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MC33067 View Datasheet(PDF) - Motorola => Freescale

Part Name
Description
Manufacturer
MC33067 Datasheet PDF : 16 Pages
1 2 3 4 5 6 7 8 9 10 Next Last
VCC
15
Enable /
UVLO Adjust
9
MC34067 MC33067
Figure 17. Undervoltage Lockout and Reference
50k
7.0k
50k
7.0k
8.0V VCC UVLO
5.1V
Reference
Vref
Vref UVLO
Vref
5
UVLO
4.2/4.0V
PERIPHERAL SUPPORT FUNCTIONS
The MC34067 Resonant Controller provides a number of
support and protection functions including a precision voltage
reference, undervoltage lockout comparators, soft–start
circuitry, and a fault detector. These peripheral circuits
ensure that the power supply can be turned on and off in a
controlled manner and that the system will be quickly
disabled when a fault condition occurs.
Undervoltage Lockout and Voltage Reference
Separate undervoltage lockout comparators sense the
input VCC voltage and the regulated reference voltage as
illustrated in Figure 17. When VCC increases to the upper
threshold voltage, the VCC UVLO comparator enables the
Reference Regulator. After the Vref output of the Reference
Regulator rises to 4.2 V, the Vref UVLO comparator switches
the UVLO signal to a logic zero state enabling the primary
control path. Reducing VCC to the lower threshold voltage
causes the VCC UVLO comparator to disable the Reference
Regulator. The Vref UVLO comparator then switches the
UVLO output to a logic one state disabling the controller.
The Enable/UVLO Adjust pin allows the power supply
designer to select the VCC UVLO threshold voltages. When
this pin is open, the comparator switches the controller on at
16 V and off at 9.0 V. If this pin is connected to the VCC
terminal, the upper and lower thresholds are reduced to
9.0 V and 8.6 V, respectively. Forcing the Enable/UVLO
Adjust pin low will pull the VCC UVLO comparator input low
(through an internal diode) turning off the controller.
The Reference Regulator provides a precise 5.1 V
reference to internal circuitry and can deliver up to 10 mA to
external loads. The reference is trimmed to better than 2%
initial accuracy and includes active short circuit protection.
Fault Detector
The high speed Fault Comparator and Latch illustrated in
Figure 18 can protect a power supply from destruction under
fault conditions. The Fault Input pin connects to the input of
the Fault Comparator. If this input exceeds the 1.0 V
threshold of the comparator, the Fault Latch is set and two
logic signals simultaneously disable the primary control path.
The signal labeled “Fault” at the output of the Fault
Comparator is connected directly to the output drivers. This
direct path reduces the propagation delay from the Fault
Input to the A and B outputs to typically 70 ns. The Fault
Latch output is OR’d with the UVLO output from the Vref
UVLO comparator to produce the logic output labeled
“UVLO+Fault”. This signal disables the Oscillator and
One–Shot by forcing both the COSC and CT capacitors to be
continually charged.
Figure 18. Fault Detector and Soft–Start
UVLO
Fault
UVLO + Fault
9.0µA
Ea Clamp
Fault Fault
R Comparator Input
Q
S
10
Fault
Latch 1.0V
Soft–Start
CSoft–Start Buffer
11
6 Ground
The Fault Latch is reset during startup by a logic “1” at the
UVLO output of the Vref UVLO comparator. The latch can
also be reset after startup by pulling the Enable/UVLO Adjust
pin momentarily low to disable the Reference Regulator.
Soft–Start Circuit
The Soft–Start circuit shown in Figure 18 forces the
variable frequency Oscillator to start at the maximum
frequency and ramp downward until regulated by the
feedback control loop. The external capacitor at the
CSoft–Start terminal is initially discharged by the UVLO+Fault
signal. The low voltage on the capacitor passes through the
Soft–Start Buffer to hold the Error Amplifier output low. After
UVLO+Fault switches to a logic zero, the soft–start
capacitor is charged by a 9.0 µA current source. The buffer
allows the Error Amplifier output to follow the soft–start
capacitor until it is regulated by the Error Amplifier inputs. The
soft–start function is generally applicable to controllers
operating below resonance and can be disabled by simply
opening the CSoft–Start terminal.
MOTOROLA ANALOG IC DEVICE DATA
9
 

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