Feb21-07 SP7661 Evaluation Manual ©2007 Sipex Corporation
Page 2 of 11
Using the Evaluation Board
1) Powering Up the SP7661EB Circuit
Connect the SP7661 Evaluation Board with an external +12V power supply. Connect with short
leads and large diameter wire directly to the “VIN” and “GND” posts. Connect a Load between
the VOUT and GND posts, again using short leads with large diameter wire to minimize
inductance and voltage drops.
2) Measuring Output Load Characteristics
VOUT ripple can best be seen by touching the probe tip to the pad for C5 and the scope GND
collar touching Output GND post – avoid a GND lead on the scope which will increase noise
pickup.
3) Using the Evaluation Board with Different Output Voltages
While the SP7661 Evaluation Board has been tested and delivered with the output set to 3.30V,
by simply changing one resistor, R2, the SP7661 can be set to other output voltages. The
relationship in the Equation 1 is based on a voltage divider from the output to the feedback pin
VFB, which is set to an internal reference voltage of 0.80V. Note, due to the common mode
voltage range of the current sense amplifier, output voltages greater than 3.3V are only possible
if the current sense is disabled. To disable current limit, remove R3 and R4. Standard 1%
metal film resistors of surface mount size 0603 are recommended.
1)
.80V
Vout
(
R1
R2
−
=
Equation 1
Where R1 = 10kΩ and for V
OUT = 0.80V setting, simply remove R2 from the board.
Furthermore, one could select the value of the R1 & R2 combination to meet the exact output
voltage setting by restricting R1 resistance range such that 10kΩ ≤ R1 ≤ 100kΩ for overall
system loop stability.
Note that since the SP7661 Evaluation Board design was optimized for 12V down conversion to
3.30V, changes of output voltage and/or input voltage will alter performance from the data given
in the Power Supply Data section. In addition, the SP7661ER provides short circuit protection by
sensing V
OUT at GND. The current limit of the converter is set to about 5.5A which is
accomplished by sensing the current through the inductor. To adjust the current limit, follow
Equations 2 and 3 to set the current limit accordingly. The current limit should be set to about
50% higher than the maximum output current that is desired. This will prevent the part from
accidentally triggering the current limit during large transient load steps.
Adjusting the current upwards is done by adjusting resistor R9.
60mV)-DCR(max
R4)R3(60mV
R9
⋅
=
I
Equation 2
Where DCR is the Inductor winding resistance
I
MAX is the desired output current
