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MIC2168A Datasheet(PDF) 8 Page - Micrel Semiconductor |
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MIC2168A Datasheet(HTML) 8 Page - Micrel Semiconductor |
8 / 14 page MIC2168A Micrel, Inc. M9999-062205 8 April 2005 VDD regulator operates in dropout mode, and it is necessary that the power MOSFETs used are low threshold and are in full conduction mode for VGS of 2.5V. For applications when VIN > 5V; logic-level MOSFETs, whose operation is specified at VGS = 4.5V must be used. It is important to note the on-resistance of a MOSFET in- creases with increasing temperature. A 75°C rise in junction temperature will increase the channel resistance of the MOS- FET by 50% to 75% of the resistance specified at 25°C. This change in resistance must be accounted for when calculating MOSFET power dissipation and in calculating the value of current-sense (CS) resistor. Total gate charge is the charge required to turn the MOSFET on and off under specified operating conditions (VDS and VGS). The gate charge is sup- plied by the MIC2168A gate drive circuit. At 1MHz switching frequency and above, the gate charge can be a significant source of power dissipation in the MIC2168A. At low output load, this power dissipation is noticeable as a reduction in efficiency. The average current required to drive the high- side MOSFET is: I Q f G[high I Q I Q - I Q I Q side](avg) I Q I Q G Sff = × I Q I Q G S G S where: IG[high-side](avg) = average high-side MOSFET gate current. G[high-side](avg) G[high-side](avg) QG = total gate charge for the high-side MOSFET taken from manufacturer’s data sheet for VGS = 5V. The low-side MOSFET is turned on and off at VDS = 0 because the freewheeling diode is conducting during this time. The switching loss for the low-side MOSFET is usu- ally negligible. Also, the gate-drive current for the low-side MOSFET is more accurately calculated using CISS at VDS = 0 instead of gate charge. For the low-side MOSFET: I C V f G[low I C I C -side](avg) I C I C ISS GS V f V fS V f V f = × I C I C ISS ISS V f V f Since the current from the gate drive comes from the input voltage, the power dissipated in the MIC2168A due to gate drive is: P V GATEDRIVE P V P VIN = + P V P VIN IN ( ) I I I I G[high G[high --side](avg) side](avg) G[low G[low --side](avg) side](avg) = + = + I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I A convenient figure of merit for switching MOSFETs is the on resistance times the total gate charge RDS(ON)×QG. Lower numbers translate into higher efficiency. Low gate-charge DS(ON) DS(ON) logic-level MOSFETs are a good choice for use with the MIC2168A. Parameters that are important to MOSFET switch selection are: • Voltage rating • On-resistance • Total gate charge The voltage ratings for the top and bottom MOSFET are essentially equal to the input voltage. A safety factor of 20% should be added to the VDS(max)oftheMOSFETstoaccount for voltage spikes due to circuit parasitics. The power dissipated in the switching transistor is the sum of the conduction losses during the on-time (PCONDUCTION) and the switching losses that occur during the period of time when the MOSFETs turn on and off (PAC when the MOSFETs turn on and off (P when the MOSFETs turn on and off (P ). P P P SW P P P PCONDUCTION AC = + P P P PCONDUCTION CONDUCTION where: P I R CONDUCTION P I P I SW(rms) SW 2 = × P I P I SW(rms) SW(rms) 22 P P P AC P P P PAC(off) AC(on) = + P P P PAC(off) AC(off) RSW = on-resistance of the MOSFET switch D = V V O IN duty cycle Makingtheassumptiontheturn-onandturn-offtransitiontimes are equal; the transition times can be approximated by: t C V C V I T ISS C V C VGS OSS C V C VIN G = × + C V C VGS GS C V C V where: CISS and COSS are measured at VDS = 0 IG = gate-drive current (1A for the MIC2168A) The total high-side MOSFET switching loss is: P (V V ) I t f AC P ( P ( IN D P ) I ) I K T t f t fS t f t f = + P ( P (V V V V V V V V V V V V × × ) I ) I D P D P ) I ) I ) I ) I K T K T t f t f where: tT = switching transition time (typically 20ns to 50ns) VD = freewheeling diode drop, typically 0.5V fS it the switching frequency, nominally 1MHz The low-side MOSFET switching losses are negligible and can be ignored for these calculations. Inductor Selection Values for inductance, peak, and RMS currents are required to select the output inductor. The input and output voltages and the inductance value determine the peak-to-peak induc- tor ripple current. Generally, higher inductance values are used with higher input voltages. Larger peak-to-peak ripple currents will increase the power dissipation in the inductor and MOSFETs. Larger output ripple currents will also require more output capacitance to smooth out the larger ripple cur- rent. Smaller peak-to-peak ripple currents require a larger inductance value and therefore a larger and more expensive inductor. A good compromise between size, loss and cost is to set the inductor ripple current to be equal to 20% of the maximum output current. The inductance value is calculated by the equation below. L V (V m V ) V m f 0.2 I OUT V ( V ( IN V m V m OUT V ) V ) IN V m V m S ff OUT = × − V ( V (V m V m V m V m V m V m × × ff SS ffff × ( ) V m V max ax ( ) V m V max ax × − × − V m V m V m V max ax ax ax ( ) V m V max ax ( ) max max where: fS = switching frequency, 1MHz 0.2 = ratio of AC ripple current to DC output current VIN(max) = maximum input voltage The peak-to-peak inductor current (AC ripple current) is: |
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