SC411 Semtech Corporation, SC411 Datasheet - Page 15

• Manufacturer Part Number: SC411
• Description: Synchronous Buck Pseudo-Fixed Frequency Power Supply Controller
• Manufacturer: Semtech Corporation

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Where TOL
POSLIM
The minimum output capacitance is calculated as
follows:
This calculation assumes the absolute worst case condi-
tion of a full-load to no load step transient occurring when
the inductor current is at its highest. The capacitance
required for smaller transient steps may be calculated by
substituting the desired current for the I
For our example:
C
We will select 440μF, using two 220μF, 25mΩ capacitors
in parallel. For smaller load release overshoot, 660μF
may be used. Alternatively, one 15mΩ or 12mΩ, 220μF,
330μF or 470μF capacitor may be used (with the appro-
priate change to the calculation for C
the load transient requirements.
Next we calculate the RMS input ripple current, which is
largest at the minimum battery voltage:
For our example:
I
Input capacitors should be selected with suffi cient ripple
current rating for this RMS current, for example a 10μF,
1210 size, 25V ceramic capacitor can handle approxi-
mately 3A
derate appropriately.
© 2007 Semtech Corp.
IN(RMS)
POWER MANAGEMENT
Application Information (Cont.)
OUT(MIN)
C
COUT(MIN)
= 2.14A
= 626μF.
TR
I
IN
(
= 1.296V
RMS
RMS
TR
)
is the transient tolerance. For our example:
. Refer to manufacturer’s data sheets and
= L
=
RMS
V
OUT
POSLIM
I
•
OUT
(
V
BAT
+
(
MIN
I
TR 2
)
RIPPLE_VBAT(MAX)
−
V
OUT
2
V
OUT_ST_POS 2
)
•
TOP
V
), depending upon
BAT
I
OUT
OUT
_
MIN
term.
2
A
RMS
F
15
Finally, we calculate the current limit resistor value. As
described in the current limit section, the current limit
looks at the “valley current”, which is the average output
current minus half the ripple current. We use the maxi-
mum room temperature specifi cation for MOSFET R
at V
The ripple at low battery voltage is used because we want
to make sure that current limit does not occur under nor-
mal operating conditions.
For our example:
I
We select the next lowest 1% resistor value: 7.68kΩ
Thermal Considerations
The junction temperature of the device may be calculated
as follows:
Where:
The power dissipation may be calculated as follows:
Where:
VCCA
I
VDDP = gate drive supply voltage (V)
VALLEY
VCCA
GS
= 5.13A, R
= 4.5V for purposes of this calculation:
T
P
θ
= chip supply voltage (V)
= operating current (A)
A
D
JA
= ambient temperature (°C)
= power dissipation in (W)
= thermal impedance junction to ambient
R
ILIM
from absolute maximum ratings (°C/W)
P
+
D
V
I
VALLEY
=
DS(ON)
=
g
T
•
(
VCCA
I
J
VALLEY
Q
=
g
=
= 9mΩ and R
T
•
A
f
I
OUT
•
•
+
+
I
1
VCCA
VBST
P
2 .
−
D
)
I
•
+
RIPPLE
•
θ
VDDP
R
•
JA
10
1
DS
mA
_
(
2
VBAT
ON
•
°
ILIM
10
)
C
•
•
•
(
I
D
MIN
VDDP
1
−
6
= 7.76kΩ
4 .
)
W
A
Ohms
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SC411
DS(ON)