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LTC3716 Datasheet(PDF) 22 Page - Linear Technology |
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LTC3716 Datasheet(HTML) 22 Page - Linear Technology |
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22 / 28 page  22 LTC3716 2) Transition losses apply only to the topside MOSFET(s), and are significant only when operating at high input voltages (typically 12V or greater). Transition losses can be estimated from: Transition Loss V I Cf IN OMAX RSS = (. ) () 17 2 2 3) INTVCC current is the sum of the MOSFET driver and control currents. The MOSFET driver current results from switching the gate capacitance of the power MOSFETs. Each time a MOSFET gate is switched from low to high to low again, a packet of charge dQ moves from INTVCC to ground. The resulting dQ/dt is a current out of INTVCC that is typically much larger than the control circuit current. In continuous mode, IGATECHG = (QT + QB), where QT and QB are the gate charges of the topside and bottom side MOSFETs. Supplying INTVCCpower through the EXTVCC switch input from an output-derived source will scale the VIN current required for the driver and control circuits by the ratio (Duty Factor)/(Efficiency). For example, in a 20V to 5V application, 10mA of INTVCC current results in approxi- mately 3mA of VIN current. This reduces the mid-current loss from 10% or more (if the driver was powered directly from VIN) to only a few percent. 4) The VIN current has two components: the first is the DC supply current given in the Electrical Characteristics table, which excludes MOSFET driver and control cur- rents; the second is the current drawn from the differential amplifier output. VIN current typically results in a small (<0.1%) loss. Other “hidden” losses such as copper trace and internal battery resistances can account for an additional 5% to 10% efficiency degradation in portable systems. It is very important to include these “system” level losses in the design of a system. The internal battery and input fuse resistance losses can be minimized by making sure that CIN has adequate charge storage and a very low ESR at the switching frequency. A 50W supply will typically require a minimum of 200 µF to 300µF of output capaci- tance having a maximum of 10m Ω to 20mΩ of ESR. The LTC3716 2-phase architecture typically halves the input APPLICATIO S I FOR ATIO and output capacitance requirements over competing solutions. Other losses including Schottky conduction losses during dead-time and inductor core losses gener- ally account for less than 2% total additional loss. Checking Transient Response The regulator loop response can be checked by looking at the load transient response. Switching regulators take several cycles to respond to a step in DC (resistive) load current. When a load step occurs, VOUT shifts by an amount equal to ∆ILOAD(ESR), where ESR is the effective series resistance of COUT(∆ILOAD) also begins to charge or discharge COUT generating the feedback error signal that forces the regulator to adapt to the current change and return VOUT to its steady-state value. During this recovery time VOUT can be monitored for excessive overshoot or ringing, which would indicate a stability problem. The availability of the ITH pin not only allows optimization of control loop behavior but also provides a DC coupled and AC filtered closed loop response test point. The DC step, rise time, and settling at this test point truly reflects the closed loop response. Assuming a predominantly second order system, phase margin and/or damping factor can be estimated using the percentage of overshoot seen at this pin. The bandwidth can also be estimated by examining the rise time at the pin. The ITH external components shown in the Figure 1 circuit will provide an adequate starting point for most applications. The ITH series RC-CC filter sets the dominant pole-zero loop compensation. The values can be modified slightly (from 0.2 to 5 times their suggested values) to optimize transient response once the final PC layout is done and the particular output capacitor type and value have been determined. The output capacitors need to be decided upon first because the various types and values determine the loop gain and phase. An output current pulse of 20% to 80% of full-load current having a rise time of <2 µs will produce output voltage and ITH pin waveforms that will give a sense of the overall loop stability without breaking the feedback loop. The initial output voltage step resulting from the step change in output current may not be within the bandwidth of the feedback loop, so this signal cannot be used to determine phase margin. This is why it is better to look at the Ith pin signal which is in the feedback |
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Similar Description - LTC3716 |
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