 |
Electronic Components Datasheet Search |
|
|
Electronic Components Datasheet Search |
|
SC4901ITSTRT Datasheet(PDF) 8 Page - Semtech Corporation |
|
||||||||||||||||||||||||||||||
SC4901ITSTRT Datasheet(HTML) 8 Page - Semtech Corporation |
|
|
8 / 20 page  8 2005 Semtech Corp. www.semtech.com SC4901 POWER MANAGEMENT Application Information Introduction The SC4901 is a voltage mode PWM controller for implementing secondary side synchronous rectification and simultaneous post regulation in forward convertors. Multiple outputs can be derived off the same transformer winding. Each output is independently regulated by modulating the on time of the forward MOSFET pair which acts as a bidirectional switch. An introduction to this unique Combi Sync topology is presented later in this section. The device takes the transformer secondary voltage as the clock input and generates an internal ramp. The switching frequency and maximum duty cycle are always determined by the transformer waveform. The output current is sensed as a low level differential input signal. An amplified current signal is put out by the device for further use in current share and hot swap applications. Overload protection is achieved by peak current limiting followed by hiccup mode under short circuit. A buffered 2.5V reference is also put out for system monitoring and other purposes. The controller implements the classical trailing edge modulation and the outputs are complementary PWM signals that drive the forward and rectifying MOSFETs. The high side outputs can source up to + 2A peak current and can be easily configured for driving a pulse transformer with duty ratios greater than 50%. Undervoltage, Soft Start and Enable The SC4901 has an operating range of 4.5V to 18V with undervoltage lockout. Two conditions must be met before the controller is operational. The input supply should be above the undervoltage threshold of 4.5V and the SSEN pin should be above 0.65V typical. If not, the device is deactivated - the outputs are held active low, the SSEN and the VEA pins are also held low. The controller is in standby mode and draws only the quiescent current of 7 mA typical. Once the undervoltage threshold has been exceeded and the SSEN pin is released, the soft start capacitor at pin 5 is charged by a constant current of 10 µA. The VEA follows the SSEN voltage and the output gradually ramps up. Once the SSEN pin charges to 3.3V, the soft start cycle is complete and the device is fully operational. ZCD and RCT Since the SC4901 is specifically designed for secondary side control, the oscillator is always derived from the transformer winding. Special care must be taken to ensure that the transformer voltage, with all its variations in rise time, shape, magnitude and duty ratio is translated into a steady ramp on a cycle by cycle basis. SC4901 provides two pins, ZCD and RCT, for this purpose. The transformer voltage is sensed at the Zero Crossover Detect or the ZCD pin. A limiting resistor, RZ in the Typical Application Circuit, from the winding to ZCD pin is required to limit the peak input current into the pin to 5 mA under all conditions. The ZCD pin has two comparators with hysteretic thresholds. One comparator has lower thresholds of 0.75/0.25V and the other has 3.5V/2.5V. Together they ensure proper turn on and turn off timings for the forward and the rectifying FETs. There is also an internal clamp which prevents the pin voltage from going below zero. Ensure that the ZCD pin is not forced into any negative voltage by external circuits. During the rise, ZCD pin voltage is also internally clamped to one diode drop above the 3.5V threshold. Fig 1. Generating the Zero Crossover Detect (ZCD) and the Timing Ramp (RCT) signals The waveform at the ZCD pin should be held tightly in phase with the transformer secondary voltage. In some cases it may be useful to have a small capacitor in parallel with RZ to speed up the rise of ZCD input. QS QF QR •• T1 L1 C2 RZ RT T2 D1 U? SC4901 SINGLE ENDED FORWARD CONVERTOR RTN VO UT C3 CT |
Similar Part No. - SC4901ITSTRT |
|
Similar Description - SC4901ITSTRT |
|
|
|
Link URL |
| Privacy Policy |
| ALLDATASHEET.NET |
| Does ALLDATASHEET help your business so far? [ DONATE ] |
About Alldatasheet | Advertisement | Contact us | Privacy Policy | Link Exchange | Manufacturer List All Rights Reserved©Alldatasheet.com |
| Russian : Alldatasheetru.com | Korean : Alldatasheet.co.kr | Spanish : Alldatasheet.es | French : Alldatasheet.fr | Italian : Alldatasheetit.com Portuguese : Alldatasheetpt.com | Polish : Alldatasheet.pl | Vietnamese : Alldatasheet.vn Indian : Alldatasheet.in | Mexican : Alldatasheet.com.mx | British : Alldatasheet.co.uk | New Zealand : Alldatasheet.co.nz |
|
Family Site : ic2ic.com |
icmetro.com |
English ▼
English
Chinese
German
Japanese
Russian
Korean
Spanish
French
Italian
Portuguese
Polish
Vietnamese
Indian
Mexican
British
New Zealand
