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MIC39151-1.65WT Datasheet(PDF) 11 Page - Micrel Semiconductor |
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MIC39151-1.65WT Datasheet(HTML) 11 Page - Micrel Semiconductor |
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11 / 14 page  Micrel, Inc. MIC39150/39151/39152 October 2009 11 M9999-102309-A promote stability. Ultralow ESR capacitors (<100mΩ), such as ceramic chip capacitors may promote instability. These very low ESR levels may cause an oscillation and/or underdamped transient response. A low-ESR solid tantalum capacitor works extremely well and provides good transient response and stability over temperature. Aluminum electrolytics can also be used, as long as the ESR of the capacitor is < 2Ω. The value of the output capacitor can be increased without limit. Higher capacitance values help to improve transient response and ripple rejection and reduce output noise. Input Capacitor An input capacitor of 1µF or greater is recommended when the device is more than 4 inches away from the bulk ac supply capacitance, or when the supply is a battery. Small, surface-mount, ceramic chip capacitors can be used for the bypassing. The capacitor should be placed within 1" of the device for optimal performance. Larger values will help to improve ripple rejection by bypassing the input to the regulator, further improving the integrity of the output voltage. Transient Response and 3.3V to 2.5Vor 2.5V to 1.8V Conversion The MIC39150/1/2 has excellent transient response to variations in input voltage and load current. The device has been designed to respond quickly to load current variations and input voltage variations. Large output capacitors are not required to obtain this performance. A standard 10µF output capacitor, preferably tantalum, is all that is required. Larger values help to improve performance even further. By virtue of its low-dropout voltage, this device does not saturate into dropout as readily as similar NPN-based designs. When converting from 3.3V to 2.5V, or 2.5V to 1.8V, the NPN-based regulators are already operating in dropout, with typical dropout requirements of 1.2V or greater. To convert down to 2.5V without operating in dropout, NPN-based regulators require an input voltage of 3.7V at the very least. The MIC39150/1 regulator will provide excellent performance with an input as low as 3.0V or 2.5V, respectively. This gives the PNP-based regulators a distinct advantage over older, NPN-based linear regulators. Minimum Load Current The MIC39150 regulator is specified between finite loads. If the output current is too small, leakage currents dominate and the output voltage rises. A 10mA minimum load current is necessary for proper regulation. Error Flag The MIC39151 version features an error flag circuit which monitors the output voltage and signals an error condition when the voltage 5% below the nominal output voltage. The error flag is an open-collector output that can sink 10mA during a fault condition. Low output voltage can be caused by a number of problems, including an overcurrent fault (device in current limit) or low input voltage. The flag is inoperative during overtemperature shutdown. When the error flag is not used, it is best to leave it open. A pull-up resistor from FLG to either VIN or VOUT is required for proper operation. Enable Input The MIC39151/2 features an enable input for on/off control of the device. The enable input’s shutdown state draws “zero” current (only microamperes of leakage). The enable input is TTL/CMOS compatible for simple logic interface, but can be connected to up to 20V. When enabled, it draws approximately 15µA. Adjustable Regulator Design Figure 2. Adjustable Regulator with Resistors The MIC39152 allows programming the output voltage anywhere between 1.24V and 15.5V. Two resistors are used. The resistor values are calculated by: ⎟⎟ ⎠ ⎞ ⎜⎜ ⎝ ⎛ − × = 1 1.240 V R2 R1 OUT Where VOUT is the desired output voltage. Figure 2 shows component definition. Applications with widely varying load currents may scale the resistors to draw the minimum load current required for proper operation (see Minimum Load Current section). |
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