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CRCW06030000ZOEA Datasheet(PDF) 10 Page - Vishay Siliconix |
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CRCW06030000ZOEA Datasheet(HTML) 10 Page - Vishay Siliconix |
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10 / 24 page  www.vishay.com 10 Document Number: 63388 S13-0248-Rev. B, 04-Feb-13 Vishay Siliconix SiC414, SiC424 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 questions, contact: powerictechsupport@vishay.com On-Time One-Shot Generator (tON) and Operating Frequency The figure 2 shows the on-chip implementation of on-time generation. 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. This method automatically produces an on-time that is proportional to VOUT and inversely proportional to VIN. Under steady-state conditions, the switching frequency can be determined from the on-time by the following equation. The SIC414 and SiC424 uses an external resistor to set the ontime which indirectly sets the frequency. The on-time can be programmed to provide operating frequency from 200 kHz to 1 MHz using a resistor between the tON pin and ground. The resistor value is selected by the following equation. The maximum RtON value allowed is shown by the following equation. Immediately after the on-time, the DL (drive signal for the low side FET) output drives high to turn on the low-side MOSFET. DL has a minimum high time of ~ 320 ns, after which DL continues to stay high until one of the following occurs: •VFB falls below the 750 mV reference. • The zero cross detector senses that the voltage on the LX node is below ground. Power save is activated when a zero crossing is detected. tON limitations and V5V Supply Voltage For V5V below 4.5 V, the tON accuracy may be limited by the input voltage. The original RtON equation is accurate if VIN satisfies the below relation over the entire VIN range: VIN < (V5V - 1.6 V) x 10 If VIN exceeds (V5V - 1.6 V) x 10, for all or part of the VIN range, the RtON equation is not accurate. In all cases where VIN > (V5V - 1.6 V ) x 10, the RtON equation must be modified as follows. Note that when VIN > (V5V - 1.6 V) x 10, the actual on-time is fixed and does not vary with VIN. When operating in this condition, the switching frequency will vary inversely with VIN rather than approximating a fixed frequency. VOUT Voltage Selection The switcher output voltage is regulated by comparing VOUT as seen through a resistor divider at the FB pin to the internal 750 mV reference voltage, see figure 3. Note that this control method regulates the valley of the output ripple voltage, not the DC value. The DC output voltage VOUT is offset by the output ripple according to the following equation. VOUT = 0.75 x (1 + R1/R2) + VRIPPLE/2 Enable and Power-Save Inputs The EN/PSV and ENL inputs are used to enable or disable the switching regulator and the LDO. When EN/PSV is low (grounded), the switching regulator is off and in its lowest power state. When off, the output of the switching regulator soft-discharges the output into a 10 internal resistor via the VOUT pin. When EN/PSV is allowed to float, the pin voltage will float to 33 % of the voltage at V5V. The switching regulator turns on with power-save disabled and all switching is in forced continuous mode. For V5V < 4.5 V, it is recommended to force 33 % of the V5V voltage on the EN/PSV pin to operate in forced continuous mode. When EN/PSV is high (above 45 % of the voltage at V5V) for SiC414, the switching regulator turns on with ultrasonic Figure 2 - On-Time Generation FB V REF - + VOUT VIN Rton On-time = K x Rton x (VOUT/VIN) FB comparator One-shot timer Gate drives DH DL Q1 Q2 L Q1 ESR FB VOUT COUT VLX + V IN fSW = VOUT tON x VIN R tON = 1 25 pF x f sw - 400 Ω x V IN V OUT Rton_MAX = VIN_MIN 15 µA Figure 3 - Output Voltage Selection R tON = 1 25 pF x f sw - 400 Ω x (V5V - 1.6 V) x 10 V OUT VOUT R1 R2 To FB pin |
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