Datasheet

10
UCC2750
UCC3750
reset (and to transfer power to the output). Operation in
quadrants 3 and 4 is symmetrical to the first two quad
-
rants with Q2 and Q3 interchanged. Note that the output
signal for Q2 is logically inverted to allow for driving the
p-channel switch. An n-channel switch can also be used
for Q2, but the drive circuit must be transformer isolated
and the polarity inverted. The outputs are designed for
high peak current drive and low internal impedance. In
isolated systems, GD1 must be coupled to Q1 using a
gate-drive transformer.
DC Current Limit
The DC current limit function provides protection against
short circuit conditions by limiting the maximum current
level and shutting off the PWM function when the limit
point is reached.
The DC limit is activated when DC out is below 0.5 V
CM
or above 1.5 • V
CM
. The DC current limit can be pro
-
grammed by setting:
R5
R6
=3
.
With this ratio, a symmetric DC limit with thresholds of
±0.5V is obtained. For other ratios, the positive and neg
-
ative voltage thresholds for current sense signal are
given by:
V (POS) =
V
4
•1-
R6
R5
SENSE
CM






V(NEG)=
V
4
•1-
5R6
R5
SENSE
CM






Even though the DC current is typically sensed in the
secondary, the currentl limit is applied fo the active PWM
switch at the time. For example, if Q1 is the PWM switch
and DCLIM is activated, the UCC3750 will prevent
trun-on for Q1 during the negative slope of the ramp (Fig.
2). The DC limit is functional on a cycle-by-cycle basis.
Charge Pump and Reference
The UCC3750 is designed to work on the secondary side
of an isolated power supply. It requires a 5V power sup
-
ply with respect to its GND pin to operate. Note that the
GND pin of the IC is also the reference point of the ring
signal that is generated by the converter. If the converter
output is connected in series with any other voltage, it
should be ensured that the available supply voltage is
referenced to the converter output return. The IC along
with its associated charge pump components shown in
Fig. 9 generates all the other voltages the system re-
quires. The UCC3750 typically requires about 5mA to op-
erate without any loads on the drive outputs. The charge
pump capacitor should be large enough to keep the VCP
fairly constant when driving Q1-Q3 in the converter.
12
11
+
–
V
CM
TO
PWM
NEGDC
OUTDC
DC
R5
R6
R
SENSE
R6
4
Figure 8. Current limiting.
APPLICATION INFORMATION (cont.)
Figure 9. Charge pump circuits.
UDG-96170-1
-20
0
20
40
60
80
100
120
1 10 100 1000 10000 100000 100000
0
1E+07
FREQUENCY (Hz)
MAGNITUDE (dB)
0
20
40
60
80
100
120
140
160
180
MA G .
PHASE
10k
100k 1M
10M
PHASE (Degree)
1000
10010
1
Figure 10. Frequency response to error amplifiers.
Revision Date Comment
C 08/2005 Changed amplifier input bias
current maximum limit from 600
nA to 100 nA and Typical from
500 nA to 15 nA.
Table II. Revision History