SC2618 Semtech, SC2618 Datasheet - Page 5

• Manufacturer Part Number: SC2618
• Description: Miniature PWM Controller for Buck Regulator
• Manufacturer: Semtech

Output Filter Components
Output Filter Components
General design guideline of switching power supplies can
be applied to the component selection for SC2618.
Output Filter Components
Output Filter Components
Output Filter Components
The purpose of soft start is to control inductor current
during start up, this in turn controls the amount of ex-
cess charge the output capacitor will receive and there-
fore the output voltage overshoot that will occur at the
end of soft start.
Normally, controllers with an external soft start capaci-
tor, have soft start set so long that overshoot is unde-
tectable. Since the soft start for the SC2618 is internal,
it is necessarily limited and the maximum values of L and
Co should meet the constraints of:
where t
percentage overshoot allowable.
L
POWER MANAGEMENT
Component Selection
 2004 Semtech Corp.
⋅
v
i
C
C
Fig. 3. Soft start, inductor current and overshoot
L
0
0
O
=
ss
t
SS
2
is soft start time, 100us typically, and Xs is the
50
⋅
Fig. 2. Output filter components
X
t
S
VIN
GND
V
Q1
Q2
O
V
P
t
SS
L
t
2618-02
P
I
L
Co
VO
GND
V
REF
5
Induct
Induct
Inductor and MOSFET
Induct
Induct
The selection of inductor and MOSFETs should meet ther-
mal requirement because they are power loss dominant
components. Pick an inductor with as high inductance
as possible if without adding extra cost and size. Higher
inductance, lower ripple current, smaller core loss and
higher efficiency. However, too high inductance slows
down output transient response. It is recommended to
choose the inductance that gives the inductor ripple cur-
rent to be approximate 20% of maximum load current.
So choose inductor value from:
The MOSFETs are selected from their R
and package. The SC2618 provides 0.5A gate drive cur-
rent. To drive a 25nC gate charge MOSFET gives 25nC/
0.5A=50ns switching time. The switching time ts contrib-
utes to the top MOSFET switching loss:
There is no switching loss for the bottom MOSFET be-
cause of its zero voltage switching. The conduction losses
of the top and bottom MOSFETs are given by:
If the requirement of total power losses for each MOSFET
is given, the above equations can be used to calculate
the values of R
using above equations, then the devices can be deter-
mined accordingly. The solution should ensure the
MOSFET is within its maximum junction temperature at
highest ambient temperature.
Output Capacitor(s)
Output Capacitor(s)
Output Capacitor(s)
Output Capacitor(s)
Output Capacitor(s)
The output capacitors should be selected to meet both
output ripple and transient response criteria. The output
capacitor ESR causes output ripple V
inductor ripple current flowing in. To meet output ripple
criteria, the ESR value should be:
L
P
P
P
R
C
C
S
ESR
=
_
_
=
TOP
BOT
I
<
I
O
O
⋅
or and MOSFET
or and MOSFETs s s s s
or and MOSFET
=
L
or and MOSFET
5
=
⋅
V
f
V
⋅
I
osc
I
O
IN
O
f
2
O
2
OSC
⋅
⋅
⋅
⋅
⋅
1 (
V
R
t
R
S
O
dson
−
dson
⋅
dson
V
⋅
⋅
V
V
1 (
f
RIPPLE
OSC
⋅
IN
O
⋅
D
−
1 (
)
and gate charge can be calculated
V
−
V
IN
O
D
)
)
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dson
RIPPLE
SC2618
www.semtech.com
, gate charge,
during the