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ZXRD1050PQ16 Datasheet(PDF) 7 Page - Diodes Incorporated |
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ZXRD1050PQ16 Datasheet(HTML) 7 Page - Diodes Incorporated |
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7 / 28 page  Applications Note: Component names refer to designators shown in the application circuit diagrams. Output Capacitors Output capacitors are a critical choice in the overall performance of the solution. They are required to filter the output and supply load transient current. They are also affected by the switching frequency, ripple current, di/dt and magnitude of transient load current. ESR plays a key role in determining the value of capacitor to be used. Combination of both high frequency, low value ceramic capacitors and low ESR bulk storage capacitors optimised for switching applications provide the best response to load transients and ripple requirements. Electrolytic capacitors with low ESR are larger and more expensive so the ultimate choice is always a compromise between size, cost and performance. Care must also be taken to ensure that for large capacitors, the ESL of the leads does not become an issue. Excellent low ESR tantalum or electrolytic capacitors are available from Sanyo OS-CON, AVX, Sprague and Nichicon. The output capacitor will also affect loop stability, transient performance. The capacitor ESR should preferably be of a similar value to the sense resistor. Parallel devices may be required. IRIPPLE(RMS) = 0.29 VOUT (VIN−VOUT) L f VIN where L= output filter inductance f= switching frequency For output voltage ripple it is necessary to know the peak ripple current which is given by: Ipk−pk = VOUT ( VIN−VOUT) L f VIN Voltage ripple is then:- VRIPPLE = Ipk −pk ∗ ESR Input Capacitors The input capacitor is chosen for its RMS current and voltage rating. The use of low ESR electrolytic or tantalum capacitors is recommended. Tantalum capacitors should have their voltage rating at 2VIN (max), electrolytic at 1.4VIN(max). IRMS can be approximated by: IRMS = IOUT √ (VOUT(VIN−VOUT)) VIN Underspecification of this parameter can affect long term reliability. An additonal ceramic capacitor should be used to provide high frequency decoupling at VIN. Also note that the input capacitance ESR is effectively in series with the input and hence contributes to efficiency losses related to IRMS 2 * ESR of the input capacitor. MOSFET Selection The ZXRD1000 family can be configured in circuits where either N or P channel MOSFETs are employed as the main switch. If an N channel device is used, the corresponding N phase controller must be chosen. Similarly, for P channel main switch a P phase controller must be used. The ordering information has a clear identifier to distinguish between N and P phase controllers. The MOSFET selection is subject to thermal and gate drive considerations. Care also has to be taken to allow for transition losses at high input voltages as well as RDS(ON) los s e s for the m a in MO SF ET . It is recommended that a device with a drain source breakdown of at least 1.2 times the maximum VIN should be used. For optimum efficiency , two N channel low RDS(on) devices are required. MOSFETs should be selected with the lowest RDS(ON) consistent with the output current required. As a guide, for 3-4A output, <50m Ω devices would be optimum, provided the devices are low gate threshold and low gate charge. Typically look for devices that will be fully enhanced with 2.7V VGS for 4-5A capability. Zetex offers a range of low RDS(ON)logic level MOSFETs which are specifically designed with DC-DC power conversion in mind. Packaging includes SOT23, SOT23-6 and MSOP8 options. Ideal examples of optimum devices would be Zetex ZXM64N03X and ZXM64N02X (N channel). Contact your local Zetex office or Zetex web page for further information. 7 ZXRD1000 SERIES ISSUE 4 - OCTOBER 2000 |
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