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© 2010 Exar Corporation

16/27

Rev. 1.0.3

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50

Output Voltage Lower Than 0.9V

The XRP7708 can be programmed to regulate an output voltage lower than 0.9V. However, in this

case the specification of +-2% output voltage accuracy may be exceeded.

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

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