UCC28C43-EP Datasheet, PDF (7/23 Pages) Texas Instruments – BiCMOS LOW-POWER CURRENT-MODE PWM CONTROLLERS

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UCC28C4x-EP
BiCMOS LOW-POWER CURRENT-MODE PWM CONTROLLERS
SGLS352B – DECEMBER 2006 – REVISED MAY 2007
APPLICATION INFORMATION
This device is a pin-for-pin replacement of the bipolar UC2842 family of controllers—the industry standard PWM
controller for single-ended converters. Familiarity with this controller family is assumed.
The UCC28C4x series is an enhanced replacement with pin-to-pin compatibility to the bipolar UC284x and
UC284xA families. The new series offers improved performance when compared to older bipolar devices and
other competitive BiCMOS devices with similar functionality. Note that these improvements discussed below
generally consist of tighter specification limits that are a subset of the older product ratings, maintaining drop-in
capability. In new designs these improvements can be utilized to reduce the component count or enhance circuit
performance when compared to the previously available devices.
Advantages
This device increases the total circuit efficiency whether operating off-line or in dc input circuits. In off-line
applications the low start-up current of this device reduces steady state power dissipation in the startup resistor,
and the low operating current maximizes efficiency while running. The low running current also provides an
efficiency boost in battery-operated supplies.
Low-Voltage Operation
Two members of the UCC28C4x family are intended for applications that require a lower start-up voltage than
the original family members. The UCC28C40 and UCC28C41 have a turn-on voltage of 7 V typical and exhibit
hysteresis of 0.4 V for a turn-off voltage of 6.6 V. This reduced start-up voltage enables use in systems with
lower voltages, such as 12 V battery systems that are nearly discharged.
High-Speed Operation
The BiCMOS design allows operation at high frequencies that were not feasible in the predecessor bipolar
devices. First, the output stage has been redesigned to drive the external power switch in approximately
one-half the time of the earlier devices. Second, the internal oscillator is more robust, with less variation as
frequency increases. In addition, the current sense to output delay has been reduced by a factor of three, to 45
ns typical. These features combine to provide a device capable of reliable high-frequency operation.
The UCC28C4x family oscillator is true to the curves of the original bipolar devices at lower frequencies, yet
extends the frequency programmability range to at least 1 MHz. This allows the device to offer pin-to-pin
capability where required, yet capable of extending the operational range to the higher frequencies typical of
latest applications. When the original UC2842 was released in 1984, most switching supplies operated between
20 kHz and 100 kHz. Today, the UCC28C4x can be used in designs cover a span roughly ten times higher than
those numbers.
Start/Run Current Improvements
The start-up current is only 60 µA typical, a significant reduction from the bipolar device's ratings of 300 µA
(UC284xA). For operation over the temperature range of –55°C to 125°C, the UCC28C4x devices offer a
maximum startup current of 100 µA, an improvement over competitive BiCMOS devices. This allows the
power-supply designer to further optimize the selection of the start-up resistor value to provide a more efficient
design. In applications where low component cost overrides maximum efficiency the low run current of 2.3 mA
typical may allow the control device to run directly through the single resistor to (+) rail, rather than needing a
bootstrap winding on the power transformer, along with a rectifier. The start/run resistor for this case must also
pass enough current to allow driving the primary switching MOSFET, which may be a few milliamps in small
devices.
±1% Initial Reference Voltage
The BiCMOS internal reference of 2.5 V has an enhanced design and utilizes production trim to allow initial
accuracy of ±1% at room temperature and ±2% over the full temperature range. This can be used to eliminate
an external reference in applications that do not require the extreme accuracy afforded by the additional device.
This is very useful for nonisolated dc-to-dc applications where the control device is referenced to the same
common as the output. It is also applicable in offline designs that regulate on the primary side of the isolation
boundary by looking at a primary bias winding, or perhaps from a winding on the output inductor of a
buck-derived circuit.
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