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SP6125EK1L Datasheet(PDF) 7 Page - Sipex Corporation |
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SP6125EK1L Datasheet(HTML) 7 Page - Sipex Corporation |
7 / 12 page Mar22-07 RevC SP6125: TSOT-6 PFET Buck Controller 2007 Sipex Corporation 7 Power MOSFET Selection Select the Power MOSFET for Voltage rating BVDSS, On resistance RDS(ON), and thermal resistance Rthja. BVDSS should be about twice as high as VIN in order to guard against switching transients. Recommended MOSFET voltage rating for VIN of 5V, 12V and 24V is 12V, 30V and 40V respectively. RDS(ON), must be selected such that when operating at peak current and junction temperature the Overcurrent threshold of the SP6125 is not exceeded. Allowing 50% for temperature coefficient of RDS(ON) and 15% for inductor current ripple, the following expression can be used: × × ≤ Iout mV ON RDS 15 . 1 5 . 1 300 ) ( Within this constraint, selecting MOSFETs with lower RDS(ON) will reduce conduction losses at the expense of increased switching losses. As a rule of thumb select the highest RDS(ON) MOSFET that meets the above criteria. Switching losses can be assumed to roughly equal the conduction losses. A simplified expression for conduction losses is given by: × × = Vin Vout ON RDS Iout Pcond ) ( MOSFET’s junction temperature can be estimated from: ( ) Tambient Rthja Pc T + × × = 2 Schottky Rectifier selection Select the Schottky for Voltage rating VR, Forward voltage Vf, and thermal resistance Rthja. Voltage rating should be selected using the same guidelines outlined for MOSFET voltage selection. For a low duty cycle application such as the circuit shown on first page, the Schottky is conducting most of the time and its conduction losses are the largest component of losses in the converter. Conduction losses can be estimated from: − × × = Vin Vout Iout Vf Pc 1 where: Vf is diode forward voltage at IOUT Schottky’s AC losses due to its switching capacitance are negligible. Inductor Selection Select the Inductor for inductance L and saturation current Isat. Select an inductor with Isat higher than the programmed overcurrent. Calculate inductance from: ( ) × × × − = Irip f Vin Vout Vout Vin L 1 1 where: VIN is converter input voltage VOUT is converter output voltage f is switching frequency IRIP is inductor peak-to-peak current ripple (nominally set to 30% of IOUT) Keep in mind that a higher IRIP results in a smaller inductor which has the advantages of small size, low DC equivalent resistance DCR, high saturation current Isat and allows the use of a lower output capacitance to meet a given step load transient. A higher Irip, however, increases the output voltage ripple and increases the current at which converter enters Discontinuous Conduction Mode. The output current at which converter enters DCM is ½ of IRIP . Note that a negative current step load that drives the converter into DCM will result in a large output voltage transient. Therefore the lowest current for a step load should be larger than ½ of IRIP. Output Capacitor Selection Select the output capacitor for voltage rating, capacitance and Equivalent Series Resistance (ESR). Nominally the voltage rating is selected to be twice as large as the output voltage. Select the capacitance to satisfy the specification for output voltage overshoot or undershoot caused by current step load. A steady-state output current IOUT corresponds to inductor stored energy of ½ L IOUT 2. A sudden decrease in IOUT forces the energy surplus in L to be absorbed by COUT. General Overview |
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