Solved by
SP7663
TM
Evaluation Board Manual
Easy Evaluation for the SP7663ER
0 to 22V Input, 0 to 6A Output
Synchronous Buck Converter
Built-in low R
DS
(
ON
) Power FETs
UVLO Detects Both VCC and VIN
High Integrated Design, Minimal
Components
High Efficiency: 90%
Feature Rich:
UVIN, Programmable Soft Start, Built-in
VCC Supply, Current
Limiting and
Output Short Circuit Protection
SP7663EB SCHEMATIC
SP7663 Evaluation Board Manual
©2006 Sipex Corporation
USING THE EVALUATION BOARD
1) Powering Up the SP7663EB Circuit
Connect the SP7663 Evaluation Board with an external +12V power supply. Connect
with short leads and large diameter wire directly to the “VIN” and “GND2” posts. Connect
a Load between the VOUT and GND1 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 all scope and digital meters using the Star GND post in the
center of the board. VOUT ripple can best be seen by touching the probe tip to the pad
for C3 and the scope to the 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 SP7663 Evaluation Board has been tested and delivered with the output set to
3.30V, by simply changing one resistor, R2, the SP7663 can be set to other output
voltages. The relationship in Equation 1 is based on a voltage divider from the output to
the feedback pin V
FB
, 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.
R2
=
R1
VOUT
(
−
1)
.80V
Equation1
Where R1 = 68.1KΩ and for V
OUT
= 0.80V setting, simply remove R2 from the board.
Furthermore, one could select the value of the 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 SP7663 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
SP7663ER provides short circuit protection by sensing V
OUT
at GND. The current limit of
the converter is set to about 9A 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
•
( R3
+
R4)
R9
=
Equation 2
I
max
•
( DCR - 60mV)
Where:
DCR is the Inductor winding resistance
I
MAX
is the desired output current
Mar12-07 RevD
SP7663 Evaluation Board Manual
Page 2 of 16
©2007 Sipex Corp.
Adjusting the current downwards is controlled by adjusting R8.
(Vout - 60mV)
+
(Imax
•
DCR)
R8
=
R
4
⋅
60mV - (Imax
•
DCR)
Equation 3
Where:
DCR is the Inductor winding resistance
I
MAX
is the desired output current
Further details on the current limit can be found in the SP7663 data sheet.
POWER SUPPLY DATA
The SP7663ER is designed with an accurate 2.0% reference over line, load and
temperature. Figure 1 data shows a typical SP7663 evaluation board efficiency plot, with
efficiencies up to 90% and output currents up to 6A. The output voltage ripple of less
than 50mV at full load and the LX node are shown in figure 2. Figures 3 and 4 illustrate a
3A-to-6A and 0A-to-6A Load Step. Short circuit and current limit are shown in Figures 5
and 6. Typical startup characteristics into a full load and no load are shown in figure 7
and 8. All data was taken at 12V
IN
.
While data on individual power supply boards may vary, the capability of the
SP7663ER of achieving high accuracy over a range of load conditions shown here is
quite impressive and desirable for accurate power supply design.
SP7663 Efficiency
12Vin - 3.3Vout - 6Amp
95.00
90.00
Efficiency .
85.00
80.00
75.00
70.00
0.00
1.00
2.00
3.00
4.00
5.00
6.00
Iout (Amps)
Figure 1. Efficiency vs. Load
Mar12-07 RevD
SP7663 Evaluation Board Manual
Page 3 of 16
©2007 Sipex Corp.
V
out
V
outripple
I
out
(2A/div)
Figure 2. LX node output ripple voltage
Figure 3. Load Step Response: 3->6A
V
out
V
out
SoftStart
I
out
(2A/div)
I
out
(5A/div)
Figure 4. Load Step Response 0->6A
Figure 5. Current Limit set point 9A
V
out
Vout
SoftStart
Soft Start
Iout (5A/div)
Vin
Iout (5A/div)
Figure 6. Output load Short Circuit
Figure 7. Startup into full load
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SP7663 Evaluation Board Manual
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©2007 Sipex Corp.
Iout (5A/div)
Figure 8. Startup into no load
TYPE III LOOP COMPENSATION DESIGN
The open loop gain of the SP7663EB can be divided into the gain of the error
amplifier
Gamp(s),
PWM modulator
Gpwm,
buck converter output stage
Gout(s),
and
feedback resistor divider
Gfbk.
In order to cross over at the selecting frequency
fco,
the
gain of the error amplifier must compensate for the attenuation caused by the rest of the
loop at this frequency. The goal of loop compensation is to manipulate the open loop
frequency response such that its gain crosses over 0dB at a slope of –20dB/dec. The
open loop crossover frequency should be higher than the ESR zero of the output
capacitors but less than 1/5 to 1/10 of the switching frequency
fs
to insure proper
operation. Since the SP7663EB is designed with Ceramic Type output capacitors, a
Type III compensation circuit is required to give a phase boost of 180° in order to
counteract the effects of the output
LC
underdamped resonance double pole frequency.
Mar12-07 RevD
SP7663 Evaluation Board Manual
Page 5 of 16
©2007 Sipex Corp.
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