Datasheet

OUT(nom)

VOUT _ SCALE _ LOOP

( )

V VOUT _ TRIM VOUT _ SCALE _ LOOP 0.6= ´ +

( )

V VO UT _ M ARGIN _ HIGH VOUT _ TRIM VOUT _ SCALE _ LO OP= + ´

TPS40400

SLUS930B –APRIL 2011– REVISED OCTOBER 2011

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Output voltage adjustment. The nominal output voltage of the converter can be adjusted using the

VOUT_TRIM command. See the VOUT_TRIM command description for the format of this command as used in

the TPS40400. The adjustment range is ±25% from the nominal output voltage. The VOUT_TRIM command is

typically used to trim the final output voltage of the converter without relying on high precision resistors being

used in Figure 17. The resolution of the adjustment is 7 bits, with a resulting minimum step size of approximately

0.4%. Note that the output margining is accomplished using this same 7 bit structure so the total combined

deviation from the nominal output for margining and VOUT_TRIM is still limited to ±25%. Exceeding this range

causes errors.

In order for the PMBus output voltage adjustments to function correctly, the VOUT_SCALE_LOOP parameter

must be set properly. VOUT_SCALE_LOOP is a PMBus command (see Supported PMBus Commands) that tells

the controller what the ratio of the voltage divider that sets the nominal output voltage is. The data for this

command is the ratio of the divider that is used to set the output voltage. From Figure 17, VOUT_SCALE_LOOP

parameter can be calculated using Equation 6.

(6)

The resolution of the VOUT_SCALE_LOOP command is 0.00195, or slightly under 0.2% due to the data format

of the command (the linear data mode exponent is fixed at -9 for this command). This granularity affects the

accuracy of adjustments to the output voltage made using the PMBus (VOUT_TRIM, VOUT_MARGIN_HIGH and

VOUT_MARGIN_LOW) as well as setting the over and under voltage fault and warning levels. These commands

use the VOUT_SCALE_LOOP parameter to calculate what the reference voltage needs to be for the requested

output voltage or the thresholds referenced to the FB pin need to be for the requested warning and fault levels.

Once the VOUT_SCALE_LOOP parameter has been properly set, the commands that adjust the output voltage

functions properly. There are three possible states that the TPS4040 can be in when considering what the actual

output voltage is:

• No output margin

• Margin high

• Margin low

These output states are set using the OPERATION command. The FB pin reference voltage is calculated as

follows in each of these states.

No margin voltage:

(7)

Margin high voltage state:

(8)

Product Folder Link(s) :TPS40400