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NUC5158 Datasheet(PDF) 8 Page - Micrel Semiconductor |
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NUC5158 Datasheet(HTML) 8 Page - Micrel Semiconductor |
8 / 13 page MIC5156/5157/5158 Micrel, Inc. MIC5156/5157/5158 8 August 2005 Charge Pump (MIC5157/5158 only) The charge pump tripler creates a dc voltage across reservoir capacitor C3. External capacitors C1 and C2 provide the nec- essary storage for the stages of the charge pump tripler. The tripler’s approximate dc output voltage is: VCP ≈ 3 (VDD – 1) where: VCP = charge pump output voltage VDD = supply voltage The VCP clamp circuit limits the charge pump voltage to 16V above VDDbygatingthechargepumposcillatorONorOFFas required. The charge pump oscillator operates at 160kHz. The error amplifier uses the charge pump voltage to drive the gate of the external MOSFET. It provides a constant load of about 1mA to the charge pump. The error amplifier output can swing to within 1V of VCP. Although the MIC5157/8 is designed to provide gate drive using its internal charge pump, an external gate drive sup- ply voltage can be applied to VCP . When using an external gate drive supply, VCP must not be forced more than 14V higher than VDD. When constant loads are driven, the ON/OFF switching of the charge pump may be evident on the output waveform. This is caused by the charge pump switching ON and rapidly increasing the supply voltage to the error amplifier.The period ofthissmallchargepumpexcitationisdeterminedbyanumber of factors: the input voltage, the 1mA op-amp load, any dc leakage associated with the MOSFET gate circuit, the size of the charge pump capacitors, the size of the charge pump reservoircapacitor,andthecharacteristicsoftheinputvoltage and load. The period is lengthened by increasing the charge pump reservoir capacitor (C3). The amplitude is reduced by weakening the charge pump—this is accomplished by reduc- ing the size of the pump capacitors (C1 and C2). If this small burst is a problem in the application, use a 10µF reservoir capacitor at C3 and 0.01µF pump capacitors at C1 and C2. Note that the recovery time to repetitive load transients may be affected with small pump capacitors. Gate-to-Source Clamp A gate-to-source protective voltage clamp of 16.6V protects the MOSFET in the event that the output voltage is suddenly forced to zero volts. This prevents damage to the external MOSFET during shorted load conditions. Refer to “Charge Pump” for normal clamp circuit operation. The source connection required by the gate-to-source clamp is not available on the adjustable version of the MIC5156. Output Regulation At start-up, the error amplifier feedback voltage (EA), or internal feedback on fixed versions, is below nominal when compared to the internal 1.235V bandgap reference. This forces the error amplifier output high which turns on exter- nal MOSFET Q1. Once the output reaches regulation, the controller maintains constant output voltage under changing input and load conditions by adjusting the error amplifier output voltage (gate enhancement voltage) according to the feedback voltage. Out-of-Regulation Detection When the output voltage is 8% or more below nominal, the open-collector FLAG output (normally high) is forced low to signal a fault condition. The FLAG output can be used to signal or control external circuitry. The FLAG output can also be used to shut down the regulator using the EN control. Current Limiting Super LDO Regulators perform constant-current limiting (not foldback). To implement current limiting, a sense resistor (RS) must be placed in the “power” path between VDD and D (drain). If the voltage drop across the sense resistor reaches 35mV, the current limit comparator reduces the error amplifier out- put. The error amplifier output is decreased only enough to reduce the output current, keeping the voltage across the sense resistor from exceeding 35mV. Application Information MOSFET Selection Standard N-channel enhancement-mode MOSFETs are ac- ceptable for most Super LDO regulator applications. Logic-level N-channel enhancement-mode MOSFETs may be necessary if the external gate drive voltage is too low (MIC5156), or the input voltage is too low, to provide adequate charge pump voltage (MIC5157/8) to enhance a standard MOSFET. Circuit Layout For the best voltage regulation, place the source, ground, and error amplifier connections as close as possible to the load. See figures (1a) and (1b). GND S G MIC515x VIN Figure 1a. Connections for Fixed Output |
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