MC33368D データシートの表示（PDF） - Motorola => Freescale
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MC33368D Datasheet PDF : 16 Pages
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MC33368
Multiplier
A single quadrant, two input multiplier is the critical
element that enables this device to control power factor. The
ac haversines are monitored at Pin 5 with respect to ground
while the Error Amplifier output at Pin 4 is monitored with
respect to the Voltage Feedback Input threshold. A graph of
the Multiplier transfer curve is shown in Figure 1. Note that
both inputs are extremely linear over a wide dynamic range,
0 to 3.2 V for Pin 5 and 2.5 to 4.0 V for Pin 4. The Multiplier
output controls the Current Sense Comparator threshold as
the ac voltage traverses sinusoidally from zero to peak line.
This has the effect of forcing the MOSFET on–time to track
the input line voltage, thus making the preconverter load
[ ǒ Ǔ appear to be resistive.
Pin 6 Threshold 0.55 VPin 4 – VPin 3 VPin 5
Zero Current Detector
The MC33368 operates as a critical conduction current
mode controller, whereby output switch conduction is
initiated by the Zero Current Detector and terminated when
the peak inductor current reaches the threshold level
established by the Multiplier output. The Zero Current
Detector initiates the next on–time by setting the RS Latch at
the instant the inductor current reaches zero. This critical
conduction mode of operation has two significant benefits.
First, since the MOSFET cannot turn–on until the inductor
current reaches zero, the output rectifier’s reverse recovery
time becomes less critical allowing the use of an inexpensive
rectifier. Second, since there are no deadtime gaps between
cycles, the ac line current is continuous thus limiting the peak
switch to twice the average input current
The Zero Current Detector indirectly senses the inductor
current by monitoring when the auxiliary winding voltage falls
below 1.2 V. To prevent false tripping, 200 mV of hysteresis is
provided. The Zero Current Detector input is internally
protected by two clamps. The upper 10 V clamp prevents
input overvoltage breakdown while the lower –0.7 V clamp
prevents substrate injection. An external resistor must be
used in series with the auxiliary winding to limit the current
through the clamps to 5.0 mA or less.
Current Sense Comparator and RS Latch
The Current Sense Comparator RS Latch configuration
used ensures that only a single pulse appears at the Drive
Output during a given cycle. The inductor current is
converted to a voltage by inserting a ground–referenced
sense resistor R7 in series with the source of output switch.
This voltage is monitored by the Current Sense Input and
compared to a level derived from the Multiplier output. The
peak inductor current under normal operating conditions is
controlled by the threshold voltage of Pin 6 where:
+ Ipk
Pin 6 Threshold
R7
Abnormal operating conditions occur when the
preconverter is running at extremely low line or if output
voltage sensing is lost. Under these conditions, the Current
Sense Comparator threshold will be internally clamped to
1.5 V. Therefore, the maximum peak switch current is:
+ Ipk(max)
1.5 V
R7
With the component values shown in Figure 15, the
Current Sense Comparator threshold, at the peak of the
haversine, varies from 110 mV at 90 Vac to 100 mV at
268 Vac. The Current Sense Input to Drive Output
propagation delay is typically 200 ns.
Timer
A watchdog timer function was added to the IC to
eliminate the need for an external oscillator when used in
stand alone applications. The Timer provides a means to
automatically start or restart the preconverter if the Drive
Output has been off for more than 385 µs after the inductor
current reaches zero.
Undervoltage Lockout and Quickstart
The MC33368 has a 5.0 V internal reference brought out
to Pin 1 and capable of sourcing 10 mA typically. It also
contains an Undervoltage Lockout (UVLO) circuit which
suppresses the Gate output at Pin 11 if the VCC supply
voltage drops below 8.5 V typical.
A Quickstart circuit has been incorporated to optimize
converter startup. During initial startup, compensation
capacitor C1 will be discharged, holding the Error Amplifier
output below the Multiplier’s threshold. This will prevent Drive
Output switching and delay bootstraping of capacitor C4 by
diode D6. If Pin 4 does not reach the multiplier threshold
before C4 discharges below the lower SMPS UVLO
threshold, the converter will hiccup and experience a
significant startup delay. The Quickstart circuit is designed to
precharge C1 to 1.7 V. This level is slightly below the Pin 4
Multiplier threshold, allowing immediate Drive Output
switching.
Restart Delay
A restart delay pin is provided to allow hiccup mode fault
protection in case of a short circuit condition and to prevent
the SMPS from repeatedly trying to restart after the input line
voltage has been removed. When power is first applied, there
is no startup delay, but subsequent cycling of the VCC voltage
will result in delay times that are programmed by an external
resistor and capacitor. The Restart Delay, Pin 2, is a high
impedance, so that an external capacitor can provide delay
times as long as several seconds.
If the SMPS output is short circuited, the transformer
winding, which provides the VCC voltage to the control IC and
the MC33368, will be unable to sustain VCC to the control
circuits. The restart delay capacitor at Pin 2 of the MC33368
prevents the high voltage startup transistor within the IC from
maintaining the voltage on C4. After VCC drops below the
UVLO threshold in the SMPS, the SMPS switching
transistors are held off for the time programmed by the values
of the restart capacitor (C9) and resistor (R8). In this manner,
the SMPS switching transistors are operated at very low duty
cyles, preventing their destruction. If the short circuit fault is
removed, the power supply system will turn on by itself in a
normal startup mode after the restart delay has timed out.
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MOTOROLA ANALOG IC DEVICE DATA
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