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SC4808CMSTRT Datasheet(PDF) 7 Page - Semtech Corporation |
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SC4808CMSTRT Datasheet(HTML) 7 Page - Semtech Corporation |
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7 / 24 page  7 2006 Semtech Corp. www.semtech.com SC4808C POWER MANAGEMENT VCC UNDER VOLTAGE LOCK OUT Depending on the application and the voltages available, the SC4808C can be used to provide the VCC undervoltage lock out function to ensure the converters controlled start up. Before the VCC UVLO has been reached, the internal refer- ence, oscillator, OUTA/OUTB drivers, and logic are disabled. LINE UNDER VOLTAGE LOCK OUT The SC4808C also provides a line undervoltage (LUVLO = Vref) function. The LUVLO pin is programmed via an exter- nal resistor divider connected as shown below. The actual start-up voltage can be calculated by using the equation below: () 33 R 33 R 23 R V V REF Startup + × = R27 15k R24 10k 200p C31 VCC R26 2.2k R28 10 2.2u,16V C26 82p C29 15 REF 0.1u,25V C33 R25 18 R33 10k SYNC U4 SC4808 4 5 3 2 10 1 6 7 8 9 FB REF CS RC LUVLO SYNC GND OUTB OUTA VCC R23 56.2k Vin REFERENCE A 3.125V(SC4808C) reference voltage is available that can be used to source a typical current of 5mA to the external circuitry. The Vref can be used to provide the oscillator RC network with a regulated bias. OSCILLATOR Application Information (Cont.) The oscillator frequency is set by connecting a RC network as shown below. 0 R27 15k 0 200p C31 VCC R28 10 2.2u,16V C26 REF 0.1u,25V C33 R33 10k SYNC U4 SC4808 4 5 3 2 10 1 6 7 8 9 FB REF CS RC LUVLO SYNC GND OUTB OUTA VCC R23 56.2k Vin The oscillator has a ramp voltage of about Vref/2. The os- cillator frequency is twice the frequency of the OUTA and OUTB gate drive controls. The oscillator capacitor C31 is charged by a current sourced from the Vref through R27. Once the RC pin reaches about Vref/2, the capacitor is discharged internally by the SC4808C. It should be noted that larger capacitor values will result in a longer dead time during the down slope of the ramp. The following equation can be used as an approximation of the oscillator frequency and the Dead time: 8 . 0 C R 1 F TOT OSC OSC × ≅ where: Circuit 4808 SC OSC TOT C C C C + + = pF 22 C 4808 SC ≅ 3 REF OSC deadtime 10 3 5 . 0 V C T − ⋅ × × ≅ The recommended range of timing resistors is between 10 kOhm and 200kOhm, range of timing capacitors is between 100pF and 1000pF. Timing resistors less than 10 kOhm should be avoided. |
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