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SC2544 Datasheet(PDF) 8 Page - Semtech Corporation |
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SC2544 Datasheet(HTML) 8 Page - Semtech Corporation |
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8 / 24 page  8 2005 Semtech Corp. www.semtech.com SC2544 POWER MANAGEMENT Overview Overview Overview Overview Overview The SC2544 is a constant frequency 2-phase voltage mode step-down PWM switching controller driving all N-channel MOSFETs. The two channels of the controller operate at 180 degree out of phase from each other. Since input currents are interleaved in a two-phase converter, input ripple current is lower and smaller input capacitance can be used for filtering. Also, with lower inductor current and smaller inductor ripple current per phase, overall I2 R losses are reduced. Frequency Setting Frequency Setting Frequency Setting Frequency Setting Frequency Setting The frequency of the SC2544 is user- programmable.The oscillator of SC2544 can be pro- grammed with an external resistor from the Rosc pin to the ground. The step-down controller is ca- pable of operating up to 300KHz. The relationship between oscillation frequency versus oscillation resis- tor is shown in Figure 1. The advantages of using constant frequency operation are simple passive component selection and ease of feedback compensation. Before setting the operating frequency, 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 components 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 issues are also to be considered. The frequency bands for signal transmission should be avoided because of EM interference. 50 100 150 200 250 300 350 Rosc (KOHM) Figure 1. Switching frequency versus Rosc. This soft start scheme will ensure the duty cycle to increase slowly, therefore limiting the charging current into the output capacitor and also ensuring the inductor does not saturate. The soft start capacitor will eventually be charged up to 2.5V. The soft-start sequence is initiated when EN pin is high and Vcc >4.5V or during recovery from a fault condition ( OCP, OVP, or UVLO). The period of start up can be programed by the soft start capacitor: A V Css Tss µ 84 5 . 2 × = Soft Start Soft Start Soft Start Soft Start Soft Start During start-up, the reference voltage of the error amplifier equals 30% of the voltage on Css (soft- start capacitor) which is connected between the SS pin and ground. When the controller is enabled (by pulling EN pin high), one internal 84uA current source, I SS, (soft start current) will charge the soft- start capacitor gradually. The PWM output starts pulsing when the soft start voltage reaches 1V. Shutdown Shutdown Shutdown Shutdown Shutdown When the EN pin is pulled low, an internal 15uA current source discharges the soft-start capacitor and DRVH/DRVL signals stop pulsing. The output voltage ramps down at a rate determined by the load condition. The SC2544 can also be shutdown by pulling down directly on the SS pin. The designer needs to consider the slope of the SS pin voltage and choose a suitable pull down resistor to prevent the output from undershooting. Shutdown can also be triggered when an OCP condi- tion occurs. When an OCP condition is detected, DRVH and DRVL will stop pulsing and enter a “tri- state shutdown” with the output voltage ramping down at a rate determined by the load condition. The internal 15uA current source will begin dis- charging the soft-start capacitor and when the soft-start voltage reaches 0.65V, DRVL will go high. Applications Information |
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