Surface-Mount Ceramic Tokin 1E475ZY5U-C304 REVERSE CURRENT PATH The P-channel Pass transistor used in the LM4140 has an inherent diode connected between the V IN and VREF pins (see diagram below). 10107903 Forcing the output to voltages higher than the input, or pulling V IN below voltage stored on the output capacitor by more than a V be, will forward bias this diode and current will flow from the V REF terminal to VIN. No damage to the LM4140 will occur under these conditions as long as the current flowing into the output pin does not exceed 50mA. ON/OFF OPERATION The LM4140 is designed to quickly reduce both V REF and IQ to zero when turned-off. V REF is restored in less than 200µs when turned-on. During the turn-off, the charge across the output capacitor is discharged to ground through internal circuitry. The LM4140 is turned-off by pulling the enable input low, and turned-on by driving the input high. If this feature is not to be used, the enable pin should be tied to the V IN to keep the reference on at all times (the enable pin must not be left floating). To ensure proper operation, the signal source used to drive the enable pin must be able to swing above and below the specified high and low voltage thresholds which guarantee an ON or OFF state (see Electrical Characteristics). The ON/OFF signal may come from either a totem-pole output, or an open-collector output with pull-up resistor to the LM4140 input voltage. This high-level voltage may exceed the LM4140 input voltage, but must remain within the Abso- lute Maximum Rating for the enable pin. OUTPUT ACCURACY Like all references, either series or shunt, the after assembly accuracy is made up of primarily three components: initial accuracy itself, thermal hysteresis and effects of the PCB assembly stress. LM4140 provides an excellent output initial accuracy of 0.1% and temperature coefficient of 6ppm/˚C (B Grade). For best accuracy and precision, the LM4140 junction tem- perature should not exceed 70˚C. The thermal hysteresis curve on this datasheet are perfor- mance characteristics of three typical parts selected at ran- dom from a sample of 40 parts. Parts are mounted in a socket to minimize the effect of PCB’s mechnical expansion and contraction. Readings are taken at 25˚C following multiple temperature cycles to 0˚C and 70˚C. The labels on the X axis of the graph indicates the device temperature cycle prior to measurement at 25˚C. The mechanical stress due to the PCB’s mechanical and thermal stress can cause an output voltage shift more than the true thermal coefficient of the device. References in surface mount packages are more susceptible to these stresses because of the small amount of plastic molding which support the leads. Following the recommendations on PCB Layout Consider- ation section can minimize the mechanical stress on the device. PCB LAYOUT CONSIDERATION The simplest ways to reduce the stress related shifts are: 1. Mounting the device near the edges or the corners of the board where mechanical stress is at its minimum. The center of the board generally has the highest mechani- cal and thermal expansion stress. 2. Mechanical isolation of the device by creating an island by cutting a U shape slot on the PCB for mounting the device. This approach would also provide some thermal isolation from the rest of the circuit. Figure 5 is a recommended printed board layout with a slot cut on three sides of the circuit layout to serve as a strain relief. 10107933 FIGURE 4. Typical Thermal Hysteresis www.national.com 10 |
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