UCC18500/1/2/3
UCC28500/1/2/3
UCC38500/1/2/3
PRELIMINARY
BiCMOS PFC/PWM Combination Controller
FEATURES
• Combines PFC and 2nd Stage Down
DESCRIPTION
The UCC18500 family provides all of the functions necessary for an ac-
tive power factor corrected preregulator and a second stage DC-to-DC
converter. The controller achieves near-unity power factor by shaping
the AC input line current waveform to correspond to the AC input line
voltage using average current mode control. The DC-to-DC converter
uses peak current mode control to perform the step down power con-
version.
Converter Function
• Controls Boost PWM to Near-unity Power
Factor
• Accurate Power Limiting
• Average Current Mode Control in PFC
Stage
The PFC stage is leading edge modulated while the second stage is
trailing edge synchronized to allow for minimum overlap between the
boost and PWM switches. This reduces ripple current in the bulk output
capacitor.
• Peak Current Mode Control in Second
Stage
• Programmable Oscillator
In order to operate with a three to one range of input line voltages, a
line feedforward (VFF) in used to keep input power constant with vary-
ing input voltage. Generation of VFF is done using IAC in conjunction
with an external single pole filter. This not only reduces external parts
count, but avoids the use of high voltage components offering a lower
cost solution. The multiplier then divides the line current by the square
• Leading Edge/Trailing Edge Modulation
for Reduced Output Ripple Using
SmartSync™
• Low Startup Supply Current
• Synchronized Second Stage Start-up,
with Programmable Soft-start
of VFF
.
(continued)
• Programmable Second Stage Shut-down
BLOCK DIAGRAM
ISENSE2
8
SS2
13
VCC
9
GND
6
VERR
7
7.5V
REFERENCE
SECOND STAGE
SOFT START
20 VREF
6.75V
UVLO2
1.5V
UVLO
16V/10
VCC
OVP/ENBL
4
ENABLE
–
+
I
LIMIT
10
GT2
R
R
S
1.5V
1.3V
8.0V
+
–
PFCOVP
Q
ZERO
VAOUT
1
3
PWM
CLK2
POWER
VOLTAGE
ERROR AMP
0.25V
–
+
VSENSE
–
+
X
VCC
MULT
CURRENT AMP
÷
–
+
–
X
Q
S
PWM
PWM
+
7.5V
12 GT1
2
VFF 19
OSC
LATCH
(V
)
FF
R
R
CLK1
CLK2
MIRROR
2:1
11 PWRGND
CLK1
CLK2
I
LIMIT
OSCILLATOR
IAC 18
14 PKLMT
–
+
MOUT 17
16
15
2
5
ISENSE1 CAOUT
RT
CT
UDG-98189
SLUS419 - AUGUST 1999