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MIC2159 Datasheet(PDF) 9 Page - Micrel Semiconductor |
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MIC2159 Datasheet(HTML) 9 Page - Micrel Semiconductor |
9 / 17 page Micrel MIC2159 Application information MOSFET Selection The MIC2159 controller works from input voltages of 3V to 14.5V and has an internal 5V regulator to provide power to turn the external N-Channel power MOSFETs for high- and low-side switches. For applications where VIN < 5V, the internal VDD regulator operates in dropout mode, and it is necessary that the power MOSFETs used are sub-logic level 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. For the lower (<5v) applications, the VDD supply can be connected directly to Vin to help increase the driver voltage to the MOSFET. It is important to note the on-resistance of a MOSFET increases with increasing temperature. A 75°C rise in junction temperature will increase the channel resistance of the MOSFET 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 supplied by the MIC2159 gate-drive circuit. At 400kHz switching frequency and above, the gate charge can be a significant source of power dissipation in the MIC2159. 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: IG[high-side](avg)=QG x FS Where: IG[high-side](avg)=Average high-side MOSFET gate current QG = total gate charge for the high-side MOSFET taken from manufacturer’s data sheet for VGS = 5V. FS = Switching Frequency (400kHz) 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 usually 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: IG[low-side](avg) = CISS × VGS x FS Since the current from the gate drive comes from the input voltage, the power dissipated in the MIC2159 due to gate drive is: PGATEDRIVE = VIN.(IG[high-sde](avg) + IG[low-side](avg)) 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 logic-level MOSFETs are a good choice for use with the MIC2159. 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) of the MOSFETs to account for voltage spikes due to circuit parasitic elements. 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). AC CONDUCTION SW P P P + = Where: SW RMS SW CONDUCTION R I P ⋅ = 2 ) ( ) ( ) ( on AC off AC AC P P P + = RSW = on-resistance of the MOSFET switch IN OUT V V cyle duty D = = _ Making the assumption the turn-on and turn-off transition times are equal; the transition times can be approximated by: G IN OSS GS ISS T I V C V C t ⋅ ⋅ ⋅ = where: CISS and COSS are measured at VDS = 0 IG = gate-drive current (1.4A for the MIC2159) The total high-side MOSFET switching loss is: ( ) S T PK D IN AC F t I V V P ⋅ ⋅ ⋅ + = Where: tT = Switching transition time (~20ns) VD = Freewheeling diode drop (0.5v) FS = Switching Frequency (400kHz) 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 inductor ripple current. Generally, higher October 2006 9 M9999-101206 |
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