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SIC403 Datasheet(PDF) 10 Page - Vishay Siliconix |
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SIC403 Datasheet(HTML) 10 Page - Vishay Siliconix |
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10 / 26 page  www.vishay.com 10 Document Number: 66550 S12-0628-Rev. C, 19-Mar-12 Vishay Siliconix SiC403 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 For technical support, please contact: analogswitchtechsupport@vishay.com APPLICATIONS INFORMATION SiC403 Synchronous Buck Converter The SiC403 is a step down synchronous buck DC/DC converter with integrated power FETs and programmable LDO. The SiC403 is capable of 6 A operation at very high efficiency in a tiny 5 mm x 5 mm - 32 pin package. The programmable operating frequency range of 200 kHz to 1 MHz, enables the user to optimize the solution for minimum board space and optimum efficiency. The buck controller employs pseudo-fixed frequency adaptive on-time control. This control scheme allows fast transient response thereby lowering the size of the power components used in the system. Input Voltage Range The SiC403 requires two input supplies for normal operation: VIN and VDD. VIN operates over the wide range from 3 V to 28 V. VDD requires a supply voltage between 3 V to 5 V that can be an external source or the internal LDO configured from VIN. Power Up Sequence The SIC403 initiates a start up when VIN, VDD, and EN/PSV pins are above the applicable thresholds. When using an external bias supply for the VDD voltage, it is recommended that the VDD is applied to the device only after the VIN voltage is present because VDD cannot exceed VIN at any time. A 10 resistor must be placed between the external VDD supply and the VDD pin to avoid damage to the device during power-up and or shutdown situations where VDD could exceed VIN unexpectedly. Shut-Down The SIC403 can be shut-down by pulling either VDD or EN/PSV pin below its threshold. When using an external supply voltage for VDD, the VDD pin must be deactivated while the VIN voltage is still present. A 10 resistor must be placed between the external VDD supply and the VDD pin to avoid damage to the device. When the VDD pin is active and EN/PSV is at low logic level, the output voltage discharges through an internal FET. Pseudo-Fixed Frequency Adaptive On-Time Control The PWM control method used for the SiC403 is pseudo-fixed frequency, adaptive on-time, as shown in figure 1. The ripple voltage generated at the output capacitor ESR is used as a PWM ramp signal. This ripple is used to trigger the on-time of the controller. The adaptive on-time is determined by an internal oneshot timer. When the one-shot is triggered by the output ripple, the device sends a single on-time pulse to the highside MOSFET. The pulse period is determined by VOUT and VIN; the period is proportional to output voltage and inversely proportional to input voltage. With this adaptive on-time arrangement, the device automatically anticipates the on-time needed to regulate VOUT for the present VIN condition and at the selected frequency. The adaptive on-time control has significant advantages over traditional control methods used in the controllers today. • Reduced component count by eliminating DCR sense or current sense resistor as no need of a sensing inductor current. • Reduced saves external components used for compensation by eliminating the no error amplifier and other components. • Ultra fast transient response because of fast loop, absence of error amplifier speeds up the transient response. • Predictable frequency spread because of constant on-time architecture. • Fast transient response enables operation with minimum output capacitance Overall, superior performance compared to fixed frequency architectures. On-Time One-Shot Generator (tON) and Operating Frequency The SiC403 have an internal on-time one-shot generator which is a comparator that has two inputs. The FB Comparator output goes high when VFB is less than the internal 750 mV reference. This feeds into the gate drive and turns on the high-side MOSFET, and also starts the one-shot timer. The one-shot timer uses an internal comparator and a capacitor. One comparator input is connected to VOUT, the other input is connected to the capacitor. When the on-time begins, the internal capacitor charges from zero volts through a current which is proportional to VIN. When the capacitor voltage reaches VOUT, the on-time is completed and the high-side MOSFET turns off. The figure 2 shows the on-chip implementation of on-time generation. Figure 1 - Output Ripple and PWM Control Method VIN CIN VLX Q1 Q2 L ESR + FB VLX tON VFB COUT VOUT FB threshold |
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