Datenblatt-Suchmaschine für elektronische Bauteile |
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CS5127GDW16 Datenblatt(PDF) 7 Page - Cherry Semiconductor Corporation |
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CS5127GDW16 Datenblatt(HTML) 7 Page - Cherry Semiconductor Corporation |
7 / 24 page The feedback (VFB) leads are connected to external resistor dividers to set the output voltage. The on-chip error ampli- fier is referenced to 1.275V, and the resistor divider values are determined by selecting the desired output voltage and the value of the divider resistor connected between the VFB lead and ground. Resistor R1 is chosen first based on a design trade-off of system efficiency vs. output voltage accuracy. Low values of divider resistance consume more current which decreas- es system efficiency. However, the VFB lead has a 1µA maximum bias current which can introduce errors in the output voltage if large resistance values are used. The approximate value of current sinking through the resistor divider is given by IV(FB) = The output voltage error that can be expected due to the bias current is given by Error Percentage = ´ 100% where R1 is given in ohms. For example, setting R1 = 5K yields an output voltage error of 0.39% while setting the feedback divider current at 255µA. Larger currents will result in reduced error. Figure 3: Feedback resistor divider. R2 can be sized according to the following formula once the desired output voltage and the value of R1 have been determined: R2 = R1 -1 There are many factors to consider when choosing the inductor. Maximum load current, core losses, winding losses, output voltage ripple, short circuit current, satura- tion, component height, EMI/EMC and cost are all variables the designer must consider. Inductance values between 1µH and 50µH are suitable for use with the CS5127. Low values within this range minimize the component size and improve transient response, but larger values reduce ripple current. Choosing the inductor value requires the designer to make some choices early in the design. Output current, output voltage and the input voltage range should be known in order to make a good choice. The input voltage range is bracketed by the maximum and minimum expected values of VIN. Most computer applica- tions use a fairly well-regulated supply with a typical output voltage tolerance on the order of ±5%. The values of VIN(MAX) and VIN(MIN) are used to calculate peak current and minimum inductance value, respectively. However, if the supply is well-regulated, these calculations may both be made using the typical input voltage value with very little error. Current in the inductor while operating in the continuous current mode (CCM) is defined as the load current plus the inductor ripple current: IL = IOUT + IRIPPLE The ripple current waveform is triangular, and the current is a function of the voltage across the inductor, the switch on-time and the inductor value. Switch on-time is the duty cycle divided by the operating frequency, and duty cycle can be defined as the ratio of VOUT to VIN, such that IRIPPLE = The peak current can be described as the load current plus half of the ripple current. Peak current must be less than the maximum rated switch current. This limits the maxi- mum load current that can be provided. It is also important that the inductor can deliver the peak current without saturating. IOUT(MAX) = ISWITCH(MAX) - Since the peak inductor current must be less than or equal to the peak switch current, the minimum value of induc- tance can be calculated: LMIN = The theoretical limit on load current transient response is a function of the inductor value, the load transient and the voltage across the inductor. In conventionally-controlled regulators, the actual limit is the time required by the con- trol loop. Conventional current-mode and voltage-mode control loops adjust the switch duty cycle over many oscil- lator periods, often requiring tens or even hundreds of Load Current Transient Response (VIN(MIN) - VOUT)VOUT f ´ VIN(MIN) ´ ISWITCH(MAX) (VIN(MAX) - VOUT)VOUT 2f ´ L ´ VIN(MAX) (VIN - VOUT)VOUT f ´ L ´ VIN Selecting the Inductor ) VOUT 1.275 ( COMP VFB GATE R2 Output Driver - + 1.275V VOUT R1 (1E - 6) ´ R1 1.275 1.275V R1 Selection of Feedback Lead Divider Resistor Values 7 Applications Information |
Ähnliche Teilenummer - CS5127GDW16 |
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Ähnliche Beschreibung - CS5127GDW16 |
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