2
USING THE EVALUATION BOARD
1) Powering Up the SP7650EB Circuit
Connect the SP7650 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 GND2 posts, again using short leads with large
diameter wire to minimize inductance and voltage drops.
2) Measuring Output Load Characteristics
It’s best to GND reference scope and digital meters using the Star GND post in the
center of the board. VOUT ripple can best be seen touching probe tip to the pad for C3
and scope GND collar touching Star 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 SP7650 Evaluation Board has been tested and delivered with the output set
to 3.30V, by simply changing one resistor, R2, the SP7650 can be set to other output
voltages. The relationship in the following formula 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.
Standard 1% metal film resistors of surface mount size 0603 are recommended.
Vout = 0.80V ( R1 / R2 + 1 ) => R2 = R1 / [ ( Vout / 0.80V ) – 1 ]
Where R1 = 68.1KΩand for Vout = 0.80V setting, simply remove R2 from the board.
Furthermore, one could select the value of R1 and R2 combination to meet the exact
output voltage setting by restricting R1 resistance range such that 50KΩ≤R1 ≤100KΩ
for overall system loop stability.
Note that since the SP7650 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
SP7650ER provides short circuit protection by sensing Vout at GND.
POWER SUPPLY DATA
The SP7650ER is designed with a very accurate 1.0% reference over line, load and
temperature. Figure 1 data shows a typical SP7650 Evaluation Board Efficiency plot,
with efficiencies to 90% and output currents to 3A. SP7650ER Load Regulation is
shown in Figure 2 of only 0.1% change in output voltage from no load to 3A load.
Figures 3 and 4 illustrate a 1.5A to 3A and 0A to 3A Load Step. Start-up Response in
Figures 5, 6 and 7 show a controlled start-up with different output load behavior when
power is applied where the input current rises smoothly as the Softstart ramp increases.
In Figure 8 the SP7650ER is configured for hiccup mode in response to an output dead
short circuit condition and will Softstart until the over-load is removed. Figure 9 and 10
show output voltage ripple less than 40mV at no load to 3A load.
While data on individual power supply boards may vary, the capability of the SP7650ER
of achieving high accuracy over a range of load conditions shown here is quite
impressive and desirable for accurate power supply design.
