XRP7720 Datasheet, PDF (21/28 Pages) Exar Corporation – Quad Output Universal Customizable PMIC with PFM

XRP7720
Quad Output Universal Customizable PMIC with PFM
OCP
A user defined OCP fault level is set with
10mA increments in PA 5.1. PA 5.1 uses
calculations to give the user the approximate
DC output current entered in the current limit
field. However the actual current limit trip
value programmed into the part is limited to
280mV as defined in the electrical
characteristics. The maximum value the user
can program is limited by Rdson of the
synchronous Power FET and current
monitoring ADC range. For example, using a
synchronous FET with Rdson of 30mΩ and
wider ADC range the maximum current limit
programmed would be:
280𝑚𝑉
𝑂𝐶𝑃 𝑀𝑎𝑥(𝐴) = _{30𝑚Ω} = 9.33𝐴
The current is sampled approximately 30ns
before the low side MOSFET turns off so the
actual measured DC output current in this
example would be 9.33A plus approximately
half the inductor ripple.
An OCP Fault is considered to have occurred
only if the fault threshold has been tripped in
four consecutive switching cycles. When the
switching frequency is set to the 4x multiplier
the current is sampled every other cycle. As
a result it can take as many as 8 switching
cycles for an over current event to be
detected. When operating in 4x mode an
inductor with a soft saturation characteristic is
recommended.
In addition, OCP fault can be monitored
through HW Flags on GPIO0. The OCP
warning level is calculated by PA 5.1 as 85%
of the OCP fault level.
A user can choose one of three options in
response to an OCP event: shut down the
faulting channel, shut down faulting channel
and perform auto-restart of the channel, or
restart the chip.
The output current reported by the XRP7720
is processed through a 7 sample median filter
in order to reduce noise. The OCP limit is
compared against unfiltered ADC output.
In the case of Shutdown and Auto-restart
Channel the user has an option to specify
startup timeout (the time in which the fault is
validated) and hiccup timeout (the period
after which the controller will try to restart
the channel) periods in 1 msec increments
with a maximum value of 255 msec.
Note: The Channel Fault Action response is
the same for either the OVP or OCP event.
Start-up Time-out Fault
A channel will be at Start-up Time-out Fault if
it does not come-up in a time period specified
in the “Startup Timeout” box. In addition a
channel is at Start-up Timeout Fault if its pre-
bias configuration voltage is within a defined
value too close to the target.
LDO5 OCP
When current is drawn from the LDO5 that
exceeds the LDO5 current limit the controller
will be reset.
V5EXT SWITCHOVER
The V5EXT gives the user an opportunity to
supply an external 5 Volt rail to the controller
in order to reduce the controller’s power
dissipation. The 5 Volt rail can be an
independent power rail present in a system or
any of 7720 channels regulated to 5 Volts and
routed back to the V5EXT pin. It is important
to note that voltage to Vcc must be applied all
the time even after the switchover, in which
© 2014 Exar Corporation
21/28
Rev. 1.0.0