MAX8743
Dual, High-Efficiency, Step-Down
Controller with High Impedance in Shutdown
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Double-pulsing occurs due to noise on the output or
because the ESR is so low that there is not enough volt-
age ramp in the output voltage signal. This “fools” the
error comparator into triggering a new cycle immedi-
ately after the 400ns minimum off-time period has
expired. Double-pulsing is more annoying than harmful,
resulting in nothing worse than increased output ripple.
However, it may indicate the possible presence of loop
instability, which is caused by insufficient ESR.
Loop instability can result in oscillations at the output
after line or load perturbations that can trip the overvolt-
age protection latch or cause the output voltage to fall
below the tolerance limit.
The easiest method for checking stability is to apply a
very fast zero-to-max load transient (refer to the
MAX8743 EV kit manual) and carefully observe the out-
put-voltage-ripple envelope for overshoot and ringing. It
helps to simultaneously monitor the inductor current with
an AC current probe. Do not allow more than one cycle of
ringing after the initial step-response under- or overshoot.
Input Capacitor Selection
The input capacitor must meet the ripple current
requirement (IRMS) imposed by the switching currents.
Nontantalum chemistries (ceramic, aluminum, or OS-
CON) are preferred due to their resistance to power-up
surge currents:
Power MOSFET Selection
Most of the following MOSFET guidelines focus on the
challenge of obtaining high load-current capability
(>5A) when using high-voltage (>20V) AC adapters.
Low-current applications usually require less attention.
For maximum efficiency, choose a high-side MOSFET
(Q1) that has conduction losses equal to the switching
losses at the optimum battery voltage (15V). Ensure
that the conduction losses at the minimum input volt-
age do not exceed the package thermal limits or violate
the overall thermal budget. Ensure that conduction
losses plus switching losses at the maximum input
voltage do not exceed the package ratings or violate
the overall thermal budget.
Choose a low-side MOSFET (Q2) that has the lowest
possible RDS(ON), comes in a moderate to small pack-
age (i.e., SO-8), and is reasonably priced. Ensure that
the MAX8743 DL gate driver can drive Q2; in other
words, check that the gate is not pulled up by the high-
side switch turning on due to parasitic drain-to-gate
capacitance, causing cross-conduction problems.
Switching losses are not an issue for the low-side
MOSFET since it is a zero-voltage switched device
when used in the buck topology.
MOSFET Power Dissipation
Worst-case conduction losses occur at the duty cycle
extremes. For the high-side MOSFET, the worst-case-
power dissipation (PD) due to resistance occurs at min-
imum battery voltage:
Generally, a small high-side MOSFET is desired to
reduce switching losses at high input voltages.
However, the RDS(ON) required to stay within package
power-dissipation limits often limits how small the
MOSFET can be. Again, the optimum occurs when the
switching (AC) losses equal the conduction (RDS(ON))
losses. High-side switching losses do not usually
become an issue until the input is greater than approxi-
mately 15V.
Switching losses in the high-side MOSFET can become
an insidious heat problem when maximum AC adapter
voltages are applied, due to the squared term in the
CV2f switching-loss equation. If the high-side MOSFET
chosen for adequate RDS(ON) at low battery voltages
becomes extraordinarily hot when subjected to
VIN(MAX), reconsider the choice of MOSFET.
Calculating the power dissipation in Q1 due to switch-
ing losses is difficult since it must allow for difficult
quantifying factors that influence the turn-on and turn-
off times. These factors include the internal gate resis-
tance, gate charge, threshold voltage, source
inductance, and PC board layout characteristics. The
following switching-loss calculation provides only a
very rough estimate and is no substitute for bench eval-
uation, preferably including a verification using a ther-
mocouple mounted on Q1:
where CRSS is the reverse transfer capacitance of Q1,
and IGATE is the peak gate-drive source/sink current
(1A typ).
For the low-side MOSFET, Q2, the worst-case power
dissipation always occurs at maximum battery voltage: