Datasheet

1 1
83
6 1
2
150
m= » ´
´
+
S RT
RT
f I Hz / A
ns A
ns
I
UCC25600
RT
Maximum
frequency limiting
Minimum
frequency limiting
R1
R2
6
1
150
2
f max
max
ns
I
ns
f
=
-
6
1
150
2
f min
min
ns
I
ns
f
=
-
1 2
1 1
2 5
æ ö
= +
ç ÷
è ø
f max
I . V
R R
2
2 5
=
f min
. V
I
R
UCC25600
SLUS846B –SEPTEMBER 2008– REVISED JULY 2011 www.ti.com
Oscillator
With variable switching frequency control, UCC25600 relies on the internal oscillator to vary the switching
frequency. The oscillator is controlled by the current flowing out of RT pin. Except during soft start, the
relationship between the gate signal frequency and the current flowing out of RT pin can be represented as:
(10)
Since the switching frequency is proportional to the current, by limiting the maximum and minimum current
flowing out of RT pin, the minimum and maximum switching frequency of the converter could be easily limited.
As shown in Figure 18, putting a resistor from RT pin to ground limits the minimum current and putting a resistor
in series with the opto-coupler limits the maximum current.
Figure 18. Maximum and Minimum Frequency Setting for UCC25600
The frequency limiting resistor can be calculated based on following equations.
(11)
(12)
(13)
(14)
14 Copyright © 2008–2011, Texas Instruments Incorporated