UC3844, 45 UC2844, 45
Undervoltage Lockout
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
terminal (VCC and the reference output (Vref) are each
monitored by separate comparators. Each has built–in
hysteresis to prevent erratic output behavior as their
respective thresholds are crossed. The VCC comparator
upper and lower thresholds are 16 V/10 V for the UCX844,
and 8.4 V/7.6 V for the UCX845. The Vref comparator upper
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
36 V zener is connected as a shunt regulator from VCC to
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.
The SO–14 surface mount package provides separate
pins for VC (output supply) and Power Ground. Proper
implementation will significantly reduce the level of switching
transient noise imposed on the control circuitry. This
becomes particularly useful when reducing the Ipk(max) clamp
level. The separate VC supply input allows the designer
added flexibility in tailoring the drive voltage independent of
VCC. A zener clamp is typically connected to this input when
driving power MOSFETs in systems where VCC is greater the
20 V. Figure 22 shows proper power and control ground
connections in a current sensing power MOSFET
application.
Reference
The 5.0 V bandgap reference is trimmed to
±
1.0%
tolerance at TJ = 25°C on the UC284X, and
±
2.0% on the
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.
Design Considerations
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
bypass capacitors (0.1
µF)
connected directly to VCC, VC,
and Vref may be required depending upon circuit layout. This
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.
Figure 17. External Clock Synchronization
Vref
8(14)
RT
R
Bias
R
Osc
0.01
CT
47
4(7)
+
–
2(3)
1(1)
5(9)
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=
EA
+
5.0k
5
2
2R
R
C
RA
Figure 18. External Duty Cycle Clamp and
Multi–Unit Synchronization
8(14)
8
4
R
Bias
R
Osc
RB
6
External
Sync
Input
5.0k
+
–
+
–
R
Q
S
3
7
4(7)
+
–
2(3)
1(1)
EA
+
5.0k
MC1455
1
2R
R
1.44
(RA + 2RB)C
RB
Dmax =
RA + 2RB
To Additional
UCX84XA’s
5(9)
MOTOROLA ANALOG IC DEVICE DATA
9
FREESCALE [ FREESCALE SEMICONDUCTOR, INC ]
