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SC251 Datasheet(PDF) 10 Page - Semtech Corporation |
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SC251 Datasheet(HTML) 10 Page - Semtech Corporation |
10 / 19 page 10 © 2006 Semtech Corp. www.semtech.com SC251 POWER MANAGEMENT Applications Information (Cont.) Inductor Selection The SC251 is designed for use with a 4.7μH inductor. The magnitude of the inductor current ripple is dependent on the inductor value and can be determined by the following equation: OSC N I OUT OUT L f L V V 1 V I × ⎟ ⎠ ⎞ ⎜ ⎝ ⎛ − = Δ The inductor should have a low DC Resistance (DCR) to minimize the conduction losses and maximize efficiency. As a minimum requirement, the DC current rating of the inductor should be equal to the maximum load current plus half of the inductor current ripple as shown by the following equation: 2 I I I L ) MAX ( OUT LPK Δ + = Final inductor selection will depend on various design considerations such as efficiency, EMI, size and cost. Table 1 lists the manufacturers of practical inductor options. C IN Selection The source input current to a buck converter is non- continuous. To prevent large input voltage ripple a low ESR ceramic capacitor is required. A minimum value of 10μF should be used for sufficient input voltage filtering and a 22μF should be used for improved input voltage filtering. C OUT Selection The internal compensation is designed to work with a certain output filter corner frequency defined by the equation: OUT C C L 2 1 f × π = This single pole filter is designed to operate with an output capacitor value of 4.7μF. Output voltage ripple is a combination of the voltage ripple from the inductor current charging and discharging the output capacitor and the voltage created from the inductor current ripple through the output capacitor ESR. Selecting an output capacitor with a low ESR reduces the bias supply for power amplifiers. This output provides a regulated 2.85V with output current capability up to 10mA. The 2.85V output is guaranteed for input supply voltages in excess of 2.95V. When input voltages below 2.95V are used, V REF is equal to VIN - VREF DO. This reference supply is controlled by the same enable pin as the switching regulator. Protection Features The SC251 provides the following protection features: Thermal shutdown Current limit Under voltage lockout Thermal Shutdown The device has a thermal shutdown feature to protect the device if the junction temperature exceeds 160°C. In thermal shutdown the on-chip power devices are disabled, effectively tri-stating the LX output. Switching will resume when the temperature drops by 15°C. Short Circuit Protection The PMOS and NMOS power devices of the buck switcher stage are protected by current limit functions. In the event of a short to ground on the output, the LX pin will switch with minimum duty cycle. The duty cycle is short enough to allow the inductor to discharge during each cycle, thereby preventing the inductor current from “staircasing”. The pass-through PMOS is protected by a current limit function. When the part is enabled in pass-through, the output capacitor charges up with a large surge current. In order to support this surge current and to protect against short circuits, an internal timer is used. A short circuit condition must exist for more than 128 clock cycles before the pass-through device is disabled. After an additional 2048 clock cycles, the pass-through device will turn back on. This cycle will continue until the short circuit is removed. This method allows the part to manage thermal dissipation and recover when the fault condition is removed. Under Voltage Lockout The part will turn itself off if the input supply voltage falls below 2.4V typical. The device is allowed to turn on again when the input supply voltage increases above the lockout voltage. Hysteresis is included to prevent chattering. • • • Applications Information (Cont.) |
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