X

0

Q

© 2012 Exar Corporation

16/28

Rev. 1.2.2

The Output Voltage setting is controlled by the SET_VOUT_TARGET_CHx register. This register

allows the user to set the output voltage with a resolution of 50mV for output voltages between 0

and 2.5V and with a resolution of 100mV for output voltages between 2.6V and 5.1V. Output

voltages higher than 5.1V can be achieved by adding an external voltage divider network. The

output voltage of a particular channel can be read back using the READ_VOUTx register.

Output Voltage from 0.9V to 5.1V

Per the equation below, for values between 0.9V and 5.1V the output voltage is equal to the binary

number stored in the SET_VOUT_TARGET_CHx register multiplied by 50mV. When programming an

output voltage from 2.6V to 5.1V, odd binary values should be avoided. As a result, the set

resolution for an output voltage higher than 2.5V is 100mV.

Output VOUT Higher Than 5.1V

To set the output voltage higher than 5.1V, the user needs to add an external voltage divider. The

resistors used in the voltage divider should be below 10kΩ. The SET_VOUT_TARGET_CHx register

should be set to 0x32 which is equivalent to an output voltage of 2.5V without the external divider

network. The output voltage regulation in this case might exceed 2% due to extra error from the

resistor divider. R1 and R2 follows the definition below.

Voutx pin

Vout>5.1V

R1

R2

Fig.21: External divider network for high output voltage

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