MAX8576–MAX8579
3V to 28V Input, Low-Cost, Hysteretic
Synchronous Step-Down Controllers
16 ______________________________________________________________________________________
chosen based on a prototype or evaluation circuit. As a
general rule, a smaller current ripple results in less out-
put voltage ripple. Since the inductor ripple current is a
factor of the inductor value and input voltage, the output
voltage ripple decreases with larger inductance and
increases with higher input voltages. For reliable and
safe operation, ensure that the capacitor’s voltage and
ripple-current ratings exceed the calculated values.
The response of the MAX8576–MAX8579 to a load
1transient depends on the selected output capacitors.
After a load transient, the output voltage instantly
changes by ESR times ΔI
LOAD
. Before the controller
can respond, the output voltage deviates further
depending on the inductor and output capacitor val-
ues. The controller responds immediately as the output
voltage deviates from its regulation limit (see the
Typical Operating Characteristics).
The MAX8576–MAX8579 are compatible with both alu-
minum electrolytic and ceramic output capacitors. Due
to the limited capacitance of a ceramic capacitor, it is
typically used for a higher switching frequency and
lower output current. Aluminum electrolytic is more
applicable to frequencies up to 300kHz and can sup-
port higher output current with its much higher capaci-
tance value.
Due to the much higher ESL and ESR of the aluminum
electrolytic capacitor, an RC filter (R7 and C12 of Figure
2) is required to prevent excessive ESL and ESR ripple
from tripping the feedback threshold prematurely.
MOSFET Snubber Circuit
Fast-switching transitions cause ringing because of
resonating circuit parasitic inductance and capaci-
tance at the switching nodes. This high-frequency ring-
ing occurs at LX’s rising and falling transitions and can
interfere with circuit performance and generate EMI. To
dampen this ringing, a series RC snubber circuit is
added across each switch. Below is the procedure for
selecting the value of the series RC circuit:
1) Connect a scope probe to measure V
LX
to GND,
and observe the ringing frequency, f
R
.
2) Find the capacitor value (connected from LX to
GND) that reduces the ringing frequency by half.
The circuit parasitic (C
PAR
) at LX is then equal to 1/3
the value of the added capacitance above. The circuit
parasitic inductance (L
PAR
) is calculated by:
The resistor for critical dampening (R
SNUB
) is equal to
2π x f
R
x L
PAR
. Adjust the resistor value up or down to tai-
lor the desired damping and the peak voltage excursion.
The capacitor (C
SNUB
) should be at least 2 to 4 times
the value of C
PAR
to be effective. The power loss of the
snubber circuit is dissipated in the resistor (P
RSNUB
)
and can be calculated as:
where V
IN
is the input voltage and f
SW
is the switching
frequency. Choose an R
SNUB
power rating that meets
the specific application’s derating rule for the power
dissipation calculated.
Feed-Forward Capacitor
The feed-forward capacitor, C8 (Figure 2, MAX8576/
MAX8577 with aluminum electrolytic output capacitor),
or C19 (Figure 3, MAX8578/MAX8579 with ceramic out-
put capacitor), dominantly affects the switching fre-
quency. Choose a ceramic X7R capacitor with a value
given by:
or
where F
S
is the desired switching frequency, and R
FB
is the parallel combination of the two feedback divider-
resistors (R1 and R3 of Figure 2, and R9 and R11 of
Figure 3).
Select the closest standard value to C8 and C19 as
possible.
The output inductor and output capacitor also affect the
switching frequency, but to a much lesser extent.
The equations for C8 and C19 above should yield with-
in ±30% of the desired switching frequency for most
applications. The values of C8 and C19 can be
increased to lower the frequency, or decreased to raise
the frequency for better accuracy.
Application Information
PC Board Layout Guidelines
Careful PC board layout is critical to achieve low
switching losses and clean, stable operation. The
switching power stage requires particular attention.
Follow these guidelines for good PC board layout:
