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SC4612STRT Datasheet(PDF) 15 Page - Semtech Corporation

Part # SC4612STRT
Description  Wide Input Range High Performance Synchronous Buck Switching Controller
PDF  24 Pages
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Manufacturer  SEMTECH [Semtech Corporation]
Direct Link  http://www.semtech.com
Logo SEMTECH - Semtech Corporation

SC4612STRT Datasheet(HTML) 15 Page - Semtech Corporation

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 2007 Semtech Corp.
www.semtech.com
SC4612
POWER MANAGEMENT
COMPONENT SELECTION:
SWITCHING SECTION
OUTPUT CAPACITORS - Selection begins with the most
critical component. Because of fast transient load current
requirements in modern microprocessor core supplies, the
output capacitors must supply all transient load current
requirements until the current in the output inductor ramps
up to the new level. Output capacitor ESR is therefore one
of the most important criteria. The maximum ESR can be
simply calculated from:
step
current
Transient
I
excursion
voltage
transient
Maximum
V
Where
I
V
R
t
t
t
t
ESR
=
=
≤
For example, to meet a 100mV transient limit with a 10A
load step, the output capacitor ESR must be less than
10m
Ω. To meet this kind of ESR level, there are three
available capacitor technologies.
y
g
o
l
o
n
h
c
e
T
h
c
a
E
r
o
ti
c
a
p
a
C
y
t
Q
.
d
q
R
l
a
t
o
T
C
)
F
u
(
R
S
E
m
(
Ω)
C
)
F
u
(
R
S
E
m
(
Ω)
m
u
l
a
t
n
a
T
R
S
E
w
o
L0
3
30
66
0
0
0
20
1
N
O
C
-
S
O0
3
35
23
0
9
93
.
8
m
u
n
i
m
u
l
A
R
S
E
w
o
L0
0
5
14
45
0
0
5
78
.
8
The choice of which to use is simply a cost/performance
issue, with low ESR Aluminum being the cheapest, but
taking up the most space.
INDUCTOR - Having decided on a suitable type and value
of output capacitor, the maximum allowable value of
inductor can be calculated. Too large an inductor will
produce a slow current ramp rate and will cause the output
capacitor to supply more of the transient load current for
longer - leading to an output voltage sag below the ESR
excursion calculated above.
The maximum inductor value may be calculated from:
()
O
IN
t
ESR
V
V
I
C
R
L
−
≤
The calculated maximum inductor value assumes 100%
duty cycle, so some allowance must be made. Choosing
an inductor value of 50 to 75% of the calculated maximum
will guarantee that the inductor current will ramp fast
enough to reduce the voltage dropped across the ESR at a
faster rate than the capacitor sags, hence ensuring a good
recovery from transient with no additional excursions. We
must also be concerned with ripple current in the output
inductor and a general rule of thumb has been to allow
10% of maximum output current as ripple current. Note
that most of the output voltage ripple is produced by the
inductor ripple current flowing in the output capacitor ESR.
Ripple current can be calculated from:
OSC
IN
L
f
L
4
V
I
RIPPLE
⋅
⋅
=
Ripple current allowance will define the minimum permitted
inductor value.
POWER FETS - The FETs are chosen based on several
criteria with probably the most important being power
dissipation and power handling capability.
TOP FET - The power dissipation in the top FET is a
combination of conduction losses, switching losses and
bottom FET body diode recovery losses.
a) Conduction losses are simply calculated as:
IN
O
)
on
(
DS
2
O
COND
V
V
cycle
duty
=
D
where
D
R
I
P
≈
⋅
⋅
=
b) Switching losses can be estimated by assuming a
switching time, If we assume 100ns then:
SW
IN
O
SW
T
ns
100
V
I
P
⋅
⋅
=
or more generally,
2
f
)
t
t
(
V
I
P
OSC
f
r
IN
O
SW
⋅
+
⋅
⋅
=
c) Body diode recovery losses are more difficult to estimate,
but to a first approximation, it is reasonable to assume
Application Information (Cont.)



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