XR77128 Datasheet, PDF (22/28 Pages) Exar Corporation – Quad Output Digital PWM/PFM

XR77128
troller. The host then can use an I²C command to check
which channel is at fault.
LDO5 OCP
When current is drawn from LDO5 that exceeds LDO5 cur-
rent limit, the controller gets reset.
LDOOUT OCP
When current drawn from LDOOUT exceeds LDOOUT cur-
rent limit the regulator gets shut down, a fault is generated,
and the host will be notified by the SUPPLY_-
FAULT_EVENT interrupt generated by the controller. The
host then can through an I²C command check which chan-
nel/regulator is at fault. Once the fault condition is removed,
the host needs to turn the regulator on again.
V5EXT Switchover and Control
The XR77128 has a function called V5EXT that allows an
external 5V power supply to be used to supply its internal
5V (LD05) housekeeping voltage instead of using its inter-
nal 5V LDO. This function can notably reduce the control-
ler’s power dissipation and improve overall system
operating efficiency (particularly in the PFM mode).
When enabled in PA5.2, LD05 will automatically switch over
to the external supply and disable the internal LDO when
the V5EXT voltage reaches a programmable threshold volt-
age. The switchover thresholds are programmable in 50mV
steps with a total range of 200mV. Hysteresis to switch the
external 5 Volt supply in and out is 150mV. LDO5 automati-
cally turns off when the external voltage is switched in. This
is referred to as Forward Transfer.
There are two voltage sequencing requirements to use the
V5EXT function:
1. Power On - The chip’s VCC voltage has to be applied
prior to the V5EXT voltage. If the V5EXT voltage however is
present before VCC is applied, adding a simple logic FET to
the V5EXT input will meet this requirement. This is
described in detail in the “Operating from a System 5V” sec-
tion.
2. Power Off - Once the forward transfer is complete, the
controller’s internal circuits are then operating on the exter-
nal 5V supply, so the V5EXT voltage has to remain in regu-
lation, above the switchover threshold, as long as any
output is operating. Once all outputs are disabled the
V5EXT supply can be turn off (Off Last). This will ensure
that no false faults are indicated during shutdown.
When using V5EXT, an additional good quality 22uF
capacitor needs to be placed from the LDO5 pin to ground
to filter out any potential noise from getting into the internal
analog blocks.
If the V5EXT function is not used, the V5EXT pin should be
either grounded or left floating, in conjunction with making
sure the function gets disabled through PA 5.2.
Operating from a Generated 5V Output
It is recommended when using the 5VEXT function, that the
5V rail be generated by one of the XR77128 outputs and
routed back to the V5EXT pin. This meets the Power On
sequence requirement. Power down sequencing requires
that the V5EXT supply remains on until the all outputs are
powered down. Therefore, the 5V rail should be defined in
PA 5.2 as the last to be shut down. If the 5V output has no
requirement for being actively shut down, then set the
“Active Shutdown Threshold” to 5V.
Operating from a System 5V
The system 5V rail must be regulated to a value of 5V, -5%/
+10% and the V5EXT switchover threshold must be set to
4.8V in PA 5.2.
VCC must be applied to the chip before connecting the Sys-
tem 5V to the V5EXT pin. If the system 5V rail is an always
on rail, a simple way to meet the Power On sequencing
requirement is to add a logic NFET, with the Source con-
nected to the V5EXT pin, the Gate to VCC pin and the
Drain to the system 5V supply. For proper operation the
VCC input voltage must be greater than 5V plus the NFET
threshold voltage. In this configuration, the ENABLE pin of
the XR77128 should be left floating. Again the V5EXT sup-
ply has to remain on until all the output have been turned
off.
Fault Management
Fault management settings, when V5EXT is XR77128 gen-
erated require, that the other three rails follow the 5V rail if it
faults. There are three fault actions the 5V rail may take:
“Shutdown Channel”, “Shutdown and Auto-restart Channel”,
and “Restart Chip”. If “Shutdown Channel” is selected, out-
side interaction is required to restart the 5V and the other
three rails. This may be through I²C, I/Os or VCC power
cycling. If “Shutdown and Auto-restart Channel” is selected,
to restart the other channels, an I/O should be assigned as
HW Power Good or Power OK for the 5V output and con-
nected to a second I/O configured to “Enable” for the other
three outputs. Lastly, “Restart Chip” is the simplest to use
for auto recovery.
© 2014 Exar Corporation
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exar.com/XR77128
Rev 1A