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SC4524EVB Datasheet(PDF) 9 Page - Semtech Corporation |
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SC4524EVB Datasheet(HTML) 9 Page - Semtech Corporation |
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9 / 21 page  9 ã 2006 Semtech Corp. www.semtech.com SC4524 POWER MANAGEMENT Applications Information Setting the Output Voltage The regulator output voltage is set with an external resistive divider (Figure 4) with its center tap tied to the FB pin. 9 5 5 287 - = (1) The percentage error due the input bias current of the error amplifier is 9 5 5 Q$ 9 9 287 287 ÔÏ ¼ ¼ - = D . Example: Determine the output voltage error of a 9 9287 = converter with W = N 5 . From (1), W = - ¼ W = N N 5 9 N N Q$ 9 9 287 287 - = ÔÏ ¼ ¼ - = D . This error is at least an order of magnitude lower than the ratio tolerance resulting from the use of 1% resistors in the divider string. Setting the Switching Frequency The switching frequency of the SC4524 is set with an external resistor from the ROSC pin to ground. A graph of switching frequency against R OSC is shown in Typical Performance Characteristics. The switching frequency is programmable up to 1.5MHz. The switching frequency is limited by the minimum controllable on time at low duty cycles. For V IN > 20V, setting switching frequency below 500kHz makes converter output short circuit operation more robust. These will be described in more details later. Minimum On Time Consideration The operating duty cycle of a non-synchronous step-down switching regulator in continuous-conduction mode (CCM) is given by &(6$7 ' ,1 ' 287 9 9 9 9 9 ' - + + = (2) where V CESAT is the switch saturation voltage and VD is voltage drop across the rectifying diode. Duty cycle decreases with increasing 287 ,1 9 9 ratio. In peak current-mode control, the PWM modulating ramp is the sensed current ramp of the power switch. This current ramp is absent unless the switch is turned on. The intersection of this ramp with the output of the voltage feedback error amplifier determines the switch pulse width. The propagation delay time required to immediately turn off the switch after it is turned on is the minimum controllable switch on time (T ON(MIN)). Closed- loop measurement of the SC4524 with low ,1 287 9 9 ratios WPVU S! S $6 A7à à à à T8#$!# Figure 4. V OUT is set with a Resistive Divider HvvÃPÃUvrà Ã6ivrÃUrr
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r Urr
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rÃ8 U 2 1 0 ,1 Ã Figure 5. Variation of Minimum On Time with Ambient Temperature. |
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