MAX724/MAX726
5A/2A Step-Down, PWM,
Switch-Mode DC-DC Regulators
10 ______________________________________________________________________________________
0.2µs would be needed to provide a narrow enough
duty cycle that could control current when the output is
shorted. Since 0.6µs is too long (at 100kHz), the f
OSC
is lowered to 20kHz once FB (and hence the output)
drops below about 1.3V (see Frequency vs. V
FB
Voltage
graph in the
Typical Operating Characteristics
). This
way, the MAX724/MAX726's 0.6µs minimum t
ON
allows
a sufficiently small duty cycle (at the reduced f
OSC
) so
that current can still be limited.
Compensation Network
A series RC network connected from V
C
to ground
compensates the MAX724/MAX726. Compensation
R
C
values are shown in the applications circuits. R
C
and C
C
shape error-amplifier gain as follows: At DC,
R
C
and C
C
have no effect, so the error-amplifier's
gain is the product of its transconductance (approxi-
mately 5000µmhos) and an internal 400kΩ load
impedance (r
INT
) at V
C
. So at DC, A
V(DC)
= g
M
(r
INT
) =
approximately 2000µmhos. R
C
and C
C
then add a
low-frequency pole and a high-frequency zero, as
shown in Figure 5.
Output Overshoot
The MAX724/MAX726 error-amplifier design minimizes
overshoot, but precautions against overshoot should
still be exercised in sensitive applications. Worst-case
overshoot typically occurs when recovering from an
output short because V
C
slews down from its highest
voltage. This can be checked by simply shorting and
releasing the output.
Reduce objectional overshoot by increasing the com-
pensation resistor (to 3kΩ or 4kΩ) at V
C
. This allows
the error-amplifier output, V
C
, to move more rapidly in
the negative direction. In some cases, loop stability
may suffer with a high-value compensation resistor. An
option, then, is to add output filter capacitance, which
reduces short-circuit recovery overshoot by limiting out-
put rise time. Lowering the compensation capacitor to
below 0.05µF may also help by allowing V
C
to slew fur-
ther before the output rises too far.
Optional Output Filters
Though not shown in the application circuits in Figures
2, 7, and 8, additional filtering can easily be added to
reduce output ripple to levels below 2%. It is more
effective to add an LC type filter rather than additional
output capacitance alone. A small-value inductor (2µH
to 10µH) and between 47µF and 220µF of filter capaci-
tance should suffice (Figure 6). Although the inductor
does not need to be of high quality (it is not switching),
it must still be rated for the full load current.
When an LC filter is added, do not move the connection
of the feedback resistor to the LC output. It should be left
connected to the main output filter capacitor (C1 in Figure
2). If the feedback connection is moved to the LC filter
point, the added phase shift may impact stability.
L
F
C
F
TO LOAD
FEEDBACK RESISTOR
MAIN FILTER CAP
Figure 6. Optional LC Output Filter
FREQUENCY
GAIN
90° PHASE SHIFT
f
POLE
= 1/[2π(400kΩ)]C
C
-A
V(MID)
= g
M
/ (2π f C
C
)
f
ZERO
= 1 / (2π R
C
C
C
)
A
V(HI)
= g
M
R
C
A
V(DC)
= g
M
(400kΩ) ≈ 2000
Figure 5. Error-Amplifier Gain as Set by R
C
and C
C
at V
C
Pin
(6 Seiten)
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