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SP6121 Datasheet(PDF) 10 Page - Sipex Corporation |
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SP6121 Datasheet(HTML) 10 Page - Sipex Corporation |
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10 / 16 page  Date: 5/25/04 SP6121 Low Voltage, Synchronous Step Down PWM Controller © Copyright 2004 Sipex Corporation 10 supplies are connected ‘live’ to low impedance power sources. Certain tantalum capacitors, such as AVX TPS series, are surge tested. For ge- neric tantalum capacitors, use 2:1 voltage derat- ing to protect the input capacitors from surge fall-out. MOSFET Selection The SP6121 drives a PMOS MOSFET on the high side and an NMOS MOSFET synchronous rectifier on the low side. Using a PMOS switch on the high side negates the need for an external charge pump and simplifies the application cir- cuit. The losses associated with MOSFETs can be divided into conduction and switching losses. Conduction losses are related to the on resis- tance of MOSFETs, and increase with the load current. Switching losses occur on each on/off transition when the MOSFETs experience both high current and voltage. Since the bottom MOSFET switches current from/to a paralleled diode (either its own body diode or a Schottky diode), the voltage across the MOSFET is no more than 1V during switching transition. As a result, its switching losses are negligible. The switching losses are difficult to quantify due to all the variables affecting turn on/off time. How- ever, making the assumption that the turn on and turn off transition times are equal, the transition time can be approximated by: tT = CISSVIN , IG where; CISS is the PMOS’s input capacitance, or the sum of the gate-to-source capacitance, CGS, and the drain-to-gate capacitance, CGD. This param- eter can be directly obtained from the MOSFET’s data sheet IG is the gate drive current provided by the SP6121 (approximately 1A at VIN=5V) and VIN is the input supply voltage. Therefore an approximate expression for the switching losses associated with the high side MOSFET can be given as: PSH(max) = (VIN(max) + VF)IOUT(max)tTFS , where; tT = the switching transition time VF = free wheeling diode drop Switching losses need to be taken into account for high switching frequency, since they are directly proportional to switching frequency. The conduction losses associated with top and bottom MOSFETs are determined by PCH(max) = RDS(ON)IOUT(max) 2D PCL(max) = RDS(ON)IOUT(max) 2(1 - D), where; PCH(max) = conduction losses of the high side MOSFET PCL(max) = conduction losses of the low side MOSFET RDS(ON) = drain to source on resistance. The total power losses of the top MOSFET are the sum of switching and conduction losses. For synchronous buck converters of efficiency over 90%, allow no more than 4% power losses for high or low side MOSFETs. For input voltages of 3.3V and 5V, conduction losses often domi- nate switching losses. Therefore, lowering the RDS(ON) of the MOSFETs always improves efficiency even though it gives rise to higher switching losses due to increased CISS . Total gate charge is the charge required to turn the MOSFETs on and off under the specified operating conditions (VGS and VDS). The gate charge is provided by the SP6121 gate drive circuitry. (At 500kHz switching frequency, the gate charge is the dominant source of power dissipation in the SP6121.) At low output levels, this power dissipation is noticeable as a reduc- tion in efficiency. The average current required to drive the high side and low side MOSFETs is: IG(av) = QGHFS + QGLFS, where; QGH = gate charge of PMOS QGL = gate charge of NMOS Considering that the gate charge current comes from the input supply voltage VIN, the power dissipated in the SP6121 due to the gate drive is: PGATE DRIVE = VINIG(av) RDS(ON) varies greatly with the gate driver volt- age. The MOSFET vendors often specify RDS(ON) on multiple gate to source voltages (VGS), as APPLICATIONS INFORMATION: Continued |
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