Datasheet

7
UCC1581
UCC2581
UCC3581
A rather large soft start capacitor was chosen to give a
startup time of several hundred milliseconds, reducing
the input surge current while the output is coming up.
Note that for stability, the UCC3581 V
REF bypass capaci
-
tor needs to be at least 1µF. The V
DD supply also needs
some capacitance to hold it up between pulses at light
load and high line, where the frequency may drop to less
than 1kHz due to pulse skipping. Otherwise it may drop
low enough for the startup MOSFET to be biased on,
lowering efficiency.
If the sync input is used, it should not be left in a high im
-
pedance state where noise could cause false triggering.
If unused, it should be grounded.
The transformer was designed with a standard Magnetics
RM8 ferrite core using P material, gapped for an A
L of
1600mH/1000Turn
2
. The primary consists of 44 turns,
while the 5V secondary has 10 turns and the bootstrap
winding 18 turns. For simplicity, all the windings can be
#28 AWG. A two section bobbin was used to provide high
primary to secondary isolation. A much smaller design,
with reduced isolation, could have been done for this low
power level.
APPLICATION INFORMATION (cont.)
3.90
3.92
3.94
3.96
3.98
4.00
4.02
4.04
4.06
4.08
4.10
-75 -50 -25 0 25 50 75 100 125
TEMPERATURE [°C]
VREF [V]
Figure 5. Reference voltage vs. temperature.
TYPICAL CHARACTERISTIC CURVES
0
20
40
60
80
100
120
140
100 1000 10000
CT [pF]
FREQUENCY [kHz]
1M/10k
470k/47k
680k/12k
220k/10k
Figure 6. Frequency vs. CT vs. RT1 and RT2.
0
200
400
600
800
1000
1200
1400
1600
0 20 40 60 80 100
FREQUENCY [kHz]
I
DD
[uA]
1nF LOAD
NO
LOAD
Figure 8. I
DD
vs. frequency RT1 = 680k, RT2 = 12k.
30
40
50
60
70
80
90
100
0 20406080100
FREQUENCY [kHz]
DUTY CYCLE [%]
220K/10K
680K/12K
470K/47K
1M/10K
Figure 7. Duty cycle vs. frequency vs. RT1 / RT2.