SBVS028B − DECEMBER 2000 − REVISED DECEMBER 2007
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5
FUNCTIONAL DESCRIPTION
OVERVIEW
The DCR02 series offers isolation from an unregulated
power supply operating from a choice of input voltages.
This series provides a variety of regulated output voltages
at a nominal output power of 2 W.
POWER STAGE
The input supply is chopped at a frequency of 400 kHz
(internal oscillator divided by 2), which is used to drive the
center-tapped toroidal transformer.
RECTIFICATION
The transformer output is full wave rectified and smoothed
by the external capacitor connected to VREC.
REGULATOR
The internal low-dropout regulator provides a
well-regulated output voltage throughout the operating
range of the device.
OSCILLATOR AND WATCHDOG
The DCR02 uses an internal saw-tooth generator to
provide the 800−kHz onboard oscillator that is used to
drive the power switching circuit. The operation of the
oscillator is monitored by the watchdog, which three-states
the output driver circuit if the oscillator fails or if the SYNC
pin is taken low (shutdown mode). When the SYNC pin is
returned high, normal operation resumes.
SYNCHRONIZATION
If more than one DCR02 is being used, beat frequencies
and other electrical interference can be generated. This
interference is due to the small variations in switching
frequencies between the converters. The DCR02
overcomes this by allowing devices to be synchronized to
one another. Up to eight devices can be synchronized by
connecting the SYNC pins together, with care being taken
to minimize the capacitance of tracking.
Significant stray capacitance on the SYNC pin reduces the
frequency of the internal oscillator. If this reduction is large,
the DCR02 may be taken out with its optimized operating
parameters, and saturation of the magnetics may result,
damaging the device.
If devices are synchronized, it should be noted that all
devices draws maximum current simultaneously at start
up. This can cause the input voltage to dip. Should it fall
below the minimum input voltage, the devices may not
start up. A low equivalent series resistance (ESR) 2.2−µF
ceramic capacitor should be connected as close to the
device input pins as possible.
If more than eight devices are required to be synchronized,
it is recommended that external synchronization be used.
Details of this procedure are contained in application
report SBAA035, External Synchronization of the
DCP01/02 Series of DC/DC Converters, available for
download at www.ti.com.
CONSTRUCTION
The DCR02 is manufactured using the same technology
as standard IC packages. There is no substrate within the
package. The DCR02 is constructed using a driver IC,
low-dropout voltage regulator, rectifier diodes, and a
wound magnetic toroid, all mounted on a leadframe. The
DCR02 requires no special printed circuit board (PCB)
assembly processing, since there is no solder within the
package. The result is an isolated DC/DC converter with
inherently high reliability.