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SC4911 数据表(PDF) 6 Page - Semtech Corporation |
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SC4911 数据表(HTML) 6 Page - Semtech Corporation |
6 / 14 page 6 2006 Semtech Corp. www.semtech.com SC4911 POWER MANAGEMENT The SC4911 is a voltage mode controller designed mainly for the isolated or non-isolated flyback converters. As shown in the Block Diagram, it includes start-up circuit, bandgap, error amplifier, PWM comparator, logic and output stage. Line undervoltage has dual functions of line voltage lockout and shutdown. When LUVLO is pull down to ground, the SC4911 is disabled and the quies- cent current is reduced to typical 30µA. When it is con- nected to a resistor voltage divider between input volt- age and ground, it monitors the line voltage. Oscillator The switching frequency of the SC4911 is set by an ex- ternal resistor connected between RT and GND. Figure 1 shows the relationship between the resistance of the external resistor and the switching frequency. Figure 1. Timing Resistor vs Oscillator Frequency Timing Resistor vs Switching Frequency 0 50 100 150 200 250 100 200 300 400 500 Frequency ( KHz ) Line UVLO and VCC UVLO The Line UVLO monitors the input line voltage and VCC UVLO monitors the supply voltage to the SC4911. If ei- ther input line or VCC is below the lockout threshold, the output is held low and the supply current to the chip is typically 30µA. A resistor divider from the input line to GND determines the desired undervoltage lockout level. To prevent noise coupling to affect proper UVLO operation, a small capaci- tor, one hundred to a few hundred pF, is recommended to be used from Line UVLO pin to GND. Start-up As shown in Typical Applications Circuit on the front page, when input voltage is applied, C1 is trickle charged through the start-up resistor R1 until the SC4911 Vcc reaches the turn-on voltage. Applications Information The bias current consumed by the device during this pe- riod is only typical 30µA. Once Vcc exceeds the turn-on voltage, OUT starts driving the MOSFET, transferring en- ergy to the secondary and the bias output. If the bias output voltage builds to and is above the Vcc turnoff voltage, the start-up is completed and normal operation begins. The size of the start-up resistor not only affects power supply start-up time, but also power supply efficiency. The resistor dissipates continuous power in normal op- eration. Due to ultra low start-up current of the SC4911, large value resistor (several hundred Kohm to 1Mohm) could be used, particularly for off-line applications to improve efficiency considering reasonable start-up time. Current Limit The current sense resistor is selected by dividing the cur- rent sense threshold voltage 0.525V by the primary peak current at the desired current limit point, typically 120% of the primary peak current. PK SENSE I 2 . 1 V 525 . 0 R × = Care must be taken to ensure proper selection of sense resistor and good layout to prevent erratic operation. The non-inductive resistor must be used as the sense resis- tor. Parasitic inductance in series with the sense resistor must be minimized. Additional RC filter may be neces- sary to eliminate the narrow spike time on the leading edge of the primary current. The RC time constant can be chosen to be equal to the time constant of the sense resistor and the parasitic inductance. PCB Layout Long power supply and ground traces should be avoided. A 0.1µF ceramic capacitor closely placed between Vcc and GND is recommended. The timing resistor used to program switching frequency should be located close to pin RT and pin GND in order to have a stable switching frequency. MOSFET should be located near the device and a resistor in the range of several ohms could be used in series with the gate drive to damp the ringing if the trace between the drive output and MOSFET is not short enough. Star ground connection is recommended to avoid ground loops. The Power path for input filter, MOSFET and transformer should be separated from signal path for timing resistor and feedback resistor, and both should be brought to a single ground point. |
类似零件编号 - SC4911 |
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类似说明 - SC4911 |
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