XRP7708_12 Datasheet, PDF (24/28 Pages) Exar Corporation – Quad Channel Digital PWM Step Down Controllers

XRP7708 and XRP7740
Quad Channel Digital PWM Step Down Controllers
The XRP7740ILB-0X18-F has an I²C address of 0x18. The internal registers are written by sending
a data value of 0x30 and read by sending a data value of 0x31. This reflects the address being
shifted one bit to the left and the least significant bit being set to reflect a read or write operation
in order to stuff the byte correctly.
The second byte sent to the device is the location of a specific register.
Using GPIO3 to select I²C Address
GPIO3 may be used to change the LSB of the 7-bit address. This option can be enabled within the
PowerArchitect^TM software by checking the “Use GPIO3 to control LSB of I2C address” box at the
top right of the “Digital Design” tab. More about the use of this option and other methods of
changing the default I2C address of the part are available in ANP-31 “Power^XR Configuration and
Programming”.
EXTERNAL COMPONENT SELECTION
Inductor Selection
Select the Inductor for inductance L and saturation current Isat. Select an inductor with Isat higher
than the programmed over current limit. Calculate inductance from:
(𝑉𝑖𝑛 − 𝑉𝑜𝑢𝑡) × 𝑉𝑜𝑢𝑡 1 1
𝐿 =
𝑉𝑖𝑛
××
𝑓𝑠 𝐼𝑟𝑖𝑝
Where:
Vin is the converter input voltage
Vout is the converter output voltage
fs is the switching frequency
Irip is the inductor peak-to-peak current ripple (nominally set to 30% of Iout)
Keep in mind that a higher Irip results in a smaller inductance value which has the advantages of
smaller size, lower DC equivalent resistance (DCR), and allows the use of a lower output
capacitance to meet a given step load transient. A higher Irip, however, increases the output
voltage ripple, requires higher saturation current limit, and increases critical conduction. Notice
that this critical conduction current is half of Irip.
Capacitor Selection
• Output Capacitor Selection
Select the output capacitor for voltage rating, capacitance and Equivalent Series Resistance (ESR).
Nominally the voltage rating is selected to be at least twice as large as the output voltage. Select
the capacitance to satisfy the specification for output voltage overshoot/undershoot caused by the
current step load. A sudden decrease in the load current forces the energy surplus in the inductor
to be absorbed by Cout. This causes an overshoot in output voltage that is corrected by power
switch reduced duty cycle. Use the following equation to calculate Cout:
𝐶
=
𝐿
×
(𝐼₂
𝑉_{𝑂𝑆} ²
−
−
𝐼₁)²
𝑉_{𝑂𝑈𝑇} ²
Where:
L is the output inductance
I2 is the step load high current
© 2012 Exar Corporation
24/28
Rev. 1.2.2