Max1844 – Rainbow Electronics MAX1844 User Manual
Page 18
MAX1844
Output Capacitor Stability Considerations
Stability is determined by the value of the ESR zero rela-
tive to the switching frequency. The point of instability is
given by the following equation:
where:
For a typical 300kHz application, the ESR zero frequency
must be well below 95kHz, preferably below 50kHz.
Tantalum and OS-CON capacitors in widespread use at
the time of publication have typical ESR zero frequencies
of 25kHz. In the design example used for inductor selec-
tion, the ESR needed to support 60mV
P-P
ripple is
60mV/2.7A = 22m
Ω
. Two 470µF/4V Kemet T510 low-ESR
tantalum capacitors in parallel provide 22m
Ω
(max) ESR.
Their typical combined ESR results in a zero at 27kHz,
well within the bounds of stability.
Do not put high-value ceramic capacitors directly across
the feedback sense point without taking precautions to
ensure stability. Large ceramic capacitors can have a
high ESR zero frequency and cause erratic, unstable
operation. However, it’s easy to add enough series resis-
tance by placing the capacitors a couple of inches
downstream from the feedback sense point, which
should be as close as possible to the inductor.
Unstable operation manifests itself in two related but dis-
tinctly different ways: double-pulsing and fast-feedback
loop instability.
Double-pulsing occurs due to noise on the output or
because the ESR is so low that there isn’t enough volt-
age ramp in the output voltage signal. This “fools” the
error comparator into triggering a new cycle immediately
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 can 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 and carefully
observe the output voltage ripple envelope for over-
shoot and ringing. It can help to monitor simultaneously
the inductor current with an AC current probe. Don’t
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 (I
RMS
) imposed by the switching currents.
Nontantalum chemistries (ceramic, aluminum, or OS-
CON) are preferred due to their resistance to power-up
surge currents.
For optimal circuit reliability, choose a capacitor that
has less than 10°C temperature rise at the peak ripple
current.
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-cur-
rent 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). Check to
ensure that the conduction losses at
minimum
input
voltage do not exceed the package thermal limits or
violate the overall thermal budget. Check to ensure that
conduction losses plus switching losses at the
maxi-
mum
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 R
DS(ON)
, comes in a moderate to small pack-
age (i.e., SO-8), and is reasonably priced. Ensure that
the MAX1844 DL gate driver can drive Q2; in other
words, check that the gate is not pulled up by the high-
side switch turn on, due to parasitic drain-to-gate capac-
itance, 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 factor
extremes. For the high-side MOSFET, the worst-case
power dissipation due to resistance occurs at minimum
battery voltage:
PD(Q1 Resistive) = (V
OUT
/ V
IN(MIN)
)
✕
I
LOAD2
✕
R
DS(ON)
Generally, a small high-side MOSFET is desired to
reduce switching losses at high input voltages. However,
the R
DS(ON)
required to stay within package power-dissi-
I
I
V
V
- V
V
RMS
LOAD
OUT
IN
OUT
IN
=
(
)
f
f
f
1
2
R
C
ESR
ESR
ESR
OUT
=
=
Ч Ч
Ч
π
π
High-Speed Step-Down Controller with
Accurate Current Limit for Notebook Computers
18
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