Texas Instruments UCC3888N, UCC3888DTR, UCC3888D, UCC2888DTR, UCC2888D Datasheet
Off-line Power Supply Controller
•
Transformerless Off-line
Power Supply
•
Wide 100VDC to 400VDC
Allowable Input Range
•
Fixed 5VDC or Adjustable
Low Voltage Output
•
Output Sinks 200mA, Sources
150mA Into a MOSFET Gate
•
Uses Low Cost SMD Inductors
•
Short Circuit Protected
•
Optional Isolation Capability
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
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 topology also offers inherent short circuit protection since as the output voltage falls to
zero, the switch off time approaches infinity.
The output voltage is set internally to 5V. It can be programmed for other output
voltages with two external resis tors. An isolated version can be achieved with this
topology as described fu rther in Unitro de App lication N ote U-1 49.
UCC1888
UCC2888
UCC3888
3/97
FEATURES
DESCRIPTION
OPERATION
With reference to the application diagram below, when input voltage is firs t ap plied,
the current through R
ON
into TON is directed to VCC where it charges the external
capacitor, C3, connected to V
CC
. As voltage builds on VCC, 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 redirecting the current into T
ON
to the timing capacitor, CT. CT charges to a fixed threshold
with a current I
CHG
=0.8 • (VIN - 4.5V)/RON. 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.
Note: This device incorporates pate nt ed t ec hn ol og y us ed und er li cense from Lambda Electronics, Inc.
TYPICAL APPLICATION
UDG-96013
UCC1888
UCC2888
UCC3888
OPERATION (cont.)
At the end of the on time, Q1 is turned off and the current
through R
ON
is again diverted to VCC. Thus the current
through R
ON
, which charges CT during the on time, c on-
tributes to supplying power to the chip during the off time.
The power sw itch off time is controlle d by the dis charge
of C
T
which, in turn, is programmed by the regulated out-
put voltage. The relations hip between C
T
discharge cur-
rent, I
DCHG
, and output voltage is illustrated as follows:
Region 1. When V
OUT
= 0, the off time is infinite. This
feature provides inherent short circuit protection. However, to ensure output voltage
startup when the output is n ot a short, a high
value resistor, R
S
, is placed in parallel with C
T
to establish a minimum switching frequency.
Region 2. As V
OUT
rises above approximately 0.7V to its
regulated value, I
DCHG
is defined by R
OFF
,
and is equal to:
I
DCHG
= (V
OUT
- 0.7V) / R
OFF
As V
OUT
increases, I
DCHG
increases r e ducing off time.
The operating frequency increases and V
OUT
rises
quickly to its regulated value.
Region 3. In th i s region, a transconductance amplifi er re-
duces I
DCHG
in order to maintain a regulated
V
OUT
.
Region 4. If V
OUT
should rise abov e it s regu lat ion range,
I
DCHG
falls to zero and the circuit returns to
the minimum frequency established by R
S
and
C
T
.
The range of switching frequencies is established by
RON, R
OFF
, RS, and CT as follows:
Frequency = 1/(T
ON
+ T
OFF
)
T
ON
= R
ON
•
C
T
•
4.6
V/(VIN - 4.5V)
T
OFF
(max) = 1.4 • RS • CT
Regions 1 and 4
T
OFF
= R
OFF
•
C
T
• 3.7V /(V
OUT
- 0.7V)
Region 2, excluding the effects of R
S
which have a minimal impact on T
OFF
.
The above equa tions assume that V
CC
equals 9V. The
voltage at T
ON
increases from approximately 2.5V to
6.5V while C
T
is charging. To take this into account, V
IN
is adjusted by 4.5V in the calculation of TON. The voltage
at T
OFF
is approximately 0.7V.
DESIGN EXAMPLE
The UCC3888 regulate s a 5 volt , 1 Watt nonisolated DC output from AC inputs between 80 and 265 volts. In this example, the I C is prog rammed t o deliver a maximum on time gate drive puls e width of 2.2 microseconds which occurs
at 80 VAC. The corresponding switching frequency is approximately 100kHz at low line, and overall efficiency is approximately 50%. Additional design information is available in Unitrode Application Note U-149.
UDG-96014
2
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