XRP7708_12 Datasheet, PDF (17/28 Pages) Exar Corporation – Quad Channel Digital PWM Step Down Controllers

XRP7708 and XRP7740
Quad Channel Digital PWM Step Down Controllers
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
XRP7708 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:
𝑉𝑠𝑒𝑛𝑠𝑒
𝐼𝑂𝑈𝑇𝑀𝐴𝑋 = _{𝑅𝑑𝑠𝑜𝑛 × 𝐾𝑡}
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 XRP7708 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.
Over-Current Fault Handling
When an over-current condition occurs, PWM drivers in the corresponding channels are disabled.
After a 200ms timeout, the controller is re-powered and soft-start is initiated. When the over-
current condition is reached 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 similarly restarted. The controller will also set the fault flags in READ_OVC_FAULT_WARN
register.
Typically the over-current fault threshold would be set to 130-140% of the maximum desirable
output current. This will help avoid any over-current conditions caused by transients that would
shut down the output channel.
© 2012 Exar Corporation
17/28
Rev. 1.2.2