鈥?/div>
Transformerless Off-line
Applications
DESCRIPTION
The UCC1889 controller is optimized for use as an off-line, low power, low voltage,
regulated bias supply. The unique circuit topology utilized in this device can be
visualized as two cascaded flyback converters, each operating in the discontinu-
Ideal Primary-side Bias Supply
ous mode, and both driven from a single external power switch. The significant
Efficient BiCMOS Design
benefit of this approach is the ability to achieve voltage conversion ratios of 400V
to 12V with no transformer and low internal losses.
Wide Input Range
The control algorithm utilized by the UCC1889 is to force the switch on time to be
Fixed or Adjustable
inversely proportional to the input line voltage while the switch off time is made in-
Low Voltage Output
versely proportional to the output voltage. This action is automatically controlled by
Uses Low Cost SMD Inductors an internal feedback loop and reference. The cascaded configuration allows a volt-
age conversion from 400V to 12V to be achieved with a switch duty cycle greater
Short Circuit Protected
than 10%. This topology also offers inherent short circuit protection since as the
output voltage falls to zero, the switch off time approaches infinity.
Optional Isolation Capability
The output voltage can be easily set to 12V or 18V. Moreover, it can be pro-
grammed for other output voltages less than 18V with a few additional compo-
nents. An isolated version can be achieved with this topology as described further
in Unitrode Application Note U-149.
OPERATION
With reference to the application diagram below, when input voltage is first applied,
the R
ON
current into T
ON
is directed to V
CC
where it charges the external capacitor,
C3, connected to V
CC
. As voltage builds on V
CC
, an internal undervoltage lockout
holds the circuit off and the output at DRIVE low until V
CC
reaches 8.4V. At this
time, DRIVE goes high turning on the power switch, Q1, and redirecting the current
into T
ON
to the timing capacitor, C
T
. C
T
charges to a fixed threshold with a current
I
CHG
=0.8
鈥?/div>
(V
IN
- 4.5V)/R
ON
. Since DRIVE will only be high for as long as C
T
charges, the power switch on time will be inversely proportional to line voltage.
This provides a constant line voltage-switch on time product.
TYPICAL APPLICATION
Note: This device incorporates patented technology used under license from Lambda Electronics, Inc.
UDG-93060-1
2/95
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