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Transformerless Off-line
Power Supply
Wide 100VDC to 400VDC
Allowable Input Range
Fixed 5VDC or Adjustable
Low Voltage Output
DESCRIPTION
The UCC3888 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-
ous mode, both driven from a single external power switch. The significant benefit
of this approach is the ability to achieve voltage conversion ratios as high as 400V
to 2.7V with no transformer and low internal losses.
The control algorithm utilized by the UCC3888 sets the switch on time inversely
Output Sinks 200mA, Sources proportional to the input line voltage and sets the switch off time inversely propor-
150mA Into a MOSFET Gate
tional to the output voltage. This action is automatically controlled by an internal
Uses Low Cost SMD Inductors feedback loop and reference. The cascaded configuration allows a voltage conver-
sion from 400V to 2.7V to be achieved with a switch duty cycle of 7.6%. This topol-
Short Circuit Protected
ogy 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 is set internally to 5V. It can be programmed for other output
voltages with two external resistors. 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 current through R
ON
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 undervol-
tage 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 redirect-
ing 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
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(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)
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(switch on time) product.
TYPICAL APPLICATION
Note: This device incorporates patented technology used under license from Lambda Electronics, Inc.
UDG-96013
3/97
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