Datasheet SiC476, SiC477, SiC478, SiC479 (Vishay) - 10

SiC476, SiC477, SiC478, SiC479
www.vishay.com Vishay Siliconix
EXTERNAL COMPONENT SELECTION FOR THE SiC47x
This section explains external component selection for and capacitance. The maximum ESR requirement is the SiC47x family of regulators. Component reference controlled by the output ripple voltage requirement and the designators in any equation refer to the schematic shown in DC tolerance. The output voltage has a DC value that is Fig. 9. equal to the valley of the output ripple plus half of the An excel based calculator is available on the website to peak-to-peak ripple. A change in the output ripple voltage make external component calculation simple. The user will lead to a change in DC voltage at the output. The simply needs to enter required operating conditions. relationship between output voltage ripple, output capacitance and ESR of the output capacitor is shown by
Output Voltage Adjustment
the following equation: If a different output voltage is needed, simply change the value of V  1  OUT and solve for R_FB_H based on the following V = I ----------------- + ESR (1) RIPPLE RIPPLEMAX. x   formula: 8 x C x f o sw Where VRIPPLE is the maximum allowed output ripple R V - V  R _FB_L OUT FB = --------------------------- voltage; IRIPPLE(MAX.) is the maximum inductor ripple current; _FB_H VFB fsw is the switching frequency of the converter; Co is the total Where V output capacitance; ESR is the equivalent series resistance FB is 0.8 V for the SiC47x. R_FB_L should be a maximum of 10 k to prevent V of the total output capacitors. OUT from drifting at no load. In addition to the output ripple voltage requirement, the
Switching Frequency Selection
output capacitors need to meet transient requirements. A The following equation illustrates the relationship between worst case load release condition (from maximum load to no on-time, VIN, VOUT, and Rfsw value: load at the exact moment when inductor current is at the peak) determines the required capacitance. If the load VOUT release is instantaneous (load changes from maximum to R = ----------------------- _fsw –12 f  190  10 zero within 1 μs), the output capacitor must absorb all the sw energy stored in the inductor. The peak voltage on the
Inductor Selection
capacitor, VPK, under this worst case condition can be The choice of inductor is specific to each application and calculated by following equation: quickly determined with the following equations:  1  2 L x I + -- x I  OUT RIPPLE(MAX.) V 2 (2) C = ---------------------------------------- t OUT = ------------------- OUT_MIN. ON V x f V 2 - V 2 IN_max. sw PK OUT and During the load release time, the voltage across the inductor is approximately -VOUT. This causes a down-slope or falling V - V  x t IN OUT ON di/dt in the inductor. If the load di/dt is not much faster than L = ------------------------- I x K OUT_MAX. the di/dt of the inductor, then the inductor current will tend to track the falling load current. This will reduce the excess Where K is a percentage of maximum output current ripple inductive energy that must be absorbed by the output required. The designer can quickly make a choice of capacitor; therefore a smaller capacitance can be used. The inductor if the ripple percentage is decided, usually no more following can be used to calculate the required capacitance than 30 % however higher or lower percentages of IOUT can for a given diLOAD/dt. be acceptable depending on application. This device allows Peak inductor current, ILPK, is shown by the next equation: choices larger than 30 %. Other than the inductance the DCR and saturation current 1 I = I + -- x I LPK MAX. RIPPLE(MAX.) parameters are key values. The DCR causes an I2R loss 2 which will decrease the system efficiency and generate diLOAD heat. The saturation current has to be higher than the The slew rate of load current = --------- dt maximum output current plus ½ of the ripple current. In an over current condition the inductor current may be very high. I I LPK MAX. All this needs to be considered when selecting the inductor. L x ------- - ---------- x dt V dI OUT LOAD (3) C = I x --------------------------------
Output Capacitor Selection
OUT_MIN. LPK 2V - V  PK OUT The SiC47x is stable with any type of output capacitors by choosing the appropriate V Based on application requirement, either equation (2) or RAMP components. This allows the user to choose the output capacitance based on the equation (3) can be used to calculate the ideal output best trade off of board space, cost and application capacitance to meet transition requirement. Compare this requirements. calculated capacitance with the result from equation (1) and choose the larger value to meet both ripple and transition The output capacitors are chosen based upon required ESR requirement. S19-0910-Rev. C, 28-Oct-2019
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