Datasheet LT1425 (Analog Devices) - 10

LT1425
U OPERATION
may be minimized by the use of the load compensation external ROCOMP resistor to form a current that is function. subtracted from the RFB node. So the effective change in To implement the load compensation function, a voltage is VOUT target is: developed that is proportional to average output switch (RSENSE)(G) current. This voltage is then impressed across the external ∆VOUT = K1(∆IOUT) RFB ROCOMP ROCOMP resistor and the resulting current is then sub- tracted from the RFB node. As output loading increases, Expressing the product of RSENSE and G as the data sheet average switch current increases to maintain rough output value of ∆VRCCOMP/∆ISW, voltage regulation. This causes an increase in ROCOMP ∆ resistor current subtracted from the R V R FB node, through RCCOMP ROUT = K1 ) FB ) and, which feedback loop action causes a corresponding ∆ISW ROCOMP increase in target output voltage. ∆VRCCOMP ROCOMP = K1 ) RFB ) Assuming a relatively fixed power supply efficiency, Eff, ∆ISW ROUT Power Out = (Eff)(Power In) where, (V K1 = Dimensionless variable related to V OUT)(IOUT) = (Eff)(VIN)(IIN) IN, V Average primary side current may be expressed in terms OUT and efficiency as above of output current as follows: ∆VRCCOMP ) ∆ = Data sheet value for R I CCOMP pin V SW action vs switch current I OUT ) IN = IOUT (VIN)(Eff) RFB = External “feedback” resistor value R Combining the efficiency and voltage terms in a single OUT = Uncompensated output impedance variable, ∆VOUT ∆VRCCOMP = K1 ) RFB ) ∆IOUT ∆ I ISW ROCOMP IN = K1(IOUT) where, Nominal output impedance cancellation is obtained by V equating this expression with ROUT. The practical aspects K1 OUT ) = (V of applying this equation to determine an appropriate IN)(Eff) value for the ROCOMP resistor are found in the Applications Switch current is converted to voltage by a sense resistor Information section. and amplified by the current sense amplifier with associ- ated gain G. This voltage is then impressed across the 10