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UCC1888

Unitrode
Part Number UCC1888
Manufacturer Unitrode
Description Off-Line Power Supply Controller
Published Jun 9, 2005
Detailed Description UCC1888 UCC2888 UCC3888 Off-line Power Supply Controller FEATURES • • • • • • • Transformerless Off-line Power Supply W...
Datasheet PDF File UCC1888 PDF File

UCC1888
UCC1888

Overview
UCC1888 UCC2888 UCC3888 Off-line Power Supply Controller FEATURES • • • • • • • 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 discontinuous 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 proportional to the input line voltage and sets the switch off time inversely proportional 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 conversion from 400V to 2.
7V to be achieved with a switch duty cycle of 7.
6%.
This topolShort 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.
Output Sinks 200mA, Sources 150mA Into a MOSFET Gate OPERATION With reference to the application diagram below, when input voltage is first applied, the current through RON into TON is directed to VCC where it charges the external capacitor, C3, connected to VCC.
As voltage builds on VCC, an internal undervoltage lockout holds the circuit off and the output at DRIVE low until VCC reaches 8.
4V.
At this time, DRIVE goes high turning on the power switch...


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