XRP7704 Datasheet, PDF (19/31 Pages) Exar Corporation – Quad-Output Digital PWM Buck Controller

REV 1.1.0
XRP7704
Quad-Output Digital PWM Buck Controller
Over-Voltage Protection (OVP)
The Over-Voltage Protection (OVP) SET_OVVP_REGISTER sets the over-voltage condition in predefined steps
per channel. The over-voltage protection is always active even during soft-start condition. When the over-voltage
condition is tripped, the controller will shut down the channel. When the channel is shut down the controller will
then set corresponding OVP Fault bits in the READ_OVV_UVLO_OVT_FLAG register.
The VOUT OVP Threshold is 150mV to 300mV above nominal VOUT for a Voltage Target of 2.5V or less. For
the Voltage Target of 2.6V to 5.1V, the VOUT OVP Threshold is 300mV to 600mV.
Once the over-voltage Channel is disabled, the controller will check the SET_FAULT_RESP_CONFIG_LB and
SET_FAULT_RESP_CONFIG_HB to determine whether there are any “following” channels that need to be shut
down. Any following channel will be disabled when the channel with the Over Voltage Fault is disabled. The
channel(s) will remain disabled, until the Host takes action to enable the channel(s).
Any of the fault and warning conditions can also be configured to be represented using the general purpose input
output pins (GPIO) to use as an interface with non I²C compatible devices. For further information on this topic
see the “GPIO Pins” Section.
During OVP fault shutdown of the channel, the customer has the option to choose two types of shutdown for each
channel. The first shutdown is ‘passive shutdown’ where the IC merely stops outputting pulses. The second
shutdown is a ‘brute force’ shutdown where the GL remains on as the channel reaches its discharged voltage.
Note that if the ‘brute force’ method is chosen, then GL will permanently remain high until the channel is re-
enabled.
Output Current Setting and Monitoring
XRP7704 utilizes a low side MOSFET Rdson current sensing technique. The voltage drop on Rdson is measured
by dedicated current ADC. The ADC results are compared to a maximum current threshold and an over-current
warning threshold to generate the fault and warning flags.
Maximum Output Current
The maximum output current is set by the SET_VIOUT_MAX_CHx register and SET_ISENSE_PARAM_CHx
register. The SET_VIOUT_MAX_CHx register is an 8 bit register. Bits [5:0] set the maximum current threshold
and bits [7:6] set the over-current warning threshold. The LSB for the current limit register is 5 mV and the allowed
voltage range is between 0 and 315mV. To calculate the maximum current limit, the user needs to provide the
MOSFET Rdson. The maximum current can be calculated as:
Where Kt is the temperature coefficient of the MOSFET Rdson; Vsense is the voltage across Rdson; IOUTMAX is
the maximum output current.
Over-Current Warning
The XRP7704 also offers an Over-Current warning flag. This warning flag resides in the READ_OVC_FLAG
register. The warning flag bit will be set when the output current gets to within a specified value of the output
current limit threshold enabling the host to reduce power consumption. The SET_VIOUT_MAX_CHx register
allows the warning flag threshold to be set 10mV, 20mV, 30mV or 40mV below VIOUT_MAX. The warning flag
will be automatically cleared when the current drops below the warning threshold.
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