SC2621A Semtech Corporation, SC2621A Datasheet - Page 7

• Manufacturer Part Number: SC2621A
• Description: Voltage Mode PWM Controller
• Manufacturer: Semtech Corporation

Internal LDO for Gate Drive
Internal LDO for Gate Drive
COMPONENT SELECTION
COMPONENT SELECTION
Induct
Induct
1.5ms. Therefore, the maximum soft start current is de-
termined by the output inductance and output capaci-
tance. The values of output inductor and output bulk
capacitors have to be properly selected so that the soft
start peak current does not exceed the load trip point of
the short circuit protection.
Internal LDO for Gate Drive
Internal LDO for Gate Drive
Internal LDO for Gate Drive
An internal LDO is designed in the SC2621A to lower the
12V supply voltage for gate drive. An 1uF external ce-
ramic capacitor connected in between DRV pin to the
ground is needed to support the LDO. The LDO output is
connected to low gate drive internally, and has to be
connected to high gate drive through an external boot-
strap circuit. The LDO output voltage is set at 8.2V. The
manufacture data and bench tested results show that,
for low R
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 SC2621A.
Induct
Induct
Inductor and MOSFET
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 SC2621A 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:
POWER MANAGEMENT
Applications Information (Cont.)
L
P
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2007 Semtech Corp.
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I
I
O
O
or and MOSFET
or and MOSFETs s s s s
or and MOSFET
or and MOSFET
×
5
×
f
V
dson
osc
IN
×
×
V
t
MOSFETs run at applied load current, the
S
O
×
×
1 (
f
OSC
-
V
V
IN
O
)
dson
, gate charge,
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
R
R
C
C
ESR
ESR
>
_
_
TOP
BOT
L
<
<
×
T
V
=
V
L
I
=
I
V
T
during a transient load current I
T
T
T
2
×
2
I
I
O
O
2
O
f
2
OSC
×
×
×
1 (
R
R
dson
-
dson
×
dson
V
V
V
RIPPLE
×
IN
O
×
D
1 (
)
and gate charge can be calculated
-
D
)
SC2621A
RIPPLE
www.semtech.com
T
flowing in. To
during the