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SC4905B Datasheet(PDF) 8 Page - Semtech Corporation |
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SC4905B Datasheet(HTML) 8 Page - Semtech Corporation |
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8 / 24 page  SC4905A/B POWER MANAGEMENT 2006 Semtech Corp. www.semtech.com 8 VDD UNDER VOLTAGE LOCK OUT According to the application, and the voltages available, the SC4905A (UVLO = 4.4V), or the SC4905B (UVLO = 11.6V) can be used to provide the VDD undervoltage lock out function to ensure the converters controlled start up. Before the VDD UVLO has been reached, the internal ref- erence, oscillator, OUT driver, and logic are disabled. REFERENCE A 4V (SC4905A) or a 5V(SC4905B) reference voltage is available that can be used to source a typical current up to 5mA to the external circuitry. The REF can be used to provide the feed back circuitry with a regulated bias. OSCILLATOR The oscillator frequency is set by connecting a RC network as shown below. U1 SC4905 4 5 3 2 10 1 6 7 8 9 DMAX RC VFF FB REF VDD SYNC ILIM GND OUT RB 8.25k RT 280k RM 2k Rosc 499k Cosc 220p, 16V Vin The oscillator has a ramp voltage that will track the volt- age at the VFF pin (1.2V<VFF<3.6V). The oscillator peak voltage is derived by charging the oscillator capacitor (Cosc) to the VFF voltage via the oscillator resistor (Rosc). The bias current to charge the Cosc is controlled by the Rosc. Once the RC pin has reached the VFF voltage, the oscil- lator ramp is discharged by an internal switch hence cre- ating the triangle oscillator ramp. Application Information (Cont.) Since the Rosc is referenced to the input supply voltage, any variation in the supply is directly translated into a variation in the duty cycle, while maintaining the fixed frequency operation. Following equation can be used to calculate the oscilla- tor frequency: () 05 . 1 VFF C R 2 VFF Vin F OSC OSC OSC • • • − ≅ The recommended range if timing resistors is between 10 kohm and 500kohm and range of timing capacitors is between 100pF and 1000pF. Timing resistors less than 10 kohm should be avoided. |
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