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LX8211-00ISE-TR Datasheet(PDF) 5 Page - Microsemi Corporation |
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LX8211-00ISE-TR Datasheet(HTML) 5 Page - Microsemi Corporation |
5 / 9 page Microsemi Analog Mixed Signal Group 11861 Western Avenue, Garden Grove, CA. 92841, 714-898-8121, Fax: 714-893-2570 Page 5 Copyright © 2000 Rev. 1.0d, 2007-08-29 LX8211 150mA CMOS Low Dropout Regulator PRODUCTION DATA SHEET TM ® APPLICATION INFORMATION Description The LX8211 is a family of LDO (Low Drop-Out) linear regulators in a small SOT-23 Package, which provides a compact and cost effective solution. The family includes several fixed output voltages, along with an adjustable version. The internal PMOS power device provides low dropout regulation with fast line and load transient response. It also includes internal current limit and thermal shutdown circuitry. In this section you will find information about selection of external capacitors, thermal and layout consideration. Input Capacitor To improve load transient response and noise rejection a bypass capacitor is recommended, but it is not required for stability. There are no requirements for the ESR (Equivalent Series Resistor) on the input capacitor, but tolerance and temperature coefficient must be considered when selecting the capacitor to ensure the capacitor will be around 1uF over the entire operating temperature range. For LX8211 connect ceramic type capacitor (1uF) between VIN and ground. Output Capacitor To maintain a constant output voltage a feedback loop is necessary. The feedback loop causes a well known a phase shift, the amount of phase shift determines the loop stability. Therefore, LX8211 like any low-dropout regulator, requires an output capacitor with low ESR (Equivalent Series Resistor) connected between VOUT and GND to stabilize the internal control loop. For the LX8211 a ceramic, tantalum or electrolytic capacitor with the minimum recommended capacitance value at 1.0 uF with ESR between 5mΩ and 1Ω will satisfy the stability for whole operating range. Bypass Capacitor (Optional) / Adjustable Output Voltage The LX8211 includes the extra Bypass pin to help further reduce the already low output noise in the regulator. If the output noise requirement meets the specification, omit the bypass capacitor, that’s because the capacitor impacts the start up time which is inversely proportional to the size of bypass capacitor. The bypass capacitor reduces the regulator phase margin, therefore it requires a larger output capacitor to maintain stability. The LX8211-00 adjustable regulator’s output voltage can be externally set by connecting the ADJ pin to an external resistor divider (See Figure 1). The output voltage can be calculated using: ⎟ ⎠ ⎞ ⎜ ⎝ ⎛ + = 2 R 1 R 1 V OUT ADJ V Enable The Enable pin allows the LX8211 to be turned on and off. The Enable pin is compatible with standard TTL-CMOS levels. A logic zero (0.4V) on the Enable pin shuts the LX8211 off and reduces the supply current to less than 1uA. Pulling the Enable input high (2.0V) causes normal operation to resume. If the Enable feature is not used, this pin can be connected to VIN. Minimum Load Requirement The LX8211 is stable even at zero load; no minimum load is required for operation, which is especially important in keep-alive applications. However line regulation is improved with a minimum load of 100uA. Temperature Protection The thermal protection shuts the LX8211 down when the junction temperature exceeds approximately 1500C, there is no appreciable thermal hysteresis. Short Circuit Current Protection The LX8211 includes over current protection, when the output load current exceeds typically 400mA the circuit forces the regulator decrease in output. Thermal Consideration Thermal shutdown protects the integrated circuit from thermal overload caused from a rise in junction temperature during power dissipation. This means of protection is intended for fault protection only and not as a means of current or power limiting during normal application usage. Proper thermal evaluation should be done to ensure that the junction temperature dose not exceed it’s maximum rating. Operating at the maximum TJ of 150°C can impact reliability . Due to variation in individual device electrical characteristics and thermal resistance , the built in thermal overload protection may be activated at power levels slightly above or below the rated dissipation. Power dissipation for regulator can be calculated using the following equation: PD = (VIN(MAX)-VOUT(MIN)) * IOUT(MAX) (Note: power dissipation resulting from quiescent current is negligible) For the SOT23 package, thermal resistance, θJA is 2200C/W when mounted on a FR4 copper clad PCB. Junction temperature of the integrated circuit can be calculated using: TJ = TA + TRISE, substituting: TA + (PD * θJA). An example: Given conditions: TA = 60°C, VIN= 4.2V, VOUT= 2.7V, IOUT= 100mA. Calculated values: Total Power dissipation PD(total) = (4.2-2.7) V * 0.1A =0.15W Junction Temperature TJ = 60°C +(0.15W *220°C/W) = 93°C Layout Consideration: The layout must be done with low impedance paths for VIN, VOUT and Ground by applying sufficient wide traces to avoid voltage drops and pick up noise. Adding an area of PCB copper to the GND pin (pin2) will reduce the overall θ JA since it is thermally connected to the device substrate. This will lower the device junction temperature improving the overall output accuracy. The input/output capacitors must be placed as close as possible to the voltage regulator output pin. To allow the same voltage reference for all circuits, use a star configuration from the output capacitor to the different loads. |
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