UC1827-1/-2
UC2827-1/-2
UC3827-1/-2
Buck Current/Voltage Fed Push-Pull PWM Controllers
FEATURES
DESCRIPTION
• Ideal for Multiple Output and/or High
Voltage Output Voltage Converters
The UC3827 family of controller ICs provides an integrated control solution
for cascaded buck and push-pull converters. These converters are known
as current fed or voltage fed push-pull converters and are ideally suited for
multiple output and/or high voltage output applications. In both current fed
and voltage fed modes, the push-pull switches are driven at 50% nominal
duty cycles and at one half the switching frequency of the buck stage. In
the current fed mode, the two switches are driven with a guaranteed over-
lap period to prevent ringing and voltage stress on the devices. In the volt-
age fed mode, the two switches are driven with a guaranteed gap time
between the switches to prevent shorting the transformer across the en-
ergy storage capacitor and to prohibit excessive currents flowing through
the devices.
• Up to 500kHz Operation
• High Voltage, High Current Floating
Driver for Buck Converter Stage
• UC3827-1 Current Fed Controller has
Push-Pull Drivers with Overlapping
Conduction Periods
• UC3827-2 Voltage Fed Controller has
Push-Pull Drivers with
Non-overlapping Conduction Periods
The converter’s output voltage is regulated by pulse width modulation of
the buck switch. The UC3827 contains complete protection and PWM con-
trol functions for the buck converter. Easy control of the floating switch is
accomplished by the floating drive circuitry. The gate drive waveform is
level shifted to support an input voltage up to 72Vdc.
• Average Current Mode, Peak Current
Mode or Voltage Mode with Input
Voltage Feedforward Control for Buck
Power Stage
• Wide Bandwidth, Low Offset,
Differential Current Sense Amplifier
(continued)
• Precise Short Circuit Current Control
BLOCK DIAGRAM
VEAO
10
CEA+
12
CEAO
6
RAMP
5
VOLTAGE ERROR
AMPLIFIER
VEA+ 14
VEA– 16
CEA– 13
CURRENT
ERROR
AMPLIFIER
0.7V
PWM
R
S
COMPARATOR
CSAO
CSA+
CSA–
SS
7
8
9
4
D
FLYING
DRIVER
1
2
3
V+
+3V
Q
ILIM
COMPARATOR
BUCK
SRC
CURRENT
SENSE AMPLIFIER
SS
OSC
INHBT
UV
SYNC 19
CT 18
21 PGND
OSC
Q
T
DELAY
500kHz
MAX
PUSH-PULL
DRIVERS
RT 17
Q
REF 15
VCC 23
24 PUSH
22 PULL
REF
&
UVLO
UVLO
DELAY
GND 11
20
UDG-97172
DELAY
04/99
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