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SC4901ITSTRT Datasheet(PDF) 10 Page - Semtech Corporation |
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SC4901ITSTRT Datasheet(HTML) 10 Page - Semtech Corporation |
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10 / 20 page  10 2005 Semtech Corp. www.semtech.com SC4901 POWER MANAGEMENT Having chosen CT to meet dead time requirements, the resistor is chosen so that the ramp voltage is just below 3.0V peak at the end of the maximum ON time. where RT = Timing resistor in k Ω Fsw = Switching frequency in kHz D = Operating duty cycle at which Vsec is applied CT = Timing capacitor in pF If the primary side is PWM modulated in a feedback loop or has a volt second clamp, the product Vsec x D is fairly constant. If it is free running with a fixed duty cycle, use the maximum value of Vsec. The formula assumes that Vsec is much larger than the ramp voltage of 3.0V. For lower secondary voltages, use a smaller value of RT. REF Output and Error Amplifier The reference level for output voltage feedback is 0.75V bandgap. This is amplified, buffered and put out as a 2.5V REF output. The REF voltage can be used for external monitoring circuits or for regulating output voltages less than 0.75V. The output of the error amplifier can swing between 0.3V to 2.9V, just below the clamped peak of the timing ramp. The REF pin should be bypassed to AGND with a 0.1 uF ceramic capacitor. A maximum of 5 mA can be drawn from the REF output. Delays and Turn Off Sequence The delay between turn off of the rectifying MOSFET QR to the turn on of the forward MOSFETs QS and QF is determined by the discharge of the ramp capacitor CT through a 10 mA current. The other delay, between turn off of the forward MOSFETs and turn on of the rectifying MOSFET is fixed and typically 110 nS. This delay is valid when the output is in regulation and the forward FETs are turned off by the PWM comparator. During transients or if the output loses regulation, QS and QF will be turned off on the falling edge of the transformer voltage. First, the high comparator detects the 2.5V threshold and pulls down OUTA. This turns off the forward FETs. Further down, the lower comparator detects the 0.25V threshold and turns on OUTB. This ensures that the forward FET pair is off before transformer voltage goes negative. Application Information (Cont.) Current Sense and Current Limit Current sensing is done as the input of an uncommitted differential amplifier. The current limit threshold is 2.5V at the CAO pin. This allows the user considerable flexibility to design the current sensing circuit. A very low value sense resistor which results in a current signal of a few tens of millivolts or inductor drop sensing can be used for simple protection and maximum efficiency. The current amplifier gain has to be correspondingly higher. A larger current sense signal with lower gain may be preferred for better noise immunity and more precise current limit. Maximum recommended differential input is 200 mV and minimum differential gain for the current amplifier is 10, to ensure proper operation. The CAO output is typically 2V at full load. This signal can be brought out and used further for current sharing applications. The current sense pins have a maximum common mode range of AVCC - 2V. When 2.5V limit is reached at CAO pin, the bidirectional forward switch QS/QF is turned off for the rest of the cycle. The rectifying FET is kept ON throughout to discharge the high inductor current. As the overload level is increased, the ON time continues to reduce and the output drops gradually. The output is continuously monitored for undervoltage and when it falls below 50% of the nominal voltage, an abnormal condition is detected and both the outputs are shut down. The soft start capacitor is discharged by a 2 uA current sink down to 0.8V at which point a soft start cycle is initiated to restart the convertor. This effectively provides hiccup mode of protection under short circuit. Output Drivers There are two complementary outputs designated A and B. Output A and XFRA are the PWM drives that control the bidirectional forward MOSFETs. The complementary output for driving the ground referenced rectifying MOSFET QR is designated as OUTB. This drive is capable of sourcing and sinking +1A. OUTA is enabled when the transformer voltage turns on and ZCD voltage exceeds 3.5V. It is turned off on any one of the three conditions. a) the ramp voltage on RCT exceeds the error amplifier output at VEA or b) ZCD detects that the transformer voltage is turning off and gone below 2.5V or c) overcurrent is detected at 2.5V on the CAO pin. CT Fsw D 2) (Vsec 300000 RT × × − × ≅ |
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