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© 2012 Exar Corporation

17/28

Rev. 1.2.2

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

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.

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:����������������������������= ������������������������

����������������������������

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.