MC33023 Motorola, MC33023 Datasheet - Page 9

• Manufacturer Part Number: MC33023
• Description: High Speed Single-Ended PWM Controller
• Manufacturer: Motorola

Undervoltage Lockout
The first senses V CC and the second V ref . During power–up,
V CC must exceed 9.2 V and V ref must exceed 4.2 V before
the outputs can be enabled and the Soft–Start latch released.
If V CC falls below 8.4 V or V ref falls below 3.6 V, the outputs
are disabled and the Soft–Start latch is activated. When the
UVLO is active, the part is in a low current standby mode
allowing the IC to have an off–line bootstrap start–up circuit.
Typical start–up current is 500 A.
Output
specifically designed for direct drive of power MOSFETs. It is
capable of up to
and fall time of 30 ns driving a 1.0 nF load.
With proper implementation, a significant reduction of
switching transient noise imposed on the control circuitry is
possible. The separate V C supply input also allows the
designer added flexibility in tailoring the drive voltage
independent of V CC .
Reference
an initial accuracy of 1.0% at 25 C. This reference has short
circuit protection and can source in excess of 10 mA for
powering additional control system circuitry.
Design Considerations
wire–wrap or plug–in prototype boards. With high
frequency, high power, switching power supplies it is
imperative to have separate current loops for the signal paths
and for the power paths. 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. Shown in Figure 35 is
a printed circuit layout of the application circuit. Note how the
power and ground traces are run. All bypass capacitors and
snubbers should be connected as close as possible to the
specific part in question. The PC board lead lengths must be
less than 0.5 inches for effective bypassing for snubbing.
Instabilities
at any given duty cycle. The instability is caused by the
MOTOROLA ANALOG IC DEVICE DATA
There are two undervoltage lockout circuits within the IC.
The MC34023 has a high current totem pole output
Separate pins for V C and Power Ground are provided.
A 5.1 V bandgap reference is pinned out and is trimmed to
Do not attempt to construct the converter on
In current mode control, an instability can be encountered
2.0 A peak drive current with a typical rise
MC34023 MC33023
current feedback loop. It has been shown that the instability is
caused by a double pole at half the switching frequency. If an
external ramp (S e ) is added to the on–time ramp (S n ) of the
current–sense waveform, stability can be achieved.
compensation. If too much is added the system will start to
perform like a voltage mode regulator. All benefits of current
mode control will be lost. Figure 25 is an example of one way
in which external ramp compensation can be implemented.
external ramp slope necessary to add that will achieve
stability in the current loop. For the following equations, the
calculated values for the application circuit in Figure 34 are
also shown.
For the application circuit: S e
where:
Compensation S e
One must be careful not to add too much ramp
A simple equation can be used to calculate the amount of
Ramp
N P , N S
Figure 20. Ramp Compensation
V O
R S
A i
S e
L
= DC output voltage
= number of power transformer primary
=
= gain of the current sense network
=
= output inductor
= current sense resistance
+
or secondary turns
(see Figures 23 and 24)
Ramp Input
Signal S n
V O
Current
L
Ramp Compensation
N S
N P
+
= 0.115 V/ms
(R S )A
1.8
5
i
1.25 V
2
8
(0.3)(0.55)
9