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SC2542TETRT Datasheet(PDF) 11 Page - Semtech Corporation |
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SC2542TETRT Datasheet(HTML) 11 Page - Semtech Corporation |
11 / 27 page 11 © 2005 Semtech Corp. www.semtech.com SC2542 POWER MANAGEMENT Applications Information (Cont.) (b) Figure 6. Typical waveforms of SKIP mode. Frequency Setting The frequency of the SC2542 is user- programmable. The oscillator of SC2542 can be programmed with an external resistor from the Rosc pin to ground. The step- down controller is capable of operating up to 300KHz. The relationship between oscillation frequency versus oscillation resistor is shown in Figure 7. The advantages of using constant frequency operation include simple passive component selection and ease of feedback compensation. Before setting the operating fre- quency, the following trade-offs should be considered. 1) Passive component size 2) Circuitry efficiency 3) EMI condition 4) Minimum switch on time 5) Maximum duty ratio For a given output power, the sizes of the passive com- ponents are inversely proportional to the switching frequency, whereas MOSFETs/Diodes switching losses are proportional to the operating frequency. Other issues such as heat dissipation, packaging, and cost also need to be considered. The frequency bands for signal trans- mission should be avoided because of EM interference. 50 100 150 200 250 300 350 Rosc(KOHM) Figure 7. Switching frequency versus Rosc. Power on Sequence After applying the input voltage and with STBY and EN inputs high, Vcc and 5VSTBY will rise. When Vcc rises above the UVLO threshold both switcher outputs will also ramp up. By pulling the SS pins low, the two switcher outputs can be disabled, with Vcc and 5VSTBY maintain- ing regulation. Vcc will turn off when STBY is pulled low, with 5VSTBY maintaining regulation. And finally 5VSTBY will turn off when EN is pulled low, and the device will be in shutdown mode. e d o Mt u p n i c i g o L : e d o M l a m r o N n o i t a l u g e r n i s l i a r M W P h t o B n o i t a l u g e r n i l i a r Y B T S V 5 V C C n o i t a l u g e r n i l i a r h g i h c i g o l o t n i p N E g n i t t e S h g i h c i g o l o t n i p Y B T S g n i t t e S S S f o n w o d ll u p o N : e d o M y b d n a t S n o i t a l u g e r n i l i a r Y B T S V 5 h g i h c i g o l o t n i p N E g n i t t e S h g i h c i g o l o t n i p Y B T S g n i t t e S : e d o M n w o d t u h S w o l c i g o l o t n i p N E g n i t t e S Soft Start During start-up, the reference voltage of the error ampli- fier equals 30% of the voltage on Css (soft-start capaci- tor) which is connected between the SS pin and ground. When the controller is enabled (by pulling EN pin or STBY TG BG Vo Inductor Current TG BG Vo Inductor Current |
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