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LTM4650 Datasheet(PDF) 11 Page - Linear Technology |
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LTM4650 Datasheet(HTML) 11 Page - Linear Technology |
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11 / 34 page  LTM4647 11 4647fb For more information www.linear.com/LTM4647 APPLICATIONS INFORMATION 22pF to 47pF feedforward capacitor (CFF) is necessary for all ceramic output application to achieve enough phase margin.Thetypicaloutputcapacitancerangeisfrom400µF to 600µF. Additional output filtering may be required by the system designer if further reduction of output ripple or dynamic transient spikes is required. Table 5 shows a matrixofdifferentoutputvoltagesandoutputcapacitorsto minimize the voltage droop and overshoot during a 7.5A/ µs transient (at 10A/µs slew rate). The table optimizes total equivalent ESR and total output capacitance to optimize the transient performance. Multiphase operation will re- duce effective output ripple as a function of the number of phases. Application Note 77 discusses this reduction versus output ripple current cancellation. But the output capacitance should be considered carefully as a function of stability and transient response. The Linear Technology LTpowerCAD Design Tool can calculate the output ripple reductionasthenumberofimplementedphase’sincreases by N times and provide stability analysis. Burst Mode Operation The LTM4647 is capable of Burst Mode operation in which the power MOSFETs operate intermittently based on load demand, thus saving quiescent current. For applications where maximizing the efficiency at very light loads is a high priority, Burst Mode operation should be applied. To enableBurstModeoperation,simplyfloattheMODE_PLLIN pin. During Burst Mode operation, the peak current of the inductor is set to approximately one-third of the maximum peak current value in normal operation even though the voltage at the COMPa pin indicates a lower value. The voltage at the COMPa pin drops when the inductor’s aver- age current is greater than the load requirement. As the COMPa voltage drops below 0.5V, the burst comparator trips, causing the internal sleep line to go high and turn off both power MOSFETs. In sleep mode, the internal circuitry is partially turned off, reducing the quiescent current. The load current is now being supplied from the output capacitors. When the output voltage drops, causing COMPa to rise, the internal sleep line goes low, and the LTM4647 resumes normal operation. The next oscillator cycle will turn on the top power MOSFET and the switching cycle repeats. Pulse-Skipping Mode Operation In applications where low output ripple and high efficiency at intermediate currents are desired, pulse-skipping mode should be used. Pulse-skipping operation allows the LTM4647 to skip cycles at low output loads, thus increasing efficiency by reducing switching loss. Tying the MODE_PLLIN pin to INTVCC enables pulse-skipping operation. With pulse-skipping mode at light load, the internalcurrentcomparatormayremaintrippedforseveral cycles, thus skipping operation cycles. This mode has lower ripple than Burst Mode operation and maintains a higher frequency operation than Burst Mode operation. Forced Continuous Operation In applications where fixed frequency operation is more critical than low current efficiency, and where the lowest output ripple is desired, forced continuous operation should be used. Forced continuous operation can be enabled by tying the MODE_PLLIN pin to GND. In this mode, inductor current is allowed to reverse during low outputloads,theCOMPavoltageisincontrolofthecurrent comparator threshold throughout, and the top MOSFET alwaysturnsonwitheachoscillatorpulse.Duringstart-up, forced continuous mode is disabled and inductor current is prevented from reversing until the LTM4647’s output voltage is in regulation. Frequency Selection TheLTM4647deviceisoperatedoverarangeoffrequencies toimprovepowerconversionefficiency.Itisrecommended to operate the lower output voltages or lower duty cycle conversions at lower frequencies to improve efficiency by lowering power MOSFET switching losses. Higher output voltages or higher duty cycle conversions can be operated at higher frequencies to limit inductor ripple current. The efficiencygraphswillshowanoperatingfrequencychosen for that condition. See Table 1 for optimized frequency for various output voltages. The LTM4647 switching frequency can be set with an external resistor from the fSET pin to SGND. An accurate 20µA current source into the resistor will set a voltage that programs the frequency or a DC voltage can be applied.Figure2showsagraphoffrequencysettingverses programming voltage. |
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