XRP7714 Exar, XRP7714 Datasheet - Page 17

• Manufacturer Part Number: XRP7714
• Description: Quad Channel Digital PWM Step Down Controller
• Manufacturer: Exar

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XRP7714 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.
The
SET_ISENSE_PARAM_CHx register. The SET_VIOUT_MAX_CHx register is an 8 bit register.
[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.
The XRP7714 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.
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.
CHIP OPERATION AND CONFIGURATION
S
The SET_SS_RISE_CHx register is a 16 bit register which specifies the soft-start delay and the
ramp characteristics for a specific channel. This register allows the customer to program the
channel with a 250µs step resolution and up to a maximum 16ms delay.
Bits [15:10] specify the delay after enabling a channel but before outputting pulses; where each bit
represents 250µs steps. Bits [9:0] specify the rise time of the channel; these 10 bits define the
number of microseconds for each 50mV increment to reach the target voltage.
© 2010 Exar Corporation
Maximum Output Current
Over-Current Warning
Over-Current Fault Handling
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