LM5080MM datasheet(12/13 Pages) NSC

12 / 13 page

General Design Procedure

Select an appropriate sense resistor value. More sense

voltage will result in better load sharing but more effi-

ciency loss. Sense voltages of 50mV or more are rec-

ommended. 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.

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 de-

termines 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 refer-

ence 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).

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.

If RTRO > 100

Ω in either remote sense mode, a

second CTRO capacitor (~ 2nF) should be added be-

tween TRO and CSP to keep the error amplifier stable.

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