MC33060A View Datasheet(PDF) - Motorola => Freescale
Part Name
Description
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MC33060A Datasheet PDF : 16 Pages
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6
5
RT CT
Dead–Time 4
Control
MC34060A MC33060A
Figure 1. Block Diagram
Oscillator
0.12V
0.7V
≈ 0.7mA
+
1
–
Reference
10
Regulator
Dead–Time
– Comparator
+
Undervoltage
Lockout –
+
–
+ PWM.
Comparator
+
2
–
12
VTH
9
Q1 8
VCC
Ref Out
Collector
Emitter
1
2
Error Amp
1
3
Feedback/PWM
Comparator Input
13
14
Error Amp
2
7 Gnd
This device contains 46 active transistors.
Description
The MC34060A is a fixed–frequency pulse width modulation control circuit, incorporating the primary building blocks required
for the control of a switching power supply (see Figure 1). An internal–linear sawtooth oscillator is frequency–programmable by
two external components, RT and CT. The approximate oscillator frequency is determined by:
^ fosc
1.2
RT • CT
For more information refer to Figure 3.
Output pulse width modulation is accomplished by comparison of the positive sawtooth waveform across capacitor CT to either
of two control signals. The output is enabled only during that portion of time when the sawtooth voltage is greater than the control
signals. Therefore, an increase in control–signal amplitude causes a corresponding linear decrease of output pulse width. (Refer
to the Timing Diagram shown in Figure 2.)
Figure 2. Timing Diagram
Capacitor CT
Feedback/P.W.M.
Comparator
Dead–Time Control
Output Q1,
Emitter
APPLICATIONS INFORMATION
The control signals are external inputs that can be fed into
the dead–time control, the error amplifier inputs, or the
feed–back input. The dead–time control comparator has an
effective 120 mV input offset which limits the minimum output
dead time to approximately the first 4% of the sawtooth–cycle
time. This would result in a maximum duty cycle of 96%.
Additional dead time may be imposed on the output by setting
the dead time–control input to a fixed voltage, ranging
between 0 V to 3.3 V.
The pulse width modulator comparator provides a means
for the error amplifiers to adjust the output pulse width from
the maximum percent on–time, established by the dead time
control input, down to zero, as the voltage at the feedback pin
varies from 0.5 V to 3.5 V. Both error amplifiers have a
common mode input range from –0.3 V to (VCC –2.0 V), and
may be used to sense power supply output voltage and
current. The error–amplifier outputs are active high and are
ORed together at the noninverting input of the pulse–width
modulator comparator. With this configuration, the amplifier
that demands minimum output on time, dominates control of
the loop.
The MC34060A has an internal 5.0 V reference capable of
sourcing up to 10 mA of load currents for external bias
circuits. The reference has an internal accuracy of ±5% with a
typical thermal drift of less than 50 mV over an operating
temperature range of 0° to +70°C.
MOTOROLA ANALOG IC DEVICE DATA
5
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