Rainbow Electronics MAX17480 User Manual
Page 42
MAX17480
AMD 2-/3-Output Mobile Serial
VID Controller
42
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select the boost capacitors to avoid discharging the
capacitor more than 200mV while charging the high-
side MOSFETs’ gates:
where N is the number of high-side MOSFETs used for
one SMPS, and Q
GATE
is the gate charge specified in the
MOSFET’s data sheet. For example, assume two
IRF7811W n-channel MOSFETs are used on the high
side. According to the manufacturer’s data sheet, a single
IRF7811W has a maximum gate charge of 24nC (V
GS
=
5V). Using the above equation, the required boost
capacitance would be:
:
Selecting the closest standard value, this example
requires a 0.22µF ceramic capacitor.
NB SMPS Design Procedure
NB Inductor Selection
The switching frequency and operating point (% ripple
current or LIR) determine the inductor value as follows:
:
where I
LOAD3(MAX)
is the maximum current and f
SW3
is
the switching frequency of the NB regulator.
Find a low-loss inductor having the lowest possible DC
resistance that fits in the allotted dimensions. If using
a swinging inductor (where the inductance decreases
linearly with increasing current), evaluate the LIR with
properly scaled inductance values. For the selected
inductance value, the actual peak-to-peak inductor rip-
ple current (
∆I
INDUCTOR
) is defined by:
:
Ferrite cores are often the best choice, although pow-
dered iron is inexpensive and can work well at 200kHz.
The core must be large enough not to saturate at the
peak inductor current (I
PEAK3
):
NB Peak Inductor Current Limit (ILIM3)
The MAX17480 NB regulator overcurrent protection
employs a peak current-sensing algorithm that uses the
high-side MOSFET R
ON(NH3)
as the current-sense ele-
ment. Since the controller limits the peak inductor cur-
rent, the maximum average load current is less than the
peak current-limit threshold by an amount equal to half
the inductor ripple current. Therefore, the maximum
load capability is a function of the current-limit setting,
inductor value, switching frequency, and input-to-out-
put voltage difference. When combined with the output
undervoltage-protection circuit, the system is effectively
protected against excessive overload conditions.
The peak current-limit threshold is set by the ILIM3 pin
setting (see the
Offset and Current-Limit Setting for NB
SMPS (ILIM3)
section).
NB Output Capacitor Selection
The output filter capacitor must have low enough ESR to
meet output ripple and load-transient requirements. In
CPU V
CORE
converters and other applications where the
output is subject to large load transients, the output
capacitor’s size typically depends on how much ESR is
needed to prevent the output from dipping too low under a
load transient. Ignoring the sag due to finite capacitance:
:
The output capacitor’s size often depends on how
much ESR is needed to maintain an acceptable level of
output ripple voltage. The output ripple voltage of a
step-down controller equals the total inductor ripple
current multiplied by the output capacitor’s ESR. For
single-phase operation, the maximum ESR to meet the
output-ripple-voltage requirement is:
:
where f
SW3
is the switching frequency. The actual
capacitance value required relates to the physical size
needed to achieve low ESR, as well as to the chemistry
of the capacitor technology. Thus, capacitor selection
is usually limited by ESR and voltage rating rather than
by capacitance value (this is true of polymer types).
R
V
f
L
V
V
V
V
ESR
IN SW
IN
OUT
OUT
RI
≤
−
(
)
⎡
⎣
⎢
⎢
⎤
⎦
⎥
⎥
3
3 3
3
3
3
P
PPLE3
R
R
V
I
ESR
PCB
STEP
LOAD MAX
+
(
)
≤
∆
(
)
I
I
I
PEAK
LOAD MAX
INDUCTOR
3
3
2
=
+
⎛
⎝
⎜
⎞
⎠
⎟
(
)
∆
∆I
V
V
V
V
f
L
INDUCTOR
OUT
IN
OUT
IN SW
=
−
(
)
3
3
3
3
3 3
L
V
V
f
I
LIR
V
IN
OUT
SW LOAD MAX
OUT
3
3
3
3
3
=
−
⎛
⎝
⎜⎜
⎞
⎠
⎟⎟
(
)
3
3
3
V
IN
⎛
⎝
⎜
⎞
⎠
⎟
C
nC
mV
F
BST
= ×
= .
2 24
200
0 24µ
C
N Q
mV
BST
GATE
=
Ч
200