XRP7708 Datasheet, PDF (16/27 Pages) Exar Corporation – Quad Channel Digital PWM Step Down Controller

XRP7708
Quad Channel Digital PWM Step Down Controller
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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.
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
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
© 2010 Exar Corporation
16/27
Rev. 1.0.3