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SC251 Datasheet(PDF) 1 Page - Semtech Corporation |
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SC251 Datasheet(HTML) 1 Page - Semtech Corporation |
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1 / 19 page  1 SC251 Step-down DC-DC Converter with Bias LDO for WCDMA Amplifiers POWER MANAGEMENT May 8, 2006 www.semtech.com Description Applications Features The SC251 is a synchronous step-down converter designed for use as an adaptive voltage supply for WCDMA RF power amplifiers (PAs). An analog control input is used to adjust the output voltage dynamically between 0.5V and 3.4V using a non-linear transfer function. The non-linear relationship maximizes total system efficiency by providing the PA with the minimum voltage it needs to maintain linearity. For output voltages greater than 3.4V the input is connected directly to the output via an internal PMOS switch. An optional gate drive (GD) output to control an external low on-resistance PMOS switch is also provided for systems applications that require minimal voltage drop. The SC251 also provides a 2.85V LDO reference output that can be used to supply a PA bias input. Low power and high power modes are provided to match performance with dual mode PAs. In low power mode the output voltage follows an exponential relationship with the VDAC input until it reaches 3.4V. When the VMODE pin changes state, V OUT follows an alternate exponential relationship. The SC251 is capable of supplying output current up to 800mA. Standby current is <1μA when the device is disabled. The internal clock runs at 1MHz so that small surface mount inductors and capacitors can be used. V OUT exponentially proportional to VDAC for maximum efficiency (patent pending) Output range and pass-through mode - 0.5V to 3.4V Output current - 800mA Shutdown current - < 1μA LDO PA bias supply - 2.85V, 10mA Internal clock - 1MHz Continuous short circuit protection on VOUT Duty cycle mode - 100% Internal PMOS bypass transistor Gate drive available for external bypass transistor Over 90% efficiency Low and high power modes for optimum dual-mode PA efficiency Switching time (lowest to highest output) < 40μs Micro-lead frame package MLPD-10, 3mm x 3mm 3G mobile phones - RF PA power supply WCDMA power amplifier modules Wireless modems PA L1 4.7μH CIN 10μF VIN VOUT VREF GND LX VDAC PGND CREF 1μF EN COUT 4.7μF VIN 2.7 to 5V 2 8 10 7 1 4 6 9 RF Output RF Input Vcc GND BIAS ENABLE VDAC SC251 VMODE 5 VMODE GD 3 VOUT 0.5V to VIN Optional External Pass-through MOSFET Patent Pending Typical Application Circuit |
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