MIC5236 Low Quiescent Current µCap LDO Regulator - Micrel - #9

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July 2005 9 MIC5236 MIC5236 Micrel, Inc. Application Information The MIC5236 provides all of the advantages of the MIC2950: wide input voltage range, load dump (positive transients up to 60V), and reversed-battery protection, with the added advantages of reduced quiescent current and smaller package. Additionally, when disabled, quiescent current is reduced to 0.1ìA. Enable A low on the enable pin disables the part, forcing the quiescent current to less than 0.1ìA. Thermal shutdown and the error flag are not functional while the device is disabled. The maximum enable bias current is 2ìA for a 2.0V input. An open collector pull-up resistor tied to the input voltage should be set low enough to maintain 2V on the enable input. Figure 1 shows an open collector output driving the enable pin through a 200k pull-up resistor tied to the input voltage. In order to avoid output oscillations, slow transitions from low to high should be avoided. COUT VOUT VIN 5V VERR IN MIC5236 EN 200k 200k OUT GND SHUTDOWN ENABLE ERR Figure 1. Remote Enable Input Capacitor An input capacitor may be required when the device is not near the source power supply or when supplied by a battery. Small, surface mount, ceramic capacitors can be used for bypassing. Larger values may be required if the source supply has high ripple. Output Capacitor The MIC5236 has been designed to minimize the effect of the output capacitor ESR on the closed loop stability. As a result, ceramic or film capacitors can be used at the output. Figure 2 displays a range of ESR values for a 10ìF capacitor. Virtually any 10ìF capacitor with an ESR less than 3.4Ù is sufficient for stability over the entire input voltage range. Stability can also be maintained throughout the specified load and line conditions with 1ìF film or ceramic capacitors. 0 1 2 3 4 5 5 10 15 20 25 30 OUTPUT CAPACITOR ESR (Ù) INPUT VOLTAGE (V) Stable Region TJ = 25°C VOUT = 10ìF Figure 2. Output Capacitor ESR Error Detection Comparator Output The ERR pin is an open collector output which goes low when the output voltage drops 5% below it’s internally programmed level. It senses conditions such as excessive load (current limit), low input voltage, and over temperature conditions. Once the part is disabled via the enable input, the error flag output is not valid. Overvoltage conditions are not reflected in the error flag output. The error flag output is also not valid for input voltages less than 2.3V. The error output has a low voltage of 400mV at a current of 200ìA. In order to minimize the drain on the source used for the pull-up, a value of 200k to 1MÙ is suggested for the error flag pull-up. This will guarantee a maximum low voltage of 0.4V for a 30V pull-up potential. An unused error flag can be left unconnected. NOT VALID NOT VALID VALID ERROR Error Output Input Voltage Output Voltage 4.75V 0V 0V 5V 1.3V Figure 3. Error Output Timing Reverse Current Protection The MIC5236 is designed to limit the reverse current flow from output to input in the event that the MIC5236 output has been tied to the output of another power supply. See the graphs detailing the reverse current flow with the input grounded and open. Thermal Shutdown The MIC5236 has integrated thermal protection. This feature is only for protection purposes. The device should never be intentionally operated near this temperature as this may have detrimental effects on the life of the device. The thermal shutdown may become inactive while the enable input is transitioning a high to a low. When disabling the device via the enable pin, transition from a high to low quickly. This will insure that the output remains disabled in the event of a thermal shutdown. Current Limit Figure 4 displays a method for reducing the steady state short circuit current. The duration that the supply delivers current is set by the time required for the error flag output to discharge the 4.7ìF capacitor tied to the enable pin. The off time is set by the 200K resistor as it recharges the 4.7ìF capacitor, enabling the regulator. This circuit reduces the short circuit current from 280mA to 15mA while allowing for regulator restart once the short is removed.