Datasheet LTC1622 (Analog Devices) - 10

LTC1622
U U W U APPLICATIONS INFORMATION
101 is limiting the efficiency and which change would produce VREF the most improvement. Efficiency can be expressed as: 100 V Efficiency = 100% – (η1 + η2 + η3 + ...) 99 ITH where η1, η2, etc. are the individual losses as a percent- 98 age of input power. 97 Although all dissipative elements in the circuit produce NORMALIZED VOLTAGE (%) 96 losses, four main sources usually account for most of the losses in LTC1622 circuits: 1) LTC1622 DC bias current, 952.0 2.2 2.4 2.6 2.8 3.0 2) MOSFET gate charge current, 3) I2R losses, 4) voltage INPUT VOLTAGE (V) drop of the output diode and 5) transition losses. 1622 F03 1. The VIN current is the DC supply current, given in the
Figure 3. Line Regulation of VREF and VITH
electrical characteristics, that excludes MOSFET driver the maximum current sense voltage that sets the maxi- and control currents. VIN current results in a small loss mum output current. which increases with VIN. 2. MOSFET gate charge current results from switching
Setting Output Voltage
the gate capacitance of the power MOSFET. Each time The LTC1622 develops a 0.8V reference voltage between a MOSFET gate is switched from low to high to low the feedback (Pin 3) terminal and ground (see Figure 4). By again, a packet of charge dQ moves from VIN to ground. selecting resistor R1, a constant current is caused to flow The resulting dQ/dt is a current out of VIN which is through R1 and R2 to set the output voltage. The regulated typically much larger than the DC supply current. In output voltage is determined by: continuous mode, IGATECHG = f(Qp). 3. I2R losses are predicted from the DC resistances of the  R2 V MOSFET, inductor and current shunt. In continuous OUT = 0 8 . 1+  R1 mode the average output current flows through L but is “chopped” between the P-channel MOSFET in series For most applications, a 30k resistor is suggested for R1. with RSENSE and the output diode. The MOSFET RDS(ON) To prevent stray pickup, an optional 100pF capacitor is plus RSENSE multiplied by duty cycle can be summed suggested across R1 located close to LTC1622. with the resistance of the inductor to obtain I2R losses. 4. The output diode is a major source of power loss at VOUT R2 high currents and gets worse at high input voltages. LTC1622 3 VFB The diode loss is calculated by multiplying the forward 100pF R1 voltage drop times the diode duty cycle multiplied by the load current. For example, assuming a duty cycle of 1622 F04 50% with a Schottky diode forward voltage drop of
Figure 4. 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 external MOSFET and The efficiency of a switching regulator is equal to the increase with higher operating frequencies and input output power divided by the input power times 100%. It is voltages. Transition losses can be estimated from: often useful to analyze individual losses to determine what 10