UCC1580-1,-2,-3,-4
UCC2580-1,-2,-3,-4
UCC3580-1,-2,-3,-4
APPLICATION INFORMATION (cont.)
The soft start pin provides an effective means to start
the IC in a controlled manner. An internal current of
20 A begins charging a capacitor connected to SS once
the startup conditions listed above have been met. The
voltage on SS effectively controls maximum duty cycle
on OUT1 during the charging period. OUT2 is also con-
trolled during this period (see Figure 1). Negation of any
of the startup conditions causes SS to be immediately
discharged. Internal circuitry ensures full discharge of
SS (to 0.3V) before allowing charging to begin again,
provided all the startup conditions are again met.
Delay Times
1400
1.80
1.70
1.60
1.50
1.40
1.30
1.20
1.10
Delay Ratio
1200
1000
Delay2
800
600
Delay1
400
200
0
Oscillator
Simplified oscillator block diagram and waveforms are
shown in Figure 3. OSC1 and OSC2 pins are used to
program the frequency and maximum duty cycle. Capac-
itor CT is alternately charged through R1 and discharged
through R2 between levels of 1.67 V and 3.3 V. The
charging and discharging equations for CT are given by
t
0
100
200
300
400
500
600
700
800
900
1000
W
k
R3 ProgrammingResistor
Figure 2. Delay times.
æ
è
ö
ø
t
1
ç
÷
VC(charge) = V
• 1-
• e -
2
REF
3
t
t
2
2
VC(dis cha rge ) =
· VREF ·e -
3
where t1 = R1 • CT and t2 = R2 • CT. The charge time
and discharge time are given by
tCH = 0.69 • R1 • CT and tDIS = 0.69 • R2 • CT
The CLK output is high during the discharge period. It
blanks the output to limit the maximum duty cycle of
OUT1. The frequency and maximum duty cycle are
given by
1.44
Frequency =
(R1+ R2)• CT+ 27 pF
(
)
R1
R1+ R2
Maximum Duty Cycle =
Maximum Duty Cycle for OUT1 will be slightly less due
to Delay1 which is programmed by R3.
Voltage Feedforward and Volt-Second Clamp
UCC3580 has a provision for input voltage feedforward.
As shown in Figure 3, the ramp slope is made propor-
tional to input line voltage by converting it into a charging
current for CR. This provides a first order cancellation of
the effects of line voltage changes on converter perfor-
mance. The maximum volt-second clamp is provided to
protect against transient saturation of the transformer
core. It terminates the OUT1 pulse when the RAMP volt-
age exceeds 3.3V. If the feedforward feature is not used,
the ramp can be generated by tying R4 to REF. How-
ever, the linearity of ramp suffers and in this case the
maximum volt-second clamp is no longer available.
UDG-96016-1
Figure 3. Oscillator and ramp circuits.
6
TI [ TEXAS INSTRUMENTS ]
