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MC33065-H View Datasheet(PDF) - ON Semiconductor

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MC33065-H Datasheet PDF : 18 Pages
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MC34065H, L
When designing a high power switching regulator it may
be desirable to reduce the internal clamp voltage in order to
keep the power dissipation of RS to a reasonable level. A
simple method to adjust this voltage is shown in Figure 19.
The two external diodes are used to compensate the internal
diodes, yielding a constant clamp voltage over temperature.
Erratic operation due to noise pickup can result if there is an
excessive reduction of the Ipk(max) clamp voltage.
A narrow spike on the leading edge of the current
waveform can usually be observed and may cause the power
supply to exhibit an instability when the output is lightly
loaded. This spike is due to the power transformer
interwinding capacitance and output rectifier recovery time.
The addition of an RC filter on the Current Sense input with
a time constant that approximates the spike duration will
usually eliminate the instability, refer to Figure 24.
Undervoltage Lockout
Two Undervoltage Lockout comparators have been
incorporated to guarantee that the IC is fully functional
before the output stages are enabled. The positive power
supply terminal (VCC) and the reference output (Vref) are
each monitored by separate comparators. Each has builtin
hysteresis to prevent erratic output behavior as their
respective thresholds are crossed. The VCC comparator
upper and lower thresholds are 14 V/10 V for H suffix, and
8.4 V/7.6 V for L suffix. The Vref comparator upper and
lower thresholds are 3.6 V/3.4 V respectively. The large
hysteresis and low startup current of the H suffix version
makes it ideally suited in offline converter applications
where efficient bootstrap startup techniques are required
(Figure 28). The L suffix version is intended for lower
voltage dctodc converter applications. The minimum
operating voltage for the H suffix is 11 V and 8.2 V for the
L suffix.
Drive Outputs and Drive Ground
Each section contains a single totempole output stage
that is specifically designed for direct drive of power
MOSFETs. The Drive Outputs are capable of up to ±400 mA
peak current with a typical rise and fall time of 50 ns with a
1.0 nF load. Additional internal circuitry has been added to
keep the outputs in a sinking mode whenever an
Undervoltage Lockout is active. This characteristic
eliminates the need for an external pulldown resistor. The
totempole output has been optimized to minimize
crossconduction current in high speed operation. The
addition of two 10 Ω resistors, one in series with the source
output transistor and one in series with the sink output
transistor, reduces the crossconduction current to minimal
levels, as shown in Figure 13.
Although the Drive Outputs were optimized for
MOSFETs, they can easily supply the negative base current
required by bipolar NPN transistors for enhanced turnoff
(Figure 25).
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