Datasheet LTC1701, LTC1701B (Analog Devices) - 9

LTC1701/LTC1701B
U U W U APPLICATIO S I FOR ATIO
During normal operation the voltage on the ITH/RUN pin continuous mode, IGATECHG = f • QP, where QP is the gate will vary from 1.25V to 2.25V depending on the load charge of the internal MOSFET switch. current. Pulling the ITH/RUN pin below 0.8V puts the 3) I2R Losses are predicted from the DC resistances of the LTC1701 into a low quiescent current shutdown mode MOSFET and inductor. In continuous mode the average (IQ < 1µA). This pin can be driven directly from logic as output current flows through L, but is “chopped” between shown in Figures 3(a). the topside internal MOSFET and the Schottky diode. At I I low supply voltages where the switch on-resistance is TH/RUN TH/RUN higher and the switch is on for longer periods due to the R1 D1 higher duty cycle, the switch losses will dominate. Using C C a larger inductance helps minimize these switch losses. At C C C1 high supply voltages, these losses are proportional to the RC RC load. I2R losses cause the efficiency to drop at high output currents. (a) (b) 1701 F03
Figure 3. ITH/RUN Pin Interfacing
4) The Schottky diode is a major source of power loss at high currents and gets worse at low output voltages. The diode loss is calculated by multiplying the forward voltage
Efficiency Considerations
drop times the diode duty cycle multiplied by the load The percent efficiency of a switching regulator is equal to current. the output power divided by the input power times 100%. Other “hidden” losses such as copper trace and internal It is often useful to analyze individual losses to determine what is limiting the efficiency and what change would battery resistances can account for additional efficiency degradations in portable systems. It is very important to produce the most improvement. Percent efficiency can be include these “system” level losses in the design of a expressed as: system. The internal battery and fuse resistance losses %Efficiency = 100% – (L1 + L2 + L3 + ...) can be minimized by making sure that CIN has adequate where L1, L2, etc. are the individual losses as a percentage charge storage and very low ESR at the switching fre- of input power. quency. Other losses including Schottky conduction losses during dead-time and inductor core losses generally ac- Although all dissipative elements in the circuit produce count for less than 2% total additional loss. losses, 4 main sources usually account for most of the losses in LTC1701 circuits: 1) LTC1701 VIN current, 2)␣ switching losses, 3) I2R losses, 4) Schottky diode
THERMAL CONSIDERATIONS
losses. The power handling capability of the device at high ambi- ent temperatures will be limited by the maximum rated 1) The VIN current is the DC supply current given in the electrical characteristics which excludes MOSFET driver junction temperature (125°C). It is important to give careful consideration to all sources of thermal resistance and control currents. VIN current results in a small (< 0.1%) from junction to ambient. Additional heat sources mounted loss that increases with VIN, even at no load. nearby must also be considered. 2) The switching current is the sum of the internal MOSFET For surface mount devices, heat sinking is accomplished driver and control currents. The MOSFET driver current by using the heat spreading capabilities of the PC board results from switching the gate capacitance of the power MOSFET. Each time a MOSFET gate is switched from low and its copper traces. Copper board stiffeners and plated through-holes can also be used to spread the heat gener- to high to low again, a packet of charge dQ moves from VIN ated by power devices. to ground. The resulting dQ/dt is a current out of VIN that is typically much larger than the control circuit current. In 9