Datasheet SiC967 (Vishay) - 10

SiC967
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EXTERNAL COMPONENT SELECTION
This section explains external component selection for the peak) determines the required capacitance. If the load SiC967 family of regulators. Component reference release is instantaneous (load changes from maximum to designators in any equation refer to the schematic shown in zero within 1 μs), the output capacitor must absorb all the Fig. 9. energy stored in the inductor. The peak voltage on the An excel based calculator is available on the website to capacitor, VPK, under this worst case condition can be make external component calculation simple. The user calculated by following equation: simply needs to enter required operating conditions.  1  2 (2) L x I + -- x I
Output Voltage Adjustment
 OUT 2 RIPPLE(MAX.) C = OUT_MIN. ---------------------------------------- If a different output voltage is needed, simply change the V 2 - V 2 PK OUT value of V Dur OUT and solve for R_FB_H based on the following ing formula: the load release time, the voltage across the inductor is R V - V  _FB_L OUT FB approximately -V R = OUT. This causes a down-slope or falling _FB_H --------------------------- VFB di/dt in the inductor. If the load di/dt is not much faster than the di/dt of the inductor, then the inductor current will tend Where VFB is 0.8 V for the SiC967. R_FB_L should be a to track the falling load current. This will reduce the excess maximum of 10 k to prevent VOUT from drifting at no load. inductive energy that must be absorbed by the output capacitor; therefore a smaller capacitance can be used. The
Switching Frequency Selection
following can be used to calculate the required capacitance for a given di The following equation illustrates the relationship between LOAD/dt. on-time, V Peak inductor current, I IN, VOUT, and Rfsw value: LPK, is shown by the next equation: V 1 I = I + -- x I R OUT = LPK MAX. RIPPLE(MAX.) _fsw --------------------- 2 12 f  190  10 sw diLOAD
Output Capacitor Selection
The slew rate of load current = --------- dt The SiC967 is stable with any type of output capacitors by choosing the appropriate VRAMP components. This allows I I the user to choose the output capacitance based on the L x LPK ------- - MAX. ---------- x dt (3) best trade off of board space, cost and application V dI C = I x OUT LOAD -------------------------------- requirements. OUT_MIN. LPK 2V - V  PK OUT The output capacitors are chosen based upon required ESR Ba and capacitance. The maximum ESR requirement is se controlled by the output ripple voltage requirement and the d on application requirement, either equation (2) or equation DC tolerance. The output voltage has a DC value that is (3) can be used to calculate the ideal output capacitance to equal to the valley of the output ripple plus half of the meet transition requirement. Compare this calculated peak-to-peak ripple. A change in the output ripple voltage capacitance with the result from equation (1) and choose the will lead to a change in DC voltage at the output. The larger value to meet both ripple and transition requirement. relationship between output voltage ripple, output capacitance and ESR of the output capacitor is shown by
Enable Pin Voltage
the following equation: The EN pin has an internal pull down resistor and only  1  V = I ----------------- + ESR (1) requires an enable voltage. This needs to be greater than RIPPLE RIPPLEMAX. x 8 x C x f  1.4 V. An input voltage or a resistor connected across V o sw IN and EN can be used. The internal pull down resistance is Where VRIPPLE is the maximum allowed output ripple 5 M. voltage; IRIPPLE(MAX.) is the maximum inductor ripple current; fsw is the switching frequency of the converter; Co is the total output capacitance; ESR is the equivalent series resistance of the total output capacitors. In addition to the output ripple voltage requirement, the output capacitors need to meet transient requirements. A worst case load release condition (from maximum load to no load at the exact moment when inductor current is at the S23-0680-Rev. C, 28-Aug-2023
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