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UC1584J Datasheet(Fiches technique) 4 Page - Texas Instruments

Numéro de pièce UC1584J
Description  Secondary Side Synchronous Post Regulator
Télécharger  8 Pages
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Fabricant  TI1 [Texas Instruments]
Site Internet  http://www.ti.com
Logo TI1 - Texas Instruments

UC1584J Datasheet(HTML) 4 Page - Texas Instruments

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UC1584
UC2584
UC3584
PIN DESCRIPTIONS
BST1: Collector of the boost switch. This is the
connection point of the external boost inductor and boost
diode. The boost converter generates the bias supply for
the UC3584 from the regulated 5V output.
BST2: See BST1. BST2 must be connected externally
to BST1 pin.
CDLY: Delay Set.
External CDLY capacitor sets the
delay from the time Short Circuit condition is detected
and Fault Condition is asserted.
COMP: Output of the Voltage Error Amplifier.
CT: Connect the Timing Capacitor between CT and GND.
FB: Inverting Input of the Voltage Error Amplifier.
GND: Analog System Ground.
OUT: Output of the floating driver for an external,
N-channel MOSFET.
PGND: Power Ground. This is the reference node for the
boost bias supply regulator. PGND and GND must be
connected externally.
RT: A Timing Resistor connected between RT and GND
sets the discharge current of the timing capacitor.
SRC: Source connection of the floating driver to the
external switch.
SS: Soft Start. An external capacitor is connected
between SS and GND to set the duration of the Soft
Start cycle.
SYNC:
Synchronization
Pin.
The
UC3584
is
synchronized from the falling edge of the transformer?s
secondary winding. Voltage must exceed 1V at minimum
input line.
VCC: Bias supply of the chip, approximately 15V. This is
also the output of the boost regulator. The VCC pin must
be decoupled to PGND.
VFLT: Positive rail of the floating driver?s bias supply.
Decouple to SRC using a high frequency (ceramic)
capacitor.
VREG: Output of the internal 5V regulated supply. Must
be decoupled to GND.
Biasing the UC3584
Bias supply for the UC3584 is generated from the main
output of the power supply by a boost regulator. The in-
ductor, diode and capacitor of the boost converter are ex-
ternal components, while the boost switch is internal to
the chip. The boost converter operates in a burst mode
with a built-in hysteresis of approximately 1V centered at
15V. This is a bang-bang controller and when enabled
has a fixed duty cycle of 75%.
Undervoltage Detection
The UVLO circuit of the UC3584 monitors the voltage on
VCC. During power up and power down, the pulse width
modulator and the output driver are disabled and OUT is
held active low. Operation is enabled when VCC reaches
10.5V. The UVLO circuitry has a built-in hysteresis of
1.7V (10.5V to 8.8V) thus VCC must drop below 8.8V in
order to assert UVLO again.
Precision Reference
An internal precision bandgap reference provides accu-
rate voltages to the error amplifier and other control sec-
tions of the IC. A buffered 5V regulated voltage is also
available for external circuitry on the VREG pin. This pin
must be decoupled to the signal GND connection by a
good quality high frequency capacitor.
Oscillator and Trailing Edge Synchronization
The UC3584 is outfitted with a synchronizable oscillator
which also generates a ramp signal across the CT capac-
itor for the PWM comparator. For easy implementation of
the leading edge pulse width modulation technique, the
oscillator has an inverted ramp waveform as shown in
Fig. 1. The free running oscillator frequency is deter-
mined by the timing components, RT and CT, according
to the following approximate equations:
R
D
T
MAX
=
?
?
?
?
?
?
?
93
1
17
.
.
()
()
f
C
RC
OSC
T
TT
=
?
?
?
28 2 10
8
09
.
.
where
RT is the timing resistor, its value should be between
1k
? and 100k?,
CT is the timing capacitor,
DMAX is the desired maximum duty cycle, and
fOSC is the free running oscillator frequency.
Figure 2 graphically depicts the measured frequency
data.
APPLICATION INFORMATION


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