Output Sinks 200mA, Sources
150mA Into a MOSFET Gate
•
Uses Low Cost SMD Inductors
•
Short Circuit Protected
•
Optional Isolation Capability
TYPICAL APPLICATION
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
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 infin ity.
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 rth er in Un itrode App licat ion N ote U-1 49.
OPERATION
With reference to the application diagram below, when input voltage is fir st ap plied,
the current through R
capacitor, C3, connected to V
tage lockout holds the circuit off and the output at DRIVE low until V
8.4V. At this time, DRIVE goes high turning on the power switch, Q1, and redirecting the current into T
with a current I
long as C
voltage. This provides a constant (line voltage) • (switch on time) product.
CHG
charges, the power switch on time will be inversely proportional to line
T
into TON is directed to VCC where it charges the external
ON
. As voltage builds on VCC, an internal undervol-
CC
reaches
CC
to the timing capacitor, CT. CT charges to a fixed threshold
ON
=0.8 • (VIN - 4.5V)/RON. Since DRIVE will only be high for as
Note: This devi ce inc orp ora tes patented tec hn ol og y us ed und er license from Lambda Electronics, Inc.
3/97
UDG-96013
OPERATION (cont.)
At the end of the on time, Q1 is turned off and the current
through R
through R
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
which, in turn, is programmed by the regulated out-
T
put voltag e. The relationship between C
rent, I
DCHG
is again diverted to VCC. Thus the current
ON
, which charges CT during the on time, c on-
ON
discharge cur-
T
, and output voltage is illustrated as follows:
UCC1888
UCC2888
UCC3888
I
As V
increases, I
OUT
DCHG
= (V
The operating frequency increases and V
quickly to its regulated value.
Region 3. In th i s r eg i on , a tr ans conductance ampl ifi er re-
duces I
V
Region 4. If V
I
DCHG
DCHG
.
OUT
should rise abov e it s regu lat ion range,
OUT
falls to zero and the circuit returns to
the minimum frequency established by R
C
.
T
The range of switching frequencies is established by
RON, R
, RS, and CT as follows:
OFF
- 0.7V) / R
OUT
increases r e duc ing off time.
DCHG
OFF
rises
OUT
in order to maintain a regulated
and
S
Region 1. When V
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
to establish a minimum switching frequency.
Region 2. As V
OUT
regulated value, I
and is equal to:
= 0, the off time is infinite. This
OUT
, is placed in parallel with C
S
rises above approxi mately 0.7V to its
is defined by R
DCHG
OFF
Frequency = 1/(T
•
T
= R
ON
T
(max) = 1.4 • RS • CT
OFF
ON
C
Regions 1 and 4
T
OFF
= R
OFF
•
Region 2, excluding the effects of R
which have a minimal impact on T
T
The above equations assume that V
voltage at T
6.5V while C
,
increases from approximately 2.5V to
ON
is charging. To take this into account, V
T
is adjusted by 4.5V in the calculation of TON. The voltage
at T
is approximately 0.7V.
OFF
ON
•
4.6
T
C
• 3.7V /(V
T
+ T
)
OFF
V/(VIN - 4.5V)
OUT
CC
- 0.7V)
S
.
OFF
equals 9V. The
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 dri ve pulse 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.
is in regulation. The two points used to calculate gm are for
OUT
3
PIN DESCRIPTIONS
ADJ:
The ADJ pin i s u sed to provide a 5V regulated supply without ad ditional ext ernal components. Other output
voltages can be obtained by connecting a resistor divi der
between V
where R1 is c onnected between V
is connected between ADJ and GND. R1 || R2 should be
less than 1kΩ to minimize the effect of the temperature
coefficient of the internal 30k resistors which also connect
to V
OUT
(timing capacitor):
C
T
peak-to-pe ak swi ng of 3.7V for 9V V
C
crosses the oscillator upper threshold, DRIVE goes
T
low. As the voltage on C
threshold, DRIVE goes high.
DRIVE:
200mA peak and sourcin g 150mA peak. The output voltage swing is 0 to V
GND (chip ground):
spect to GND.
, ADJ and GND. Use the formula
OUT
OUT
V
= 2.5V •
R1 + R2
R2
and ADJ, and R2
OUT
, ADJ, and GND. See Block Diagram.
The signal voltage at C
. As the voltage at
CC
crosses the oscillator lower
T
has a
T
This output is a CMOS stage capable of sinking
.
CC
All voltages are measured with re-
UCC1888
UCC2888
UCC3888
(regulated output control):
T
OFF
charge current of the timing capacitor through an external
resistor connected between V
(line voltage control ):
T
ON
When C
T
ON
is discharging (off time), the current through
T
is routed to V
current through T
CC.
is split 80% to set the CT charge
ON
T
When CT is chargin g (o n time), the
time and 20% to sense minimum l ine voltage which occurs for a T
age of 80V, R
The C
T
current of 220µA. For a minimum l i ne volt-
ON
is 330kΩ.
ON
voltage slightly affects the value of the charge
current during the on time. During this time, the voltage at
the T
V
device at pin V
pin increases from 2.5V to 6.5V.
ON
(chip supply voltage):
CC
is inter nall y c lampe d a t 9V. The device
CC
needs an external supply, from a source such as the rectified AC line or derived from the switching circuit. Precautions m ust be taken to ensure that total I
exceed 8mA.
(regulated output):
V
OUT
The V
nected to the power supply output voltage. When V
greater than V
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CERTAIN APPLICATIONS USING SEMICONDUCTOR PRODUCTS MAY INVOLVE POTENTIAL RISKS OF
DEATH, PERSONAL INJURY, OR SEVERE PROPERTY OR ENVIRONMENTAL DAMAGE (“CRITICAL
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BE FULLY AT THE CUSTOMER’S RISK.
In order to minimize risks associated with the customer’s applications, adequate design and operating
safeguards must be provided by the customer to minimize inherent or procedural hazards.
TI assumes no liability for applications assistance or customer product design. TI does not warrant or represent
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intellectual property right of TI covering or relating to any combination, machine, or process in which such
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Copyright 1999, Texas Instruments Incorporated
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