5
UC1886
UC2886
UC3886
OSCILLATOR
The UC3886 oscillator is a saw tooth. The rising edge
is governed by a current controlled by RT flowing into
the capacitor CT. The falling edge of the sawtooth sets
the dead time for the output. Selection of RT should be
done first, based on desired maximum duty cycle. CT
can then be chosen based on the desired frequency,
FS, and the value of RT. The design equations are:
D
V
RT
mA
MAX =
1
20
40
.
()
F
VmA RT
V
CT
V RT
mA
OSC =
20
40
20
18
4 0
..
.
..
Configuring the Current Sense Amplifier
The UC3886 Current Sense Amplifier is used to amplify
a differential current sense signal across a low value
current sense resistor, RSENSE. This amplifier must be
set up as a differential amplifier as shown.
The Current Sense Amplifier gain, GCSA, is given by the
ratio of R2/R1. The output of the Current Sense Ampli-
fier at the ISO pin is given by
VV
V
R
ISO
BUF
SENSE
=+
2
1
The Current Sense Amplifier gain, GCSA, must be pro-
grammed to be greater than or equal to 5.0 (14dB), as
this amplifier is not stable with gain below 5.0. The Cur-
rent Sense Amplifier gain is limited on the high side by
its Gain-Bandwidth product of 2.5MHz. Therefore GCSA
must be programmed between
GCSA_MIN = 5.0
and
GCSA_MAX = 2.5MHz/FSWITCH
APPLICATION INFORMATION
Figure 1. Oscillator
UDG-96022
0.88
0.90
0.92
0.94
0.96
0.98
1.00
0
20
40
60
80
100
120
R6 (k)
DM
AX
Figure 2. Programming Maximum Duty Cycle with RT
10
100
1000
0
20
40
60
80
100
RT (k)
FSWITCH
(kHz)
100pF
220pF
470pF
1nF
Figure 3. Programming Switching Frequency with CT
0.000
0.100
0.200
0.300
0.400
0.500
0.600
100
300
500
700
900
1100
CT (pF)
TD
(u
s)
RT = 5k
RT = 100k
Figure 4. Deadtime vs. CT and RT