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UC2845BL Fiches technique(PDF) 10 Page - ON Semiconductor

No de pièce UC2845BL
Description  HIGH PERFORMANCE CURRENT MODE CONTROLLERS
Download  20 Pages
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Fabricant  ONSEMI [ON Semiconductor]
Site Internet  http://www.onsemi.com
Logo ONSEMI - ON Semiconductor

UC2845BL Fiches technique(HTML) 10 Page - ON Semiconductor

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UC3844B, UC3845B, UC2844B, UC2845B
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10
Undervoltage Lockout
Two undervoltage lockout comparators have been
incorporated to guarantee 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 UCX844B,
and 8.4 V/7.6 V for the UCX845B. The Vref comparator
upper and lower thresholds are 3.6 V/3.4 V. The large
hysteresis and low startup current of the UCX844B makes
it ideally suited in off−line converter applications where
efficient
bootstrap
startup
techniques
are
required
(Figure 30). The UCX845B is intended for lower voltage
dc−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
UCX844B is 11 V and 8.2 V for the UCX845B.
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 an undervoltage lockout
is active. This characteristic eliminates the need for an
external pulldown resistor.
The SOIC−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 than 20 V. Figure 23 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 UC284XB, and ±2.0% on the
UC384XB. 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
pulse−width 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
mF) 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.
Bias
+
Osc
R
R
R
2R
EA
5(9)
1(1)
2(3)
4(7)
8(14)
RT
CT
Vref
Figure 18. External Clock Synchronization
Figure 19. External Duty Cycle Clamp and
Multi−Unit Synchronization
0.01
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.
External
Sync
Input
47
+
R
R
R
2R
Bias
Osc
EA
5(9)
1(1)
2(3)
4(7)
8(14)
To Additional
UCX84XBs
R
S
Q
8
4
6
5
2
1
C
3
7
RA
RB
5.0k
5.0k
5.0k
MC1455
f
+
1.44
(RA ) 2RB)C
D(max) +
RA
RA ) 2RB


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