Operating Description

Identical regulators connected in parallel will theoretically

share the total load current equally. However, slight mis-

matches 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 imple-

mentations for the LM5080:

Reference Adjust Mode achieves current sharing by adjust-

ing 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 con-

verter 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 con-

verter with current supplied by the RSO buffer amplifier

output.

Feedback Adjust Mode achieves current sharing by adjust-

ing 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 transconduc-

tance amplifier with inputs CSP and CSM and dual outputs,

TRO and CSO. The two outputs are identical except TRO is

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 ampli-

fier that has different uses, depending on the mode of op-

eration. 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

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 al-

lows 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.

Reference Adjustment Operation

Mode

The reference adjust or trim adjust mode configuration is

shown in Figure 1. Typically only the current share amplifier

is used, however the RSO buffer can be optionally config-

ured for boosting the transconductance to increase the cur-

rent share loop gain (Figure 2). CSP is left open and CSM is

connected to a low side current sense resistor. The TRO

output is connected to the TRIM pin of the power converter

to inject a correction that adjusts the voltage regulator.

To understand the control loop, assume for a moment the

SHR pin is disconnected from the share bus. Since both

inputs to the current share amplifier are equal, the TRO

output current (IT) is zero and independent of the sense

resistor voltage (VRS). Hence the voltage regulation loop of

each converter is unaffected by the LM5080 when the SHR

bus is open. When the SHR pins are connected in a 2 supply

system, the transfer function between the sense resistor

voltages (VRS1 & VRS2) and the current injected into each

power converter TRIM pin (IT1 & IT2) are as follows:

IT1 = 0.9 x gm x (VRS1 - VRS2)

IT2 = 0.9 x gm x (VRS2 - VRS1)

where gm = current share amplifier transconductance

(8.7mA/V).

As long as the current sharing is equal (VRS1=VRS2), the

correction to the references (IT1 & IT2) will remain un-

changed. However, any difference between VRS1 and VRS2

will drive the TRIM pin currents in opposite polarities. As a

result the power converter output voltages will be adjusted to

force VRS1=VRS2. For 3 or more channels, the same aver-

aging concept is true; the injected currents (IT) will drive the

references such that the sense voltages are nearly identical.

A capacitor from TRO to ground (CTRO) sets the dominant

pole of the current share loop. The pole location is deter-

mined by gm, CTRO and the impedance of the regulator trim

pin. The current share loop frequency response does not

need to be fast and in fact should be less than or equal to

1/10th the regulator bandwidth. Since similar regulators will

have similar transient responses to a load step, the LM5080

only needs to correct the differences in each regulator’s

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