NRND

LM5080

SNVS387C – SEPTEMBER 2005 – REVISED MARCH 2013

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GENERAL DESIGN PROCEDURE

1. Select an appropriate sense resistor value. More sense voltage will result in better load sharing but more

efficiency loss. Sense voltages of 50mV or more are recommended. In addition, the sense voltage at full load

should be less than 5% of VCC in applications that control the remote sense terminals of the power supply.

2. For the reference adjust and feedback adjust modes, determine if transconductance (gm) boosting is

required. Boosting the transconductance also boosts the TRO pin current limit. The TRO pin current limit

(approximately 10µA typical) multiplied by the reference impedance determines the maximum correction the

LM5080 can make to the reference. The LM5080 must have enough TRO current to adjust the converter

output voltage by at least the accuracy of the reference. For example, if the reference accuracy is ±2%, the

LM5080 must have the ability to adjust the reference by at least 2% (in the event one converter is 2% high

and the other 2% low).

3. Compensate the current share loop by selecting an appropriate capacitance for CTRO. The compensation of

the current share loop is dependent on the frequency response of the output voltage to the controlling node

of the converter (TRIM pin, feedback divider or remote sense pins). Given the wide variety of converter

designs and the many operating modes of the LM5080, selection of CTRO is best accomplished using a

simple iterative procedure. Start with a large capacitance in TRO (100µF or more). While monitoring the load

current in each converter with a current probe, determine the minimum CTRO required for stability by

decreasing CTRO until the current sharing becomes unstable under step loads. The step loads should be

more than 50% of the load range and applied at a frequency well below the cross over frequency of the

converter. The TRO capacitance can be further reduced by introducing some resistance (RTRO) in series

with CTRO to cancel the 2nd order poles within the converter.

4. If RTRO > 100

Ω in either remote sense mode, a second CTRO capacitor (~ 2nF) should be added between

TRO and CSP to keep the error amplifier stable.

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