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CS5127 Datasheet(PDF) 8 Page - Cherry Semiconductor Corporation |
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CS5127 Datasheet(HTML) 8 Page - Cherry Semiconductor Corporation |
8 / 24 page 8 Applications Information: continued microseconds to return to a steady-state. V2ª control uses the ripple voltage from the output capacitor and a ÒfastÓ control loop to respond to load transients, with the result that the transient response of the CS5127 is very close to the theoretical limit. Response times are defined below. tRESPONSE(INCREASING) = tRESPONSE(DECREASING) = Note that the response time to a load decrease is limited only by the inductor value. Inductor current rating is an important consideration. If the regulated output is subject to short circuit or overcur- rent conditions, the inductor must be sized to handle the fault without damage. Sizing the inductor to handle fault conditions within the maximum DC current rating helps to ensure the coil doesnÕt overheat. Not only does this pre- vent damage to the inductor, but it reduces unwanted heat generated by the system and makes thermal management easier. Selecting an open core inductor will minimize cost, but EMI/EMC performance may be degraded. This is a tough choice, since there are no guidelines to ensure these com- ponents will not prove troublesome. Core materials influence the saturation current and satura- tion characteristics of the inductor. For example, a slightly undersized inductor with a powdered iron core may pro- vide satisfactory operation because powdered iron cores have a ÒsoftÓ saturation curve compared to other core materials. Small physical size, low core losses and high temperature operation will also increase cost. Finally, consider whether an alternate supplier is an important consideration. All of these factors can increase the cost of the inductor. For light load designs, the CS5127 will operate in discon- tinuous current mode (DCM). In this regime, external components can be smaller, since high power dissipation is not an issue. In discontinuous mode, maximum output current is defined as: IOUT(MAX) = where IPK is the maximum current allowed in the switch FET. Output capacitors are chosen primarily on the value of equivalent series resistance, because this is what deter- mines how much output ripple voltage will be present. Most polarized capacitors appear resistive at the typical oscillator frequencies of the CS5127. As a rule of thumb, physically larger capacitors have lower ESR. The capaci- torÕs value in µF is not of great importance, and values from a few tens of µF to several hundreds of µF will work well. Tantalum capacitors serve very well as output capaci- tors, despite their bad reputation for spectacular failure due to excessive inrush current. This is not usually an issue for output capacitors, because the failure is not associated with discharge surges. Ripple current in the output capaci- tor is usually small enough that the ripple current rating is not an issue. The ripple current waveform is triangular, and the formula to calculate the ripple current value is: IRIPPLE = and output ripple voltage due to inductor ripple current is given by: VRIPPLE(ESR) = A load step will produce an instantaneous change in output voltage defined by the magnitude of the load step, capacitor ESR and ESL. DVO = (DIO ´ ESD) + ESL A good practice is to first choose the output capacitor to accommodate voltage transient requirements and then to choose the inductor value to provide an adequate ripple voltage. Increasing a capacitorÕs value typically reduces its ESR, but there is a limit to how much improvement can be had. In most applications, placing several smaller capacitors in parallel will result in acceptable ESR while maintaining a small PC board footprint. A warning is necessary at this point. The V2ª topology relies on the presence of some amount of output ripple voltage being present to provide the input signal for the ÒfastÓ control loop, and it is impor- tant that some ripple voltage be present at the lightest load condition in normal operation to avoid subharmonic oscil- lation. Externally generated slope compensation can be added to ensure proper operation. The VFFB lead is tied to the PWM comparatorÕs non-invert- ing input, and provides the connection for the externally-generated artificial ramp signal that is required whenever duty cycle is greater than 50%. Selecting the VFFB Lead Components DI DT (VIN - VOUT) ´ VOUT ´ ESR f ´ L ´ VIN (VIN - VOUT)VOUT f ´ L ´ VIN Selecting the Output Capacitor (IPK)2 f ´ L(VIN) 2VOUT ´ (VIN(MAX) - VOUT) Operating in Discontinuous Current Mode Other Inductor Selection Concerns L(ÆIOUT) VOUT L(ÆIOUT) (VIN - VOUT) ´ 0.85 |
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