MAX1858A/MAX1875A/MAX1876A
Dual 180° Out-of-Phase Buck Controllers with
Sequencing/Prebias Startup and POR
20 ______________________________________________________________________________________
Applications Information
Dropout Performance
When working with low input voltages, the output-volt-
age adjustable range for continuous-conduction opera-
tion is restricted by the minimum off-time (t
OFF(MIN)
).
For best dropout performance, use the lowest (100kHz)
switching-frequency setting. Manufacturing tolerances
and internal propagation delays introduce an error to
the switching frequency and minimum off-time specifi-
cations. This error is more significant at higher frequen-
cies. Also, keep in mind that transient response
performance of buck regulators operated close to
dropout is poor, and bulk output capacitance must
often be added (see the V
SAG
equation in the Design
Procedure section).
The absolute point of dropout is when the inductor cur-
rent ramps down during the minimum off-time (∆I
DOWN
)
as much as it ramps up during the maximum on-time
(∆I
UP
). The ratio h = ∆I
UP
/∆I
DOWN
is an indicator of the
ability to slew the inductor current higher in response to
increased load, and must always be greater than 1. As
h approaches 1, the absolute minimum dropout point,
the inductor current cannot increase as much during
each switching cycle and V
SAG
greatly increases
unless additional output capacitance is used.
A reasonable minimum value for h is 1.5, but adjusting
this up or down allows tradeoffs between V
SAG
, output
capacitance, and minimum operating voltage. For a
given value of h, the minimum operating voltage can be
calculated as:
where V
DROP1
is the sum of the parasitic voltage drops
in the inductor discharge path, including synchronous
rectifier, inductor, and PC board resistances; V
DROP2
is
the sum of the resistances in the charging path, includ-
ing high-side switch, inductor, and PC board resis-
tances; and t
OFF(MIN)
is from the Electrical
Characteristics. The absolute minimum input voltage is
calculated with h = 1.
If the calculated V+
(MIN)
is greater than the required
minimum input voltage, then reduce the operating fre-
quency or add output capacitance to obtain an accept-
able V
SAG
. If operation near dropout is anticipated,
calculate V
SAG
to be sure of adequate transient
response.
Dropout design example:
V
OUT
= 5V
f
SW
= 600kHz
t
OFF(MIN)
= 250ns
V
DROP1
= V
DROP2
= 100mV
h = 1.5
Calculating again with h = 1 gives the absolute limit of
dropout:
Therefore, V
IN
must be greater than 6V, even with very
large output capacitance, and a practical input voltage
with reasonable output capacitance would be 6.58V.
Improving Noise Immunity
Applications where the MAX1858A/MAX1875A/
MAX1876A must operate in noisy environments can
typically adjust their controller’s compensation to
improve the system’s noise immunity. In particular,
high-frequency noise coupled into the feedback loop
causes jittery duty cycles. One solution is to lower the
crossover frequency (see the Compensation section).
PC Board Layout Guidelines
Careful PC board layout is critical to achieve low switch-
ing losses and clean, stable operation. This is especially
true for dual converters where one channel can affect
the other. Refer to the MAX1858 EV kit or MAX1875 EV
kit data sheet for specific layout examples.
If possible, mount all the power components on the top
side of the board with their ground terminals flush
against one another. Follow these guidelines for good
PC board layout:
• Isolate the power components on the top side from
the analog components on the bottom side with a
ground shield. Use a separate PGND plane under
the OUT1 and OUT2 sides (referred to as PGND1
and PGND2). Avoid the introduction of AC currents
into the PGND1 and PGND2 ground planes. Run the
power plane ground currents on the top side only.
