TPS40400_15 Datasheet, PDF (43/82 Pages) Texas Instruments – TPS40400 3-V to 20-V PMBus Synchronous Buck Controller

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TPS40400
SLUS930C – APRIL 2011 – REVISED OCTOBER 2015
For example, with a 1.2 V nominal output voltage, the POWER_GOOD_ON command can set the lower
threshold to 1.14 V, 1.104 V or 1.08 V. Doing this automatically sets the upper thresholds to 1.26 V, 1.296 V and
1.32 V respectively.
The effective value of this command is determined by the settings of the VOUT_MODE command. In this device,
the VOUT_MODE is fixed to Linear with an exponent of –10 (decimal) so the effective lower power good turn on
threshold requested is:
V_{OUT(PGOOD _ ON)} = POWER _ GOOD _ ON ´ 2^-10
(23)
The nominal output voltage is set by external resistors and a 600-mV error amplifier reference. It is critical that
the correct value be programmed into VOUT_SCALE_LOOP in order to correctly select the desired
POWER_GOOD_ON threshold.
Normally, the POWER_GOOD_ON threshold is set higher than the POWER_GOOD_OFF threshold. If the
POWER_GOOD_ON threshold is set to a value equal to or less than the POWER_GOOD_OFF threshold, the
device:
• Sets the CML bit in the STATUS_BYTE
• Sets the Invalid data bit in STATUS_CML
• Notifies the host via SMBALRT pin
It is the user's responsibility to ensure that the chosen POWER_GOOD_ON and POWER_GOOD_OFF
thresholds are reasonable with respect to each other. For values written outside the supported ranges are
ACK'ed but causes the SMBALRT line to assert and the CML bit to be set in the STATUS_WORD. The invalid
data bit is also set in the STATUS_CML results. The actual POWER_GOOD_ON threshold is set to the nearest
supported extreme value. For instance, with VOUT_SCALE_LOOP set to 0.5 for a typical 1.2-V output supply,
setting POWER_GOOD_ON to 0.5 results in the threshold being set to the 90% value.
The POWER_GOOD_ON command has two data bytes formatted as two’s complement binary integer:
Table 37. POWER_GOOD_ON Commands
COMMAND
POWER_GOOD_ON
Format
Linear, two's complement binary
Bit Position
7
6
5
4
3
2
1
0
7
6
5
4
3
2
1
0
Access
r r/w r/w r/w r/w r/w r/w r/w r/w r/w r/w r/w r/w r/w r/w r/w
Function
High Byte
Low Byte
Default Value
0
0
0
0
0
1
0
0
0
1
1
0
1
0
1
0
The default value sets the power good turn on threshold to 1.1035V which maps to the 92% low threshold and
108% high threshold.
7.6.28 POWER_GOOD_OFF (5Fh)
The POWER_GOOD_OFF command sets the value of the output voltage at which the PGOOD output pin (open
drain output) is de-asserted low. The contents of this register can be stored to non-volatile memory using the
STORE_DEFAULT_ALL or STORE_DEFAULT_CODE commands. The actual implementation is a window
comparator with symmetrical thresholds above and below the nominal. This command sets both the upper and
lower power good threshold at the same time. The parameter passed with this command is always the lower
threshold (less than the nominal output) and is mapped to the closest supported percentages of the nominal
output voltage below:
Table 38. Supported POWER_GOOD_OFF Levels
SUPPORTED POWER_GOOD_OFF LEVELS
LOW THRESHOLD
HIGH THRESHOLD
92%
90% ⁽¹⁾
108%
110%
88%
112%
(1) Default value
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