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LT1425 Datasheet(PDF) 10 Page - Linear Technology |
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LT1425 Datasheet(HTML) 10 Page - Linear Technology |
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10 / 20 page  10 LT1425 OPERATION may be minimized by the use of the load compensation function. To implement the load compensation function, a voltage is developed that is proportional to average output switch current. This voltage is then impressed across the external ROCOMP resistor and the resulting current is then sub- tracted from the RFB node. As output loading increases, average switch current increases to maintain rough output voltage regulation. This causes an increase in ROCOMP resistor current subtracted from the RFB node, through which feedback loop action causes a corresponding increase in target output voltage. Assuming a relatively fixed power supply efficiency, Eff, Power Out = (Eff)(Power In) (VOUT)(IOUT) = (Eff)(VIN)(IIN) Average primary side current may be expressed in terms of output current as follows: IIN = IOUT VOUT (VIN)(Eff) ) ) Combining the efficiency and voltage terms in a single variable, IIN = K1(IOUT) where, K1 = VOUT (VIN)(Eff) ) ) Switch current is converted to voltage by a sense resistor and amplified by the current sense amplifier with associ- ated gain G. This voltage is then impressed across the external ROCOMP resistor to form a current that is subtracted from the RFB node. So the effective change in VOUT target is: ∆VOUT = K1(∆IOUT) RFB (RSENSE)(G) ROCOMP ) ) Expressing the product of RSENSE and G as the data sheet value of ∆VRCCOMP/∆ISW, ∆VRCCOMP ∆ISW ) ) ∆VRCCOMP ∆ISW ) ) RFB ROCOMP ) ) ROUT = K1 and, RFB ROUT ) ) ROCOMP = K1 ∆VRCCOMP ∆ISW ) ) = Data sheet value for RCCOMP pin action vs switch current where, K1 = Dimensionless variable related to VIN, VOUT and efficiency as above RFB = External “feedback” resistor value ROUT = Uncompensated output impedance ∆VRCCOMP ∆ISW ∆VOUT ∆IOUT ) ) RFB ROCOMP ) ) = K1 Nominal output impedance cancellation is obtained by equating this expression with ROUT. The practical aspects of applying this equation to determine an appropriate value for the ROCOMP resistor are found in the Applications Information section. |
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