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SC4620EVB-MLPQ Datasheet(PDF) 11 Page - Semtech Corporation |
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SC4620EVB-MLPQ Datasheet(HTML) 11 Page - Semtech Corporation |
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11 / 19 page  11 2007 Semtech Corp. www.semtech.com SC4620 POWER MANAGEMENT Application Information Overview The SC4620 is a programmable high switching frequency, integrated 2.5A MOSFET, synchronous step down regula- tor. This reduces external component count and makes it effective for applications which are low in cost and sized small. A non-overlap protection is provided for the gate drive signals to prevent shoot through of the internal MOSFET pair. The SC4620 is capable of producing an output voltage as low as 0.5V and Its operation frequency is programmable up to 2MHz by an external resistor. It features lossless current sensing of the voltage drop across the internal drain to source resistance of the high side MOSFET during its conduction period. The quiescent supply current in shutdown mode is typically lower than 1µA. An external soft start is provided to prevent output voltage overshoot during start-up. Over Temperature Protection, Power Good Indicator, External Clock Synchronization are some of the internal added features. Enable The SC4620 is enabled by applying a voltage greater than 2V (typical) to the V CC and SYNC/EN pin. The voltage on the V CC pin determines the operation of the SC4620. As V CC increases during start up, the UVLO block senses VCC and keeps the high side and low side MOSFETs off and the internal soft start voltage low until V CC reaches 2V. If no faults are present, the SC4620 will initiate a soft start when V CC exceeds 2V. A typical 120mV hysteresis in the UVLO comparator provides noise immunity during its start up. (refer to Figure 1 to 2). Shutdown The SC4620 is disabled when V CC falls below 1.88V (typical) or shutdown mode operation is invoked by clamping the SYNC/EN pin to a voltage below 0.8V. During the shutdown mode, A typical 0.2µA current draw through the V CC pin, the internal soft start voltage is held low and the internal MOSFETs are turned off. (refer to Figure 3 to 4). Soft Start The soft start function is required for step down controllers to prevent excess in-rush current through the DC bus during start up. An external capacitor is necessary for the soft start function and is connected from SS pin to AGND. During start up or restart, A typical 6µA sourcing current charges the capacitor and then the voltage of capacitor ramp up the error amp reference slowly. The closed loop creates narrow width driver pulses while the output voltage is low and allows these pulses to increase to their steady state duty cycle as the output voltage reaches its regulated value. The duration of the soft start in the SC4620 is controlled by an external capacitor. The SC4620 starts up in asynchronous mode before SS voltage reaches to 0.5V, and the bottom FET diode is used for circulating current during the top FET off time. Ths SS voltage level is clamped at V CC finally. Oscillator The FS pin is used to set the PWM oscillator frequency through an external resistor that is connected from the FS pin to the AGND. The internal ramp is a triangle at the PWM frequency with a peak voltage of 1.25V and a valley voltage of 0.25V. The approximate operating frequency is determined by the value of an external resistor as shown in Figure 15. 5 15 25 35 45 55 65 75 85 95 105 115 125 135 145 200 400 600 800 1000 1200 1400 1600 1800 2000 5VIN 2.5VIN The operation frequency can be programmed up to 2MHz, but there is a minimum on-time limitation which is around 110ns.Usersshouldtakecareofminimumlimitationonthe operating duty cycle under high frequency application. Figure 15. Switching Frequency vs. R FS Switching Frequency Setting F osc (kHz) |
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