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MIC5246-2.7BM5 Datasheet(PDF) 7 Page - Micrel Semiconductor |
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MIC5246-2.7BM5 Datasheet(HTML) 7 Page - Micrel Semiconductor |
7 / 12 page MIC5246 Micrel June 2000 7 MIC5246 Applications Information Enable/Shutdown The MIC5246 comes with an active-high enable pin that allows the regulator to be disabled. Forcing the enable pin low disables the regulator and sends it into a “zero” off-mode- current state. In this state, current consumed by the regulator goes nearly to zero. Forcing the enable pin high enables the output voltage. This part is CMOS and the enable pin cannot be left floating; a floating enable pin may cause an indetermi- nate state on the output. Input Capacitor An input capacitor is not required for stability. A 1 µF input capacitor is recommended when the bulk ac supply capaci- tance is more than 10 inches away from the device, or when the supply is a battery. Output Capacitor The MIC5246 requires an output capacitor for stability. The design requires 1 µF or greater on the output to maintain stability. The capacitor can be a low-ESR ceramic chip capacitor. The MIC5246 has been designed to work specifi- cally with the low-cost, small chip capacitors. Tantalum capacitors can also be used for improved capacitance over temperature. The value of the capacitor can be increased without bound. X7R dielectric ceramic capacitors are recommended be- cause of their temperature performance. X7R-type capaci- tors change capacitance by 15% over their operating tem- perature range and are the most stable type of ceramic capacitors. Z5U and Y5V dielectric capacitors change value by as much 50% and 60% respectively over their operating temperature ranges. To use a ceramic chip capacitor with Y5V dielectric, the value must be much higher than an X7R ceramic or a tantalum capacitor to ensure the same minimum capacitance value over the operating temperature range. Tantalum capacitors have a very stable dielectric (10% over their operating temperature range) and can also be used with this device. Error Flag The error flag output is an active-low, open-drain output that drives low when a fault condition AND an undervoltage detection occurs. Internal circuitry intelligently monitors overcurrent, overtemperature and dropout conditions and ORs these outputs together to indicate some fault condition. The output of that OR gate is ANDed with an output voltage monitor that detects an undervoltage condition. That output drives the open-drain transistor to indicate a fault. This prevents chattering or inadvertent triggering of the error flag. The error flag must be pulled up using a resistor from the flag pin to either the input or the output. Error Flag Circuit The error flag circuit was designed essentially to work with a capacitor to ground to act as a power-on reset generator, signaling a power-good situation once the regulated voltage was up and/or out of a fault condition. This capacitor delays the error signal from pulling high, allowing the downstream circuits time to stabilize. When the error flag is pulled up to the input without using a pull-down capacitor, then there can be a glitch on the error flag upon start up of the device. This is due to the response time of the error flag circuit as the device starts up. When the device comes out of the zero off mode current state, all the various nodes of the circuit power up before the device begins supplying full current to the output capacitor. The error flag drives low immediately and then releases after a few microseconds. The intelligent circuit that triggers an error detects the output going into current limit AND the output being low while charging the output capacitor. The error output then pulls low for the duration of the turn-on time. This glitch is filtered by putting a capacitor from the error flag to ground. The glitch does not occur if the error flag pulled up to the output. Transient Response The MIC5246 implements a unique output stage to dramati- cally improve transient response recovery time. The output is a totem-pole configuration with a P-channel MOSFET pass device and an N-channel MOSFET clamp. The N-channel clamp is a significantly smaller device that prevents the output voltage from overshooting when a heavy load is removed. This feature helps to speed up the transient re- sponse by significantly decreasing transient response recov- ery time during the transition from heavy load (100mA) to light load (85 µA). Active Shutdown The MIC5246 also features an active shutdown clamp, which is an N-channel MOSFET that turns on when the device is disabled. This allows the output capacitor and load to dis- charge, de-energizing the load. Thermal Considerations The MIC5246 is designed to provide 150mA of continuous current in a very small package. Maximum power dissipation can be calculated based on the output current and the voltage drop across the part. To determine the maximum power dissipation of the package, use the junction-to-ambient ther- mal resistance of the device and the following basic equation: P TT D(max) J(max) A JA = − θ T J(max) is the maximum junction temperature of the die, 125 °C, and T A is the ambient operating temperature. θJA is layout dependent; Table 1 shows examples of junction-to- ambient thermal resistance for the MIC5246. Package θ JA Recommended θ JA 1" Square θ JC Minimum Footprint Copper Clad SOT-23-5 (M5) 235 °C/W 185 °C/W 145 °C/W Table 1. SOT-23-5 Thermal Resistance |
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