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DCP021212DU Datasheet(PDF) 9 Page  Texas Instruments 


DCP021212DU Datasheet(HTML) 9 Page  Texas Instruments 
9 / 17 page www.ti.com Decoupling Ceramic Capacitors Input Capacitor and the Effects of ESR 0 f O Frequency X L Z Where: X isthereactanceduetothecapacitance. C X isthereactanceduetotheESL. f istheresonantfrequency. L O Z= (X X ) Ö  C L 2 2 +(ESR) X C DCP02 Series SBVS011K – MARCH 2000 – REVISED FEBRUARY 2008 All capacitors have losses because of internal If the input decoupling capacitor is not ceramic with equivalent series resistance (ESR), and to a lesser <20m Ω ESR, then at the instant the power transistors degree, equivalent series inductance (ESL). Values switch on, the voltage at the input pins falls for ESL are not always easy to obtain. However, momentarily. Should the voltage fall below some manufacturers provide graphs of frequency approximately 4V, the DCP detects an undervoltage versus capacitor impedance. These graphs typically condition and switches the DCP drive circuits to the show the capacitor impedance falling as frequency is off state. This detection is carried out as a precaution increased (as shown in Figure 10). As the frequency against a genuine low input voltage condition that increases, the impedance stops decreasing and could slow down or even stop the internal circuits begins to rise. The point of minimum impedance from operating correctly. A slowdown or stoppage indicates the resonant frequency of the capacitor. would result in the drive transistors being turned on This frequency is where the components of too long, causing saturation of the transformer and capacitance and inductance reactance are of equal destruction of the device. magnitude. Beyond this point, the capacitor is not Following detection of a low input voltage condition, effective as a capacitor. the device switches off the internal drive circuits until the input voltage returns to a safe value. Then the device tries to restart. If the input capacitor is still unable to maintain the input voltage, shutdown recurs. This process is repeated until the capacitor is charged sufficiently to start the device correctly. Otherwise, the device will be caught up in a loop. Normal startup should occur in approximately 1ms from power being applied to the device. If a considerably longer startup duration time is encountered, it is likely that either (or both) the input supply or the capacitors are not performing adequately. For 5V to 15V input devices, a 2.2 µF lowESR ceramic capacitor ensures a good startup Figure 10. Capacitor Impedance vs Frequency performance. For the remaining input voltage ranges, 0.47 µF ceramic capacitors are recommended. Tantalum capacitors are not recommended, since At fO, XC = XL; however, there is a 180° phase most do not have lowESR values and will degrade difference resulting in cancellation of the imaginary performance. If tantalum capacitors must be used, component. The resulting effect is that the impedance close attention must be paid to both the ESR and at the resonant point is the real part of the complex voltage as derated by the vendor. impedance; namely, the value of the ESR. The resonant frequency must be well above the 800kHz Output Ripple Calculation Example switching frequency of the DCP and DCVs. DCP020505: Output voltage 5V, Output current 0.4A. The effect of the ESR is to cause a voltage drop At full output power, the load resistor is 12.5 Ω. Output within the capacitor. The value of this voltage drop is capacitor of 1 µF, ESR of 0.1Ω. Capacitor discharge simply the product of the ESR and the transient load time 1% of 800kHz (ripple frequency): current, as shown: tDIS = 0.0125µs VIN = VPK – (ESR × ITR) (1) τ = C × R LOAD Where: τ = 1 × 106 × 12.5 = 12.5µs VIN is the voltage at the device input. VDIS = VO(1 – EXP(–tDIS/τ)) VPK is the maximum value of the voltage on the VDIS = 5mV capacitor during charge. By contrast, the voltage dropped because of ESR: ITR is the transient load current. VESR = ILOAD × ESR The other factor that affects the performance is the VESR = 40mV value of the capacitance. However, for the input and Ripple voltage = 45mV the full wave outputs (singleoutput voltage devices), ESR is the dominant factor. Copyright © 2000–2008, Texas Instruments Incorporated Submit Documentation Feedback 9 Product Folder Link(s): DCP02 Series 
