sc2621 Semtech Corporation, sc2621 Datasheet - Page 7

• Manufacturer Part Number: sc2621
• Description: Sc2621 500 Khz Voltage-mode Pwm Controller With Linear Power Regulator
• Manufacturer: Semtech Corporation

Fig. 2. Pull down resistor for current limit setting
The manufacture data and bench tested results show
that, for low R
the optimum gate drive voltage is around 8.2V, where
the total power losses of power MOSFETs are minimized.
COMPONENT SELECTION
COMPONENT SELECTION
COMPONENT SELECTION
General design guideline of switching power supplies can
be applied to the component selection for the SC2621.
Induct
Inductor and MOSFET
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 without adding extra cost and size. The higher
inductance, the lower ripple current, the smaller core loss
and the higher efficiency will be. However, too high in-
ductance slows down output transient response. It is rec-
ommended to choose the inductance that gives the in-
ductor ripple current to be approximate 20% of maxi-
mum load current. So choose inductor value from:
The MOSFETs are selected from their R
and package. The SC2621 provides 1.5A gate drive cur-
rent. To drive a 50nC gate charge MOSFET gives 50nC/
1.5A=33ns switching time. The switching time ts contrib-
utes to the top MOSFET switching loss:
COMPONENT SELECTION
COMPONENT SELECTION
Induct
Induct
POWER MANAGEMENT
Applications Information (Cont.)
L
2003 Semtech Corp.
=
350
350
325
325
300
300
275
275
250
250
225
225
200
200
175
175
150
150
I
O
0
0
or and MOSFET
or and MOSFETs s s s s
or and MOSFET
or and MOSFET
×
5
f
osc
×
V
100
100
dson
O
×
1 (
MOSFETs run at applied load current,
-
V
200
200
V
IN
O
)
Rset (k -ohm)
Rset (k -ohm)
300
300
400
400
dson
, gate charge,
500
500
600
600
7
Output Capacitor
Output Capacitor
There is no significant switching loss for the bottom
MOSFET because of its zero voltage switching. The con-
duction 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
Output Capacitor
Output Capacitor
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:
The output capacitor ESR also causes output voltage tran-
sient V
meet output transient criteria, the ESR value should be:
To meet both criteria, the smaller one of above two ESRs
is required.
The output capacitor value also contributes to load tran-
sient response. Based on a worst case where the induc-
tor energy 100% dumps to the output capacitor during
the load transient, the capacitance then can be calcu-
lated by:
C
P
P
P
R
R
S
C
C
ESR
ESR
>
_
_
=
TOP
BOT
L
<
<
I
O
×
T
V
=
V
L
I
=
I
×
V
T
during a transient load current I
T
V
T
T
×
2
2
I
I
O
IN
O
f
2
O
2
OSC
×
×
×
×
1 (
R
t
R
S
dson
-
dson
×
dson
V
×
V
V
f
RIPPLE
OSC
×
IN
O
×
D
1 (
)
and gate charge can be calculated
-
D
)
RIPPLE
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SC2621
T
flowing in. To
during the