|MC33025DWG||High Speed Double−Ended PWM Controller|
|MC33025DWG Datasheet PDF : 20 Pages |
If the voltage at this pin exceeds 1.4 V, the second
comparator is activated. This comparator sets a latch which,
in turn, causes the Soft−Start capacitor to be discharged. In
this way a “hiccup” mode of recovery is possible in the case
of output short circuits. If a current limit resistor is used in
series with the output devices, the peak current at which the
controller will enter a “hiccup” mode is given by:
There are two undervoltage lockout circuits within the IC.
The first senses VCC and the second Vref. During power−up,
VCC must exceed 9.2 V and Vref must exceed 4.2 V before
the outputs can be enabled and the Soft−Start latch released.
If VCC falls below 8.4 V or Vref 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 startup circuit.
Typical startup current is 500 mA.
The MC34025 has two high current totem pole outputs
specifically designed for direct drive of power MOSFETs.
They are capable of up to ±2.0 A peak drive current with a
typical rise and fall time of 30 ns driving a 1.0 nF load.
Separate pins for VC and Power Ground are provided.
With proper implementation, a significant reduction of
switching transient noise imposed on the control circuitry is
possible. The separate VC supply input also allows the
designer added flexibility in tailoring the drive voltage
independent of VCC.
A 5.1 V bandgap reference is pinned out and is trimmed
to 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.
Do not attempt to construct the converter on
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. 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 or snubbing.
In current mode control, an instability can be encountered
at any given duty cycle. The instability is caused by the
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 (Se) is added to the on−time ramp (Sn)
of the current−sense waveform, stability can be achieved
(see Figure 21).
One must be careful not to add too much ramp
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. Figures 29A and 29B show
examples of two different ways in which external ramp
compensation can be implemented.
Figure 21. Ramp Compensation
A simple equation can be used to calculate the amount of
external ramp necessary to add that will achieve stability in
the current loop. For the following equations, the calculated
values for the application circuit in Figure 37 are also shown.
ǒ Ǔ VO
Se + L
VO = DC output voltage
NP, NS = number of power transformer primary
= or secondary turns
Ai = gain of the current sense network
= (see Figures 26, 27 and 28)
L = output inductor
RS = current sense resistance
ǒ Ǔ For the application circuit:
+ 0.115 Vńμs
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