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SC4808BIMSTRT Datasheet(PDF) 14 Page - Semtech Corporation |
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SC4808BIMSTRT Datasheet(HTML) 14 Page - Semtech Corporation |
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14 / 31 page  14 2005 Semtech Corp. www.semtech.com SC4808A/B POWER MANAGEMENT VCC UNDER VOLTAGE LOCK OUT Depending on the application and the voltages available, the SC4808A (UVLO = 12.0V), or the SC4808B (UVLO = 4.4V) 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 SC4808 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 5V (SC4808A) or a 3.125V(SC4808B) 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 pro- vide the oscillator RC network with a regulated bias. Application Information (Cont.) OSCILLATOR 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 SC4808. 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 1 _ × ≅ TOT OSC A OSC C R F 9 . 0 1 _ × ≅ TOT OSC B OSC C R F where: Circuit SC OSC TOT C C C C + + = 4808 pF C SC 22 4808 ≅ 3 10 3 5 . 0 − ⋅ × × ≅ REF OSC deadtime 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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