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MIC38300HYHL Datasheet(PDF) 9 Page - Micrel Semiconductor |
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MIC38300HYHL Datasheet(HTML) 9 Page - Micrel Semiconductor |
9 / 12 page Micrel, Inc. MIC38300 June 2010 9 M9999-061010-D Application Information Enable Input The MIC38300 features a TTL/CMOS compatible positive logic enable input for on/off control of the device. High enables the regulator while low disables the regulator. In shutdown the regulator consumes very little current (only a few microamperes of leakage). For simple applications the enable (EN) can be connected to VIN (IN). Input Capacitor PVIN provides power to the MOSFETs for the switch mode regulator section and the gate drivers. Due to the high switching speeds, a 10µF capacitor is recommended close to PVIN and the power ground (PGND) pin for bypassing. Analog VIN (AVIN) provides power to the analog supply circuitry. AVIN and PVIN must be tied together externally. Careful layout should be considered to ensure high frequency switching noise caused by PVIN is reduced before reaching AVIN. A 1µF capacitor as close to AVIN as possible is recommended. Output Capacitor The MIC38300 requires an output capacitor for stable operation. As a µCap LDO, the MIC38300 can operate with ceramic output capacitors of 10µF or greater. Values of greater than 10µF improve transient response and noise reduction at high frequency. X7R/X5R dielectric-type ceramic capacitors are recommended because of their superior temperature performance. X7R-type capacitors change capacitance by 15% over their operating temperature range and are the most stable type of ceramic capacitors. Larger output capacitances can be achieved by placing tantalum or aluminum electrolytics in parallel with the ceramic capacitor. For example, a 100µF electrolytic in parallel with a 10µF ceramic can provide the transient and high frequency noise performance of a 100µF ceramic at a significantly lower cost. Specific undershoot/overshoot performance will depend on both the values and ESR/ESL of the capacitors. For less than 5mV noise performance at higher current loads, 20µF capacitors are recommended at LDOIN and LDOOUT. Low Pass Filter Pin The MIC38300 features a Low Pass Filter (LPF) pin for adjusting the switcher frequency. By tuning the frequency, the user can further improve output ripple without losing efficiency. Adjusting the frequency is accomplished by connecting a resistor between the LPF and SW pins. A small value resistor would increase the frequency while a larger value resistor decreases the frequency. Recommended RLPF value is 25kΩ. Please see Typical Characteristics for more details. Adjustable Regulator Design Adjustable Regulator with Resistors The adjustable MIC38300 output voltage can be programmed from 1V to 5.0V using a resistor divider from output to the FB pin. Resistors can be quite large, up to 100kΩ because of the very high input impedance and low bias current of the sense amplifier. For large value resistors (>50kΩ) R1 should be bypassed by a small capacitor (CFF = 0.1µF bypass capacitor) to avoid instability due to phase lag at the ADJ/SNS input. The output resistor divider values are calculated by: ⎟ ⎠ ⎞ ⎜ ⎝ ⎛ + = 1 2 1 1 R R V V OUT Efficiency Considerations Efficiency is defined as the amount of useful output power, divided by the amount of power supplied. 100 % _ × ⎟⎟ ⎠ ⎞ ⎜⎜ ⎝ ⎛ × × = IN IN OUT OUT I V I V Efficiency Maintaining high efficiency serves two purposes. It reduces power dissipation in the power supply, reducing the need for heat sinks and thermal design considerations and it reduces consumption of current for battery powered applications. Reduced current draw from a battery increases the devices operating time and is critical in hand held devices. There are two types of losses in switching converters; DC losses and switching losses. DC losses are simply the power dissipation of I 2R. Power is dissipated in the high side switch during the on cycle. Power loss is equal to the high side MOSFET RDSON multiplied by the Switch Current 2. During the off cycle, the low side N-channel MOSFET conducts, also dissipating power. Device operating current also reduces efficiency. The product of the quiescent (operating) current and the supply voltage is another DC loss. Over 100mA, efficiency loss is dominated by MOSFET RDSON and inductor losses. Higher input supply voltages will increase the Gate to Source threshold on the internal MOSFETs, reducing the internal RDDSON. This improves efficiency by reducing DC losses in the device. All but the inductor losses are inherent to the device. In which |
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