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MIC4417BM4 Datasheet(PDF) 8 Page - MIC GROUP RECTIFIERS |
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MIC4417BM4 Datasheet(HTML) 8 Page - MIC GROUP RECTIFIERS |
8 / 10 page MIC4416/4417 Micrel, Inc. MIC4416/4417 8 May 2005 Application Information The MIC4416/7 is designed to provide high peak current for charging and discharging capacitive loads. The 1.2A peak value is a nominal value determined under specific condi- tions. This nominal value is used to compare its relative size to other low-side MOSFET drivers. The MIC4416/7 is not designed to directly switch 1.2A continuous loads. Supply Bypass Capacitors from VS to GND are recommended to control switching and supply transients. Load current and supply lead length are some of the factors that affect capacitor size requirements. A 4.7µF or 10µF tantalum capacitor is suitable for many applications. Low-ESR (equivalent series resistance) metal- ized film capacitors may also be suitable. An additional 0.1µF ceramic capacitor is suggested in parallel with the larger capacitor to control high-frequency transients. The low ESR (equivalent series resistance) of tantalum capacitors makes them especially effective, but also makes them susceptible to uncontrolled inrush current from low impedance voltage sources (such as NiCd batteries or auto- matic test equipment). Avoid instantaneously applying volt- age, capable of very high peak current, directly to or near tantalum capacitors without additional current limiting. Nor- mal power supply turn-on (slow rise time) or printed circuit trace resistance is usually adequate for normal product usage. Circuit Layout Avoid long power supply and ground traces. They exhibit inductance that can cause voltage transients (inductive kick). Even with resistive loads, inductive transients can sometimes exceed the ratings of the MOSFET and the driver. When a load is switched off, supply lead inductance forces current to continue flowing—resulting in a positive voltage spike. Inductance in the ground (return) lead to the supply has similar effects, except the voltage spike is negative. Switching transitions momentarily draw current from VS to GND. This combines with supply lead inductance to create voltage transients at turn on and turnoff. Transients can also result in slower apparent rise or fall times when driver’s ground shifts with respect to the control input. Minimize the length of supply and ground traces or use ground and power planes when possible. Bypass capacitors should be placed as close as practical to the driver. MOSFET Selection Standard MOSFET A standard N-channel power MOSFET is fully enhanced with a gate-to-source voltage of approximately 10V and has an absolute maximum gate-to-source voltage of ±20V. The MIC4416/7’s on-state output is approximately equal to the supply voltage. The lowest usable voltage depends upon the behavior of the MOSFET. VS CTL G GND MIC4416 4.7µF +8V to +18V 1 32 4 Logic Input *Gate enhancement voltage VGS* +15V Standard MOSFET IRFZ24† † International Rectifier 100m Ω, 60V MOSFET 0.1µF Try a 15 Ω, 15W or 1k, 1/4W resistor Figure 1. Using a Standard MOSFET Logic-Level MOSFET Logic-level N-channel power MOSFETs are fully enhanced with a gate-to-source voltage of approximately 5V and have an absolute maximum gate-to-source voltage of ±10V. They are less common and generally more expensive. The MIC4416/7 can drive a logic-level MOSFET if the supply voltage, including transients, does not exceed the maximum MOSFET gate-to-source rating (10V). VS CTL G GND MIC4416 +4.5V to 10V* 1 32 4 Logic Input *Gate enhancement voltage (must not exceed 10V) VGS* +5V Logic-Level MOSFET IRLZ44† † International Rectifier 28m Ω, 60V MOSFET 4.7µF 0.1µF Try a 3 Ω, 10W or 100 Ω, 1/4W resistor Figure 2. Using a Logic-Level MOSFET At low voltages, the MIC4416/7’s internal P- and N-channel MOSFET’s on-resistance will increase and slow the output rise time. Refer to “Typical Characteristics” graphs. Inductive Loads On Off VS CTL G GND MIC4416 VSUPPLY 1 32 4 Schottky Diode VSWITCHED 4.7µF 0.1µF Figure 3. Switching an Inductive Load Switching off an inductive load in a low-side application forces the MOSFET drain higher than the supply voltage (as the inductor resists changes to current). To prevent exceeding the MOSFET’s drain-to-gate and drain-to-source ratings, a Schottky diode should be connected across the inductive load. |
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