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Moteur de recherche de fiches techniques de composants électroniques |
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Moteur de recherche de fiches techniques de composants électroniques |
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FAN5234QSCX Fiches technique(PDF) 8 Page - Fairchild Semiconductor |
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FAN5234QSCX Fiches technique(HTML) 8 Page - Fairchild Semiconductor |
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8 / 15 page  PRODUCT SPECIFICATION FAN5234 8 REV. 1.0.10 5/3/04 Figure 5. Improving current sensing accuracy More accurate sensing can be achieved by using a resistor (R1) instead of the RDS(ON) of the FET as shown in Figure 5. This approach causes higher losses, but yields greater accu- racy in both VDROOP and ILIMIT. R1 is a low value (e.g. 10m Ω) resistor. Current limit (ILIMIT) should be set sufficiently high as to allow inductor current to rise in response to an output load transient. Typically, a factor of 1.2 is sufficient. In addition, since ILIMIT is a peak current cut-off value, we will need to multiply ILOAD(MAX) by the inductor ripple current (we’ll use 25%). For example, in Figure 1 the target for ILIMIT would be: ILIMIT > 1.2 × 1.25 × 1.6 × 6A ≈ 14A (6) Duty Cycle Clamp During severe load increase, the error amplifier output can go to its upper limit pushing a duty cycle to almost 100% for significant amount of time. This could cause a large increase of the inductor current and lead to a long recovery from a transient, over-current condition, or even to a failure espe- cially at high input voltages. To prevent this, the output of the error amplifier is clamped to a fixed value after two clock cycles if severe output voltage excursion is detected, limiting the maximum duty cycle to This circuit is designed to not interfere with normal PWM operation. When FPWM is grounded, the duty cycle clamp is disabled and the maximum duty cycle is 87%. Gate Driver section The Adaptive gate control logic translates the internal PWM control signal into the MOSFET gate drive signals providing necessary amplification, level shifting and shoot-through protection. Also, it has functions that help optimize the IC performance over a wide range of operating conditions. Since MOSFET switching time can vary dramatically from type to type and with the input voltage, the gate control logic provides adaptive dead time by monitoring the gate-to-source voltages of both upper and lower MOSFETs. The lower MOSFET drive is not turned on until the gate-to-source voltage of the upper MOSFET has decreased to less than approximately 1 volt. Similarly, the upper MOSFET is not turned on until the gate-to-source voltage of the lower MOSFET has decreased to less than approximately 1 Volt. This allows a wide variety of upper and lower MOS- FETs to be used without a concern for simultaneous conduc- tion, or shoot-through. There must be a low-resistance, low-inductance path between the driver pin and the MOSFET gate for the adap- tive dead-time circuit to work properly. Any delay along that path will subtract from the delay generated by the adaptive dead-time circit and shoot-through may occur. Frequency Loop Compensation Due to the implemented current mode control, the modulator has a single pole response with -1 slope at frequency deter- mined by load where RO is load resistance, CO is load capacitance. For this type of modulator, Type 2 compensation circuit is usually sufficient. To reduce the number of external components and simplify the design task, the PWM controller has an inter- nally compensated error amplifier. Figure 6 shows a Type 2 amplifier and its response along with the responses of a current mode modulator and of the converter. The Type 2 amplifier, in addition to the pole at the origin, has a zero-pole pair that causes a flat gain region at frequencies between the zero and the pole. Figure 6. Compensation LDRV PGND ISNS R SENSE Q2 DC MAX V OUT V IN -------------- 2.4 V IN --------- + = F PO 1 2 πR OCO ---------------------- = (7) R1 R2 EA Out C1 C2 REF V IN 0 14 18 modulator F P0 F Z F P erro r am p F Z 1 2 πR 2C1 -------------------- 6kHz == (8a) F P 1 2 πR 2C2 -------------------- 600kHz == (8b) |
Numéro de pièce similaire - FAN5234QSCX |
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Description similaire - FAN5234QSCX |
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