NRND

LM5080

SNVS387C – SEPTEMBER 2005 – REVISED MARCH 2013

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OPERATING DESCRIPTION

Identical regulators connected in parallel will theoretically share the total load current equally. However, slight

mismatches in the reference voltage or feedback dividers of each regulator can cause significant imbalances in

the load current sharing. The LM5080 senses the load current of each regulator with an external sense resistor

and makes adjustments to the regulator’s output voltage to achieve nearly equal current sharing. There are four

possible implementations for the LM5080:

Reference Adjust Mode achieves current sharing by adjusting the regulator reference voltage by applying an

error current from the TRO transconductance amplifier to the trim or adjust pin of the regulator.

Remote Sense Positive Mode achieves current sharing by adjusting the positive remote sense pin of the power

converter with current supplied by the RSO buffer amplifier output.

Remote Sense Negative Mode achieves current sharing by adjusting the negative remote sense pin of the

power converter with current supplied by the RSO buffer amplifier output.

Feedback Adjust Mode achieves current sharing by adjusting the regulator feedback voltage by applying an

error current from the TRO transconductance amplifier to the feedback resistor divider.

In each mode, the LM5080 combines the regulator’s load current information with the total load information on

the share (SHR) bus to create an error current on the TRO output which is proportional to the load current

mismatch. In the reference adjust or feedback adjust modes of operation, the output of the current share

amplifier (CSA) is fed directly into the regulator reference or feedback divider. The RSO buffer can optionally be

used to boost the transconductance of the CSA if needed. In the remote sense adjust modes, the RSO and CSO

pins are tied together which reconfigures the CSA as a voltage error amplifier where the RSO buffer drives the

remote sense pins of the regulator directly.

CURRENT SHARE AMPLIFIER

The current share amplifier is a low input offset transconductance amplifier with inputs CSP and CSM and dual

outputs, TRO and CSO. The two outputs are identical except TRO is current limited to approximately ±10 µA in

order to limit the maximum correction of the regulator reference in the trim adjust and feedback adjust modes.

The outputs can operate down to 450 mV without saturating which allows the TRO output to adjust reference

voltages as low as 500 mV. A capacitor from TRO to ground (CTRO) is used for frequency compensation of the

current share loop.

In the two remote sense adjust modes, the current share amplifier is configured as a unity gain differential

voltage amplifier by tying RSO to CSO. A capacitor from TRO to ground (CTRO) is used for frequency

compensation of the amplifier and the current share loop.

RSO BUFFER

The RSO buffer is a low-offset unity-gain operational amplifier that has different uses, depending on the mode of

operation. In the remote sense adjust modes, the RSO pin is externally tied to the CSO pin to create a

differential voltage amplifier that can drive the 10

Ω input impedance of the remote sense pin of typical power

converter modules. The RSO buffer can source or sink 10mA at an output voltage as low as 20mV above

ground. With RSO load resistors of 10

Ω, the buffer can drive up to 10nF without causing amplifier instability. For

RSO loads > 1 k

Ω, max load capacitance on this node is 500pF for stable operation. In the trim adjust and

feedback adjust modes, the RSO buffer can be configured with external resistors to boost the CSA

transconductance which increases the current share loop gain.

CSP MODE COMPARATOR

The LM5080 monitors CSP & CSM with the CSP mode comparator and applies a 500 mV input offset on the

RSO buffer amplifier if CSP-CSM is less than 10% of VCC (which indicates a remote sense application mode).

This offset allows the RSO output to swing within 10 mV of ground without saturating the TRO output which

drives the RSO buffer. In the trim adjust and feedback adjust modes, the CSP pin is left open. In this

configuration CSP is internally biased such that CSP - CSM = 0.2 x VCC resulting in the removal of the 500mV

RSO buffer offset.

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