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SLDM-06D1Ax

tech.support@psbel.com

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42. PMBUS ADJUSTABLE SOFT START RISE TIME

The soft start rise time can be adjusted in the module via PMBus. When setting this parameter, make sure that the charging current for output

capacitors can be delivered by the module in addition to any load current to avoid nuisance tripping of the overcurrent protection circuitry during

startup. The TON_RISE command sets the rise time in ms, and allows choosing soft start times between 600μs and 9ms, with possible values

listed in Table 5. Note that the exponent is fixed at -4 (decimal) and the upper two bits of the mantissa are also fixed at 0

Table 5

Rise Time

Exponent

Mantissa

600μs

11100

00000001010

900μs

11100

00000001110

1.2ms

11100

00000010011

1.8ms

11100

00000011101

2.7ms

11100

00000101011

4.2ms

11100

00001000011

6.0ms

11100

00001100000

9.0ms

11100

00010010000

43. OUTPUT VOLTAGE ADJUSTMENT USING THE PMBUS

The VOUT_SCALE_LOOP parameter is important for a number of PMBus commands related to output voltage trimming, margining, over/under

voltage protection and the PGOOD thresholds. The output voltage of the module is set as the combination of the voltage divider formed by RTrim

and a 20kΩ upper divider resistor inside the module, and the internal reference voltage of the module. The reference voltage VREF is nominally

set at 600mV, and the output regulation voltage is then given by

Hence the module output voltage is dependent on the value of RTrim which is connected external to the module. The information on the output

voltage divider ratio is conveyed to the module through the VOUT_SCALE_LOOP parameter which is calculated as follows:

The VOUT_SCALE_LOOP parameter is specified using the “Linear” format and two bytes. The upper five bits [7:3] of the high byte are used to

set the exponent which is fixed at –9 (decimal). The remaining three bits of the high byte [2:0] and the eight bits of the lower byte are used for the

mantissa. The default value of the mantissa is 00100000000 corresponding to 256 (decimal), corresponding to a divider ratio of 0.5. The

maximum value of the mantissa is 512 corresponding to a divider ratio of 1. Note that the resolution of the VOUT_SCALE_LOOP command is

0.2%.

When PMBus commands are used to trim or margin the output voltage, the value of VREF is what is changed inside the module, which in turn

changes the regulated output voltage of the module.

The nominal output voltage of the module can be adjusted with a minimum step size of 0.4% over a ±25% range from nominal using the

VOUT_TRIM command over the PMBus.

The VOUT_TRIM command is used to apply a fixed offset voltage to the output voltage command value using the “Linear” mode with the

exponent fixed at –10 (decimal). The value of the offset voltage is given by

This offset voltage is added to the voltage set through the divider ratio and nominal VREF to produce the trimmed output voltage. The valid range

in two’s complement for this command is

–4000h to 3FFFh. The high order two bits of the high byte must both be either 0 or 1. If a value

outside of the +/-25% adjustment range is given with this command, the module will set it’s output voltage to the nominal value (as if VOUT_TRIM

had been set to 0), assert SMBALRT#, set the CML bit in STATUS_BYTE and the invalid data bit in STATUS_CML.

REF

OUT

V

RTrim

RTrim

V



















20000

RTrim

RTrim

LOOP

SCALE

VOUT





20000

_

_

10

)

(

2

_







TRIM

VOUT

V

offset

OUT