PRODUCT SPECIFICATION
FAN53168
REV. 1.0.0 6/9/03
23
Note that the capacitor manufacturer’s ripple current ratings
are often based on only 2000 hours of life. This makes it
advisable to further derate the capacitor, or to choose a
capacitor rated at a higher temperature than required. Several
capacitors may be placed in parallel to meet size or height
requirements in the design. In this example, the input capaci-
tor bank is formed by three 2200
μ
F, 16V Nichicon capaci-
tors with a ripple current rating of 3.5 A each.
To reduce the input-current di/dt to below the recommended
maximum of 0.1A/μs, an additional small inductor (L > 1μH
@ 15A) should be inserted between the converter and the
supply bus. That inductor also acts as a
fi
lter between the
converter and the primary power source.
TUNING PROCEDURE FOR THE FAN53168
1.
Build circuit based on compensation values computed
from design spreadsheet.
2.
Hook up dc load to circuit, turn on and verify opera-
tion.Also check for jitter at no-load and full-load.
DC Loadline Setting
3.
Measure output voltage at no-load (V
NL
). Verify it is
within tolerance.
Figure 7. Efficiency vs. Output Current (Circuit of Figure 1)
4.
Measure output voltage at full-load cold (V
FLCOLD
).
Let board soak for ~10 minutes at full-load and measure
output (V
FLHOT
). If there is a change of more than a
couple of millivolts, adjust R
CS1
and R
CS2
using
Equations 35 and 37.
5.
Repeat Step 4 until cold and hot voltage measurements
remain the same.
6.
Measure output voltage from no-load to full-load using
5A steps. Compute the loadline slope for each change
and then average to get overall loadline slope (R
OMEAS
).
7.
If R
OMEAS
is off from R
O
by more than 0.05 mW, use
the following to adjust the R
PH
values:
8.
Repeat Steps 6 and 7 to check loadline and repeat
adjustments if necessary.
9.
Once complete with dc loadline adjustment, do not
change R
PH
, R
CS1
, R
CS2
, or R
TH
for rest of procedure.
10. Measure output ripple at no-load and full-load with
scope and make sure it is within spec.
AC Loadline Setting
11. Remove dc load from circuit and hook up dynamic load.
12.
Hook up scope to output voltage and set to dc coupling
with time scale at 100
μ
s/div.
13. Set dynamic load for a transient step of about 40A at
1kHz with 50% duty cycle.
14. Measure output waveform (may have to use dc offset on
scope to see waveform). Try to use vertical scale of 100
mV/div or
fi
ner.
15. You will see a waveform that looks something like
Figure 8. Use the horizontal cursors to measure V
ACDRP
and V
DCDRP
as shown. DO NOT MEASURE THE
UNDERSHOOT OR OVERSHOOT THAT HAPPENS
IMMEDIATELY AFTER THE STEP.
I
CRMS
D
I
O
×
n
D
-------------
1
–
×
=
(34)
I
CRMS
0.125
65A
×
------------------------
1
–
×
10.5A
=
=
R
CS2 NEW
)
R
CS2 OLD
)
V
NL
V
FLHOT
–
(
----------------–
)
)
×
=
(35)
100
0
10
20
30
OUTPUT CURRENT
–
A
E
–
40
50
60
80
60
40
20
0
R
PH NEW
)
R
PH OLD
)
R
O
----------------------
×
=
(36)
CS2 OLD
CS2 NEW
CS1 OLD
)
TH 25
°
C
(
)
)
)
–
)
R
CS1 OLD
)
R
TH 25
°
C
(
)
–
(
)
×
-----------------------------------------------------------+
TH 25
°
C
(
)
------------------------
–
------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
R
CS NEW
(
)
=
(37)