UC3844, 45 UC2844, 45
Undervoltage Lockout
added flexibility in tailoring the drive voltage independent of
A zener clamp is typically connected to this input when
V
Two undervoltage lockout comparators have been
incorporated to guartantee that the IC is fully functional before
the output stage is enabled. The positive power supply
CC.
driving power MOSFETs in systems where V
is greater the
CC
20 V. Figure 22 shows proper power and control ground
connections in a current sensing power MOSFET
application.
terminal (V
and the reference output (V ) are each
CC
ref
monitored by separate comparators. Each has built–in
hysteresis to prevent erratic output behavior as their
Reference
respective thresholds are crossed. The V
comparator
CC
upper and lower thresholds are 16 V/10 V for the UCX844,
and 8.4 V/7.6 V for the UCX845. The V comparator upper
The 5.0 V bandgap reference is trimmed to ± 1.0%
tolerance at T = 25°C on the UC284X, and ± 2.0% on the
J
ref
UC384X. Its primary purpose is to supply charging current to
the oscillator timing capacitor. The reference has short circuit
protection and is capable of providing in excess of 20 mA for
powering additional control system circuitry.
and lower thresholds are 3.6 V/3/4 V. The large hysteresis
and low startup current of the UCX844 makes it ideally suited
in off–line converter applications where efficient bootstrap
startup techniques later required (Figure 29). The UCX845 is
intended for lower voltage dc–to–dc converter applications. A
Design Considerations
36 V zener is connected as a shunt regulator from V
to
CC
Do not attempt to construct the converter on
wire–wrap or plug–in prototype boards. High frequency
circuit layout techniques are imperative to prevent pulsewidth
jitter. This is usually caused by excessive noise pick–up
imposed on the Current Sense or Voltage Feedback inputs.
Noise immunity can be improved by lowering circuit
impedances at these points. The printed circuit layout should
contain a ground plane with low–current signal and
high–current switch and output grounds returning on
separate paths back to the input filter capacitor. Ceramic
ground. Its purpose is to protect the IC from excessive
voltage that can occur during system startup. The minimum
operating voltage for the UCX844 is 11 V and 8.2 V for the
UCX845.
Output
These devices contain a single totem pole output stage
that was specifically designed for direct drive of power
MOSFETs. It is capable of up to ± 1.0 A peak drive current
and has a typical rise and fall time of 50 ns with a 1.0 nF load.
Additional internal circuitry has been added to keep the
Output in a sinking mode whenever and undervoltage lockout
is active. This characteristic eliminates the need for an
external pull–down resistor.
bypass capacitors (0.1 µF) connected directly to V , V ,
CC
C
and V may be required depending upon circuit layout. This
ref
provides a low impedance path for filtering the high frequency
noise. All high current loops should be kept as short as
possible using heavy copper runs to minimize radiated EMI.
The Error Amp compensation circuitry and the converter
output voltage divider should be located close to the IC and
as far as possible from the power switch and other noise
generating components.
The SO–14 surface mount package provides separate
pins for V (output supply) and Power Ground. Proper
C
implementation will significantly reduce the level of switching
transient noise imposed on the control circuitry. This
becomesparticularlyusefulwhenreducingtheI
clamp
pk(max)
level. The separate V supply input allows the designer
C
Figure 18. External Duty Cycle Clamp and
Multi–Unit Synchronization
Figure 17. External Clock Synchronization
V
ref
8(14)
R
R
8(14)
R
R
Bias
R
R
A
R
T
Bias
4
8
B
5.0k
6
Osc
+
External
Sync
Input
Osc
+
4(7)
R
S
–
C
T
3
0.01
+
4(7)
5
2
Q
+
–
+
–
+
–
2R
R
7
EA
47
2(3)
1(1)
2R
R
5.0k
1
EA
2(3)
1(1)
C
MC1455
5(9)
5(9)
To Additional
UCX84XA’s
R
B
1.44
(R + 2R )C
The diode clamp is required if the Sync amplitude is large enough to
cause the bottom side of CT to go more than 300 mV below ground.
f =
D
=
max
R
+ 2R
B
A
B
A
9
MOTOROLA ANALOG IC DEVICE DATA
MOTOROLA [ MOTOROLA ]
