Datasheet LTC1779 (Analog Devices) - 10

LTC1779
applicaTions inForMaTion
105 Although all dissipative elements in the circuit produce VREF losses, four main sources usually account for most of the 100 losses in LTC1779 circuits: 1) LTC1779 DC bias current, 2) 95 VITH MOSFET gate charge current, 3) I2R losses and 4) voltage drop of the output diode. 90 1. The VIN current is the DC supply current, given in the 85 electrical characteristics, that excludes MOSFET driver NORMALIZED VOLTAGE (%) 80 and control currents. VIN current results in a small loss which increases with VIN. 752.0 2.2 2.4 2.6 2.8 3.0 2. MOSFET gate charge current results from switching INPUT VOLTAGE (V) the gate capacitance of the internal power MOSFET. 1779 F04 Each time the MOSFET gate is switched from low to
Figure 4. Line Regulation of VREF and VITH
high to low again, a packet of charge dQ moves from
Setting Output Voltage
VIN to ground. The resulting dQ/dt is a current out of V The LTC1779 develops a 0.8V reference voltage between IN which is typically much larger than the DC supply current. In continuous mode, I the feedback (Pin 3) terminal and ground (see Figure 5). GATECHG = f(Qp). By selecting resistor R1, a constant current is caused to 3. I2R losses are predicted from the DC resistances of flow through R1 and R2 to set the overall output voltage. the internal MOSFET, inductor and current shunt. In The regulated output voltage is determined by: continuous mode the average output current flows through L but is “chopped” between the internal P- V ⎛ R2 ⎞ OUT = 0.8 1 ⎜ + ⎟ channel MOSFET in series with RSENSE and the output ⎝ R1⎠ diode. The MOSFET RDS(ON) plus RSENSE multiplied by For most applications, an 80k resistor is suggested for duty cycle can be summed with the resistances of L R1. To prevent stray pickup, locate resistors R1 and R2 and RSENSE to obtain I2R losses. close to LTC1779. 4. The output diode is a major source of power loss at VOUT high currents and gets worse at high input voltages. LTC1779 R2 3 The diode loss is calculated by multiplying the forward VFB voltage times the diode duty cycle multiplied by the R1 load current. For example, assuming a duty cycle of 1779 F05 50% with a Schottky diode forward voltage drop of
Figure 5. Setting Output Voltage
0.4V, the loss increases from 0.5% to 8% as the load current increases from 0.5A to 2A.
Efficiency Considerations
5. Transition losses apply to the internal MOSFET and The efficiency of a switching regulator is equal to the output increase at higher operating frequencies and input power divided by the input power times 100%. It is often voltages. Transition losses can be estimated from: useful to analyze individual losses to determine what is limiting the efficiency and which change would produce Transition Loss = 2(VIN)2IO(MAX)CRSS(f) the most improvement. Efficiency can be expressed as: Other losses including CIN and COUT ESR dissipative losses, Efficiency = 100% – (η1 + η2 + η3 + ...) and inductor core losses, generally account for less than 2% total additional loss. where η1, η2, etc. are the individual losses as a percent- age of input power. 1779fa 10 For more information www.linear.com/LTC1779