Voltage Margining

Shifting the output regulation voltage up or down by a small

amount is referred to as voltage margining. In the remote

sense adjust modes and the feedback adjust modes, this

can be done by connecting all of the power converter TRIM

pins together and injecting a positive or negative current.

However, in the reference adjust mode, the TRIM pin is used

for current sharing. An alternative margining method is to

inject a current into the SHR share bus. This will simulta-

neously shift the regulation voltages of all power converter’s

while maintaining equal current sharing. The injected current

is split equally between the LM5080’s SHR inputs and added

to the TRIM pin currents creating an equal offset voltage for

all of the power converter references. The trim pin current

injected into each power converter’s reference (IT) is depen-

dent on the magnitude of the total injected current into the

SHR bus (ISHR), the number of LM5080’s on the SHR bus

(N) and any transconductance boost supplied (R1 & R2):

An alternate method to shift the regulation voltage is to tie all

the CSP pins together and inject a current into that node.

The trim pin current injected into each power converter’s

reference (IT) attributed to the current injected into the CSP

node (ICSP) is derived to be:

Figure 6 shows a margining up and down application imple-

mented using pull up resistors to VCC. Since the SHR and

CSP voltages are approximately 0.1 x VCC and 0.2 x VCC

respectively, the injected current can be independently con-

trolled with RM1 and RM2.

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FIGURE 6. One Method of Implementing Voltage Margining

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