Rainbow Electronics MAX17409 User Manual
Page 26
MAX17409
1-Phase Quick-PWM GPU Controller
26
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capacitor selection, inductor saturation rating, and
the design of the current-limit circuit. The continu-
ous load current (I
LOAD
) determines the thermal
stresses and thus drives the selection of input
capacitors, MOSFETs, and other critical heat-con-
tributing components. Modern notebook CPUs gen-
erally exhibit I
LOAD
= I
LOAD(MAX)
x 80%.
•
Switching frequency: This choice determines the
basic trade-off between size and efficiency. The
optimal frequency is largely a function of maximum
input voltage, due to MOSFET switching losses that
are proportional to frequency and V
IN
2
. The opti-
mum frequency is also a moving target, due to
rapid improvements in MOSFET technology that are
making higher frequencies more practical.
•
Inductor operating point: This choice provides
trade-offs between size vs. efficiency and transient
response vs. output noise. Low inductor values pro-
vide better transient response and smaller physical
size, but also result in lower efficiency and higher
output noise due to increased ripple current. The
minimum practical inductor value is one that causes
the circuit to operate at the edge of critical conduc-
tion (where the inductor current just touches zero
with every cycle at maximum load). Inductor values
lower than this grant no further size-reduction bene-
fit. The optimum operating point is usually found
between 20% and 50% ripple current.
Inductor Selection
The switching frequency and operating point (% ripple
current or LIR) determine the inductor value as follows:
Find a low-loss inductor having the lowest possible DC
resistance that fits in the allotted dimensions. Ferrite
and molded iron cores are often the best choice,
although powdered iron is inexpensive and can work
well at 200kHz. The core must be large enough not to
saturate at the peak inductor current (I
PEAK
):
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. The worst-case output sag voltage can
be determined by:
where t
OFF(MIN)
is the minimum off-time (see the
Electrical Characteristics
table).
The amount of overshoot due to stored inductor energy
can be calculated as:
Current-Limit Control (ILIM)
REF and ILIM are used to set the current limit. REF reg-
ulates to a fixed 2.0V and the REF-to-ILIM voltage
determines the valley current-sense threshold. When
ILIM = V
CC
, the controller uses the preset 22.5mV cur-
rent-limit threshold. In an adjustable design, ILIM is
connected to a resistive voltage-divider connected
between REF and ground. The differential voltage
between REF and ILIM sets the current-limit threshold
(V
LIMIT
), so the valley current-sense threshold is:
This allows design flexibility since the DCR sense circuit
or sense resistor does not have to be adjusted to meet
the current limit as long as the current-sense voltage
never exceeds 50mV. Keeping V
LIMIT
between 20mV to
40mV leaves room for future current-limit adjustment.
The minimum current-limit threshold must be high
enough to support the maximum load current when the
current limit is at the minimum tolerance value. The val-
ley of the inductor current occurs at I
LOAD(MAX)
minus
half the ripple current; therefore:
I
I
LIR
VALLEY
LOAD MAX
>
⎛
⎝⎜
⎞
⎠⎟
(
)
1
2
-
V
V
V
LIMIT
REF
ILIM
=
-
10
V
I
L
C
V
SOAR
LOAD MAX
OUT OUT
≈
(
)
∆
(
)
2
2
V
V
t
V
t
SAG
OUT SW
IN
OFF
=
(
)
⎛
⎝⎜
⎞
⎠⎟
+
L
I
LOAD(MAX)
2
∆
(M
MIN
OUT OUT
IN
OUT
SW
IN
C
V
V
V
t
V
)
⎡
⎣
⎢
⎤
⎦
⎥
(
)
⎛
⎝⎜
⎞
⎠⎟
2
-
- tt
OFF MIN
(
)
⎡
⎣
⎢
⎢
⎤
⎦
⎥
⎥
I
I
LIR
PEAK
LOAD MAX
=
+
⎛
⎝⎜
⎞
⎠⎟
(
)
1
2
L
V
V
f
I
LIR
V
V
IN
OUT
SW LOAD MAX
OUT
IN
=
⎛
⎝
⎜
⎞
⎠
⎟
⎛
⎝⎜
⎞
-
(
)
⎠⎠⎟