UCC28C40, UCC28C41, UCC28C42, UCC28C43, UCC28C44, UCC28C45

UCC38C40, UCC38C41, UCC38C42, UCC38C43, UCC38C44, UCC38C45

SLUS458E -- AUGUST 2001 -- REVISED OCTOBER 2010

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APPLICATION INFORMATION

This device is a pin-for-pin replacement of the bipolar UC3842 family of controllers, the industry standard PWM

controller for single-ended converters. Familiarity with this controller family is assumed.

The UCC28C4x/UCC38C4x series is an enhanced replacement with pin-to-pin compatibility to the bipolar

UC284x/UC384x and UC284xA/UC384xA 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 UCC38C4x family are intended for applications that require a lower start-up voltage than

the original family members. The UCC38C40 and UCC38C41 have a turn-on voltage of 7.0 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 which 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 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 45ns typical.

These features combine to provide a device capable of reliable high frequency operation.

The UCC38C4x family oscillator is true to the curves of the original bipolar devices at lower frequencies yet

extends the frequency programmability range to at least 1MHz. 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 UC3842 was released in 1984 most switching supplies operated between

20kHz and 100kHz. Today, the UCC38C4x 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 300uA

(UC384xA). For operation over the temperature range of --40 to 85°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 startup 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.