Datasheet SIC466, SiC467, SiC468, SiC469 (Vishay) - 10

SiC466, SiC467, SiC468, SiC469
www.vishay.com Vishay Siliconix
EXTERNAL COMPONENT SELECTION FOR THE SiC46x
This section explains external component selection for The output capacitors are chosen based upon required ESR the SiC46x family of regulators. Component reference and capacitance. The maximum ESR requirement is designators in any equation refer to the schematic shown in controlled by the output ripple voltage requirement and the Fig. 9. DC tolerance. The output voltage has a DC value that is An excel based calculator is available on the website to equal to the valley of the output ripple plus half of the make external component calculation simple. The user peak-to-peak ripple. A change in the output ripple voltage simply needs to enter required operating conditions. will lead to a change in DC voltage at the output. The relationship between output voltage ripple, output
Output Voltage Adjustment
capacitance and ESR of the output capacitor is shown by If a different output voltage is needed, simply change the the following equation: value of VOUT and solve for R_FB_H based on the following  1 formula:  V = I ----------------- + ESR (1) RIPPLE RIPPLEMAX. x 8 x C x f  o sw R V - V  _FB_L OUT FB Where VRIPPLE is the maximum allowed output ripple R = --------------------------- _FB_H V voltage; I FB RIPPLE(MAX.) is the maximum inductor ripple current; fsw is the switching frequency of the converter; Co is the total Where VFB is 0.8 V for the SiC46x. R_FB_L should be a output capacitance; ESR is the equivalent series resistance maximum of 10 k to prevent VOUT from drifting at no load. of the total output capacitors.
Switching Frequency Selection
In addition to the output ripple voltage requirement, the The following equation illustrates the relationship between output capacitors need to meet transient requirements. A on-time, V worst case load release condition (from maximum load to no IN, 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 zero within 1 μs), the output capacitor must absorb all the f  190  10 sw energy stored in the inductor. The peak voltage on the
Inductor Selection
capacitor, VPK, under this worst case condition can be calculated by following equation: The choice of inductor is specific to each application and quickly determined with the following equations:  1  2 L x I + -- x I  OUT 2 RIPPLE(MAX.) (2) V C = ---------------------------------------- OUT_MIN. t OUT = ------------------- ON  2  2 V x f V - V PK OUT IN_max. sw 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 the di/dt of the inductor, then the inductor current will tend OUT_MAX. 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 I for a given di OUT can LOAD/dt. be acceptable depending on application. This device allows Peak inductor current, ILPK, is shown by the next equation: choices larger than 30 %. 1 Other than the inductance the DCR and saturation current I = I + -- x I LPK MAX. 2 RIPPLE(MAX.) parameters are key values. The DCR causes an I2R loss 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 SiC46x is stable with any type of output capacitors by choosing the appropriate VRAMP components. This allows Based on application requirement, either equation (2) or 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 S19-0911-Rev. E, 28-Oct-2019
10
Document Number: 76044 For technical questions, contact: powerictechsupport@vishay.com THIS DOCUMENT IS SUBJECT TO CHANGE WITHOUT NOTICE. THE PRODUCTS DESCRIBED HEREIN AND THIS DOCUMENT ARE SUBJECT TO SPECIFIC DISCLAIMERS, SET FORTH AT www.vishay.com/doc?91000