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SC1453ITSK28TRT Datasheet(PDF) 7 Page - Semtech Corporation |
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SC1453ITSK28TRT Datasheet(HTML) 7 Page - Semtech Corporation |
7 / 15 page 7 2008 Semtech Corp. www.semtech.com POWER MANAGEMENT SC1453 Applications Information Theory Of Operation The SC1453 is intended for applications where very low dropout voltage, low supply current and low output noise are critical. It provides a very simple, low cost solution that uses very little pcb real estate. Only three external capacitors are required for operation (two if a low noise output is not required). The SC1453 contains a bandgap reference trimmed for optimal temperature coefficient which is fed into the inverting input of an error amplifier. The output voltage of the regulator is divided down internally using a resistor divider and compared to the bandgap voltage. The error amplifier drives the gate of a low R DS(ON) P-channel MOSFET pass device. An active high enable pin (EN) allows the regulator to be shut down. Pulling this pin low causes the device to enter a very low power shutdown mode, where it will draw typically 0.1µA from the input supply. A bypass pin (BYP) is provided to decouple the bandgap reference to reduce output noise and also to improve power supply rejection. This pin can be left open if low noise operation is not required. The regulator has its own current limit circuitry to ensure that the output current will not damage the device during output short, overload or start-up. The current limit is guaranteed to be greater than 400mA to allow fast charging of the output capacitor and high initial currents for DSP initialization. The SC1453 has a fast start-up circuit to speed up the initial charging time of the bypass capacitor to enable the output voltage to come up quicker (typically 1.3ms with C BYP = 10nF). The SC1453 includes thermal shutdown circuitry to turn off the device if T J exceeds 150°C (typical), with the device remaining off until T J drops by 20°C (typical). Reverse battery protection circuitry ensures that the device cannot be damaged if the input supply is accidentally reversed, limiting the reverse current to less than 1.5mA. Component Selection - General Output capacitor - Semtech recommends a minimum capacitance of 1µF at the output with an equivalent series resistance (ESR) of < 1 Ω over temperature. While the SC1453 has been designed to be used with ceramic capacitors, it does not have to be used with ceramic capacitors, allowing the designer a choice. Increasing the bulk capacitance will further reduce output noise and improve the overall transient response. Input capacitor - Semtech recommends the use of a 1µF ceramic capacitor at the input. This allows for the device being some distance from any bulk capacitance on the rail. Additionally, input droop due to load transients is reduced, improving overall load transient response. Bypass capacitor - Semtech recommends the use of a 10nF ceramic capacitor to bypass the bandgap reference. Increasing this capacitor to 100nF will further improve power supply rejection and overall output noise. C BYP may be omitted if low noise operation is not required. Thermal Considerations The worst-case power dissipation for this part is given by: () ) MAX ( Q ) MAX ( IN ) MAX ( OUT ) MIN ( OUT ) MAX ( IN ) MAX ( D I V I V V P • + • − = (1) For all practical purposes, equation (1) can be reduced to the following expression: () ) MAX ( OUT ) MIN ( OUT ) MAX ( IN ) MAX ( D I V V P • − = (2) Looking at a typical application, 3.3V to 2.8V at 150mA: V IN(MAX) = 3.3 + 5% = 3.465V V OUT(MIN) = 2.8V - 2% = 2.744V I OUT = 150mA T A = 85°C |
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