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SiC464ED-T1-GE3 Datasheet(PDF) 7 Page - Vishay Siliconix |
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SiC464ED-T1-GE3 Datasheet(HTML) 7 Page - Vishay Siliconix |
7 / 29 page SiC461, SiC462, SiC463, SiC464 www.vishay.com Vishay Siliconix S18-0393-Rev. K, 16-Apr-18 7 Document Number: 65124 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 pulse is generated for a fixed time. During the on-time pulse, the high side MOSFET will be turned on. Once the on-time pulse expires, the low side MOSFET will be turned on after a dead time period. The low side MOSFET will stay on for a minimum duration equal to the minimum off-time (tOFF_MIN.) and remains on until VRAMP crosses VCOMP. The cycle is then repeated. Fig. 6 illustrates the basic block diagram for voltage mode, constant on time architecture with external ripple injection, VRAMP, while Fig. 5 illustrates the basic operational principle. Fig. 5 - SiC46x Operational Principle The need for ripple injection in this architecture is explained below. First, let us understand the basic principles of this control architecture: • The reference of a basic voltage mode COT regulator is replaced with a high gain error amplifier loop. The loop ensures the DC component of the output voltage follows the internal accurate reference voltage, providing excellent regulation • A second voltage feedback path via VSNS with a VRAMP scheme ensures rapid correction of the transient perturbation • This establishes two voltage loops, one is the steady state voltage feedback path (via the FB pin) and the other is the feed forward path (via the VSNS pin). The scheme gives the user the fast transient response of a COT regulator and the stable, jitter free, line and load regulation performance of a PWM controller Choosing the Ripple Injection Component Values For stability purposes the SiC46x requires adequate ripple injection amplitude. Adequate ripple amplitude is required for two main reasons: 1. To reduce jitter due to noise coupled into the system 2. To provide stable operation. Sub harmonic oscillation can occur with constant on time ripple control if below condition is not met Therefore, when the converter design uses an all ceramic output capacitor or other low ESR output capacitors, instability can occur. In order to avoid this, a VRAMP network is used to increase the equivalent RESR in order to satisfy the above condition. The VRAMP amplitude must be large enough to avoid instability or noise sensitivity but not too large that it degrades transient performance. To ensure stable operation under CCM, DCM and ultrasonic mode, minimum VRAMP amplitude of 100 mV is recommended for the SiC46x family of regulators. A maximum VRAMP of 900 mV is recommended so as not to degrade transient response. Fig. 6 - SiC46x Control Block Diagram VRAMP amplitude is a function of frequency, VIN, and VOUT. Equation 1 VRAMP amplitude is a function of VIN, VOUT, and switching frequency and should be adjusted whenever VIN, VOUT, or switching frequency is changed. For a given buck regulator design, VOUT and switching frequency is typically fixed, while the converter may be expected to work for a wide VIN range. The VRAMP amplitude will increase as VIN is increased and increase the power dissipated by Rx. A proper selection of RX, package size and value, should take into account the maximum power dissipation at the expected operating conditions. In order to optimize the VRAMP amplitude over a desired VIN range use the following procedure to calculate Rx, Cx, and Cy. 1. The equation below calculates RX as a function of VIN, VOUT, and maximum allowable power dissipated by RX. where PRX_MAX. is the maximum allowed power dissipation in Rx. Note, the maximum power dissipation of a 0603 sized resistor is typically 25 mW. Power dissipation derating must be taken into account for high ambient temperatures 2. The equation below calculates CX_MIN. as a function of VIN and maximum allowed VRAMP amplitude. where VRAMP_MAX. = 900 mV 3. Using VRAMP equation, calculate VRAMP_MIN. at minimum VIN based on the Rx and the minimum Cx value calculated above 4. If VRAMP_MIN. is > 200 mV, set Cx to CX_MIN., otherwise set Cx to (Cx_MIN. x VRAMP_MIN./200 mV). If VRIPPLE_MIN. is < 100 mV, increase PRX_MAX. and recalculate RX and CX 5. Cy should be large enough not to distort the VRAMP and small enough not to load excessively the VRAMP network (Rx and Cx). Please use the follow formula: Cy = 1/(0.82 x fsw) This procedure allows for a maximum range of operation. In order to simplify the procedure for calculating VRAMP and compensation components, a calculator is provided (visit www.vishay.com/doc?65124). Fixed on-time V RAMP V COMP PWM ESR C OUT t ON 2 --------- C x R x L C y EA Ripple based controller R1 R2 REF R COMP C COMP Load Q1 Q2 V IN C OUT C IN V RAMP V IN V OUT – V OUT V IN f sw C x R x ------------------------------------------------------ = R x V IN_MAX. V OUT 1D – P RX_MAX. -------------------------------------------------------------------- = C X_MIN. P RX_MAX. V IN_MAX. f sw V RAMP_MAX. --------------------------------------------------------------------------- = |
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