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 = 10k

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 10k

Ω ≤ 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

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