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LMS5214IMG-3.0 Datasheet(PDF) 8 Page - National Semiconductor (TI) |
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LMS5214IMG-3.0 Datasheet(HTML) 8 Page - National Semiconductor (TI) |
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8 / 10 page  Typical Characteristics Unless otherwise specified, T A = 25˚C, VOUT = 2.8V, CL = 0.47µF (Continued) Start Up Characteristics 20043104 Application Information The LMS5214 is a low dropout, linear regulator designed primarily for battery-powered applications. The LMS5214 can be used with low cost ceramic capacitors, typical value of 470nF. The LMS5214 is an enhanced version of the LMS5213 with auto discharge function which actively discharges the output voltage to ground when the device is placed in shutdown mode As illustrated in the simplified schematics, the LMS5214 consists of a 1.25V reference, error amplifier, P-channel pass transistor and internal feedback voltage divider. The 1.25V reference is connected to the input of the error amp. The error amp compares this reference with the feedback voltage. If the feedback voltage is lower than the reference, the pass transistor gate is pulled lower allowing more current to pass and increasing the output voltage. If the feedback voltage is too high, the pass transistor gate is pulled up allowing less current to pass to the output. The output volt- age is fedback through the resistor divider. Additional blocks include short circuit current protection and thermal protec- tion. The LMS5214 features an 80mA P-channel MOSFET tran- sistor. This provides several advantages over similar designs using PNP pass transistors including longer battery life. The P-channel MOSFET requires no base drive, which re- duces quiescent current considerably. PNP based regulators waste considerable amounts of current in dropout when the pass transistor saturates. They also have high base drive currents under large loads. The LMS5214 does not suffer from these problems and consumes only the specified qui- escent current under light and heavy loads. External Capacitors Like any low-dropout regulators, the LMS5214 requires ex- ternal capacitors for regulator stability. The LMS5214 is spe- cially designed for portable applications requiring minimum board space and the smallest components. A 1µF capacitor should be placed from V IN to GND if there is more than 10 inches of wire between the input and AC filter or when a battery is used as the input. This capacitor must be located a distance of not more than 1cm from the input pin and returned to a clean analog ground. The LMS5214 is designed to work with high quality tantalum capacitors and small ceramic output capacitors. Ceramic capacitors ranging between 470nF to 4.7µF are the smallest and least expensive. No-Load Stability The LMS5214 will remain stable and in regulation with no-load (other than the internal voltage divider). This is especially important in CMOS RAM keep-alive applications. Enable Input The LMS5214 is shut off by pulling the V EN pin below 0.6V; all internal circuitry is powered off and the quiescent current is typically 10nA. Pulling the V EN high above 2V re-enables the device and allows operation. If the shut down feature is not used, the V EN pin should be tied to VIN to keep the regulator output on all the time. Thermal Behavior The LMS5214 regulator has internal thermal shutdown to protect the device from over heating. Under all operating conditions, the maximum junction temperature of the LMS5214 must be below 125˚C. Maximum power dissipation can be calculated based on the output current and the voltage drop across the part. The maximum power dissipa- tion is P D(MAX) =(TJ(MAX) -TA)/ θ JA θ JA is the junction-to-ambient thermal resistance, 478˚C/W for the LMS5214 in the SC70 package. T A is the maximum ambient temperature T J(MAX) is the maximum junction tem- perature of the die, 125˚C When operating the LMS5214 at room temperature, the maximum power dissipation is 209mW. The actual power dissipated by the regulator is P D =(VIN-VOUT)IL +VIN IGND The figure below shows the voltage and currents, which are present in the circuit. www.national.com 8 |
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