Rainbow Electronics MAX17036 User Manual
Page 35
MAX17030/MAX17036
1/2/3-Phase Quick-PWM
IMVP-6.5 VID Controllers
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35
However, it can indicate the possible presence of loop
instability due to insufficient ESR. Loop instability can
result in oscillations at the output after line or load
steps. Such perturbations are usually damped, but can
cause the output voltage to rise above or fall below the
tolerance limits.
The easiest method for checking stability is to apply a
very fast 10% to 90% max load transient and carefully
observe the output voltage ripple envelope for over-
shoot and ringing. It can help 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/overshoot.
Transient Response
The inductor ripple current impacts transient-response
performance, especially at low V
IN
- V
OUT
differentials.
Low inductor values allow the inductor current to slew
faster, replenishing charge removed from the output fil-
ter capacitors by a sudden load step. The amount of
output sag is also a function of the maximum duty fac-
tor, which can be calculated from the on-time and mini-
mum off-time. For a dual-phase controller, the
worst-case output sag voltage can be determined by:
and:
where t
OFF(MIN)
is the minimum off-time (see the
Electrical Characteristics
), T
SW
is the programmed
switching period, and η
TOTAL
is the total number of
active phases. K = 66% when N
PH
= 3, and K = 100%
when N
PH
= 2. V
SAG
must be less than the transient
droop
∆I
LOAD(MAX)
x R
DROOP
.
The capacitive soar voltage due to stored inductor
energy can be calculated as:
where η
TOTAL
is the total number of active phases. The
actual peak of the soar voltage is dependent on the
time where the decaying ESR step and rising capaci-
tive soar is at its maximum. This is best simulated or
measured. For the MAX17036 with transient suppres-
sion, contact Maxim directly for application support to
determine the output capacitance requirement.
Input Capacitor Selection
The input capacitor must meet the ripple current
requirement (I
RMS
) imposed by the switching currents.
The multiphase Quick-PWM controllers operate out-of-
phase, reducing the RMS input. For duty cycles less
than 100%/η
OUTPH
per phase, the I
RMS
requirements
can be determined by the following equation:
where η
TOTAL
is the total number of out-of-phase
switching regulators. The worst-case RMS current
requirement occurs when operating with V
IN
=
2η
TOTAL
V
OUT
. At this point, the above equation simpli-
fies to I
RMS
= 0.5 x I
LOAD
/η
TOTAL
. Choose an input
capacitor that exhibits less than +10
°C temperature rise
at the RMS input current for optimal circuit longevity.
Power-MOSFET Selection
Most of the following MOSFET guidelines focus on the
challenge of obtaining high load-current capability
when using high-voltage (> 20V) AC adapters.
High-Side MOSFET Power Dissipation
The conduction loss in the high-side MOSFET (N
H
) is a
function of the duty factor, with the worst-case power
dissipation occurring at the minimum input voltage:
where η
TOTAL
is the total number of phases.
Calculating the switching losses in the high-side
MOSFET (N
H
) 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 PCB layout characteristics. The follow-
ing switching-loss calculation provides only a very
rough estimate and is no substitute for breadboard
evaluation, preferably including verification using a
thermocouple mounted on N
H
:
where C
OSS
is the N
H
MOSFET’s output capacitance,
Q
G(SW)
is the charge needed to turn on the N
H
MOSFET, and I
GATE
is the peak gate-drive source/sink
current (2.2A typ).
PD (NH Switching) =
V I
f
IN LOAD SW
TOTAL
η
⎛
⎝⎜
⎞
⎠⎟
Q
Q
I
G SW
GATE
(
)
⎛
⎝⎜
⎞
⎠⎟
+
C
V
f
OSS IN
SW
2
2
PD (NH Resistive) =
V
V
I
OUT
IN
LOAD
TOTA
⎛
⎝⎜
⎞
⎠⎟ η
L
L
DS ON
R
⎛
⎝⎜
⎞
⎠⎟
2
(
)
I
I
V
V
V
RMS
LOAD
TOTAL IN
TOTAL OUT
IN
TOT
=
⎛
⎝⎜
⎞
⎠⎟
−
η
η
η
A
AL OUT
V
(
)
V
I
L
C
V
SOAR
LOAD MAX
TOTAL
OUT OUT
≈
(
)
∆
(
)
2
2
η
T
t
t
MIN
ON
OFF MIN
=
+
(
)
V
C
V
T
KT
SAG
TOTAL
OUT OUT
MIN
≈
(
)
×
L
I
LOAD(MAX)
2
∆
2
η
S
SW
MIN
T
−
⎡⎣
⎤⎦