sc483itstrt Semtech Corporation, sc483itstrt Datasheet - Page 14

• Manufacturer Part Number: sc483itstrt
• Description: Sc483 Dual Synchronous Buck Pseudo-fixed-frequency Power-supply Controller
• Manufacturer: Semtech Corporation

For our example:
C
We will select 440µF, using two 220µF, 25m
capacitors in parallel. For smaller load release overshoot,
660µF may be used.
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 sufficient ripple
current rating for this RMS current, for example a 10µF,
1210 size, 25V ceramic capacitor can handle
approximately 3A
sheets and derate appropriately.
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
maximum room temperature specification for MOSFET
R
The ripple at low battery voltage is used because we want
to make sure that current limit does not occur under
normal operating conditions.
For our example:
I
We select the next lowest 1% resistor value: 7.68k
I
I
R
POWER MANAGEMENT
IN(RMS)
VALLEY
IN
VALLEY
OUT(MIN)
DS(ON)
ILIM
2005 Semtech Corp.
(
RMS
)
= 5.13A, R
= 2.14A
at V
I
= 610µF.
VALLEY
I
OUT
V
GS
OUT
= 4.5V for purposes of this calculation:
I
RMS
RIPPLE
1
2 .
DS(ON)
V
BAT
RMS
_
2
VBAT
R
(
= 9m
MIN
. Refer to manufacturer’s data
10
DS
(
)
MIN
(
ON
)
10
V
)
A
OUT
1
and R
6
4 .
Ohms
V
ILIM
BAT
I
OUT
_
= 7.76k
MIN
A
RMS
14
Thermal Considerations
The junction temperature of the device may be calcu-
lated as follows:
Where:
T
P
absolute maximum ratings (°C/W)
The power dissipation may be calculated as follows:
Where:
VCCA = chip supply voltage (V)
I
VDDP = gate drive supply voltage (V)
I
V
Q
f
VBST = boost pin voltage during t
D
Inserting the following values for VBAT
this is the worst case condition for power dissipation in
the controller) as an example (OUT1 = 1.5V, OUT2 = 1.2V):
T
VCCA = VDDP = 5V
I
I
V
Q
f
f
VBAT
VBST
D
D
T
P
VCCA
VDDP
VCCA
VDDP
x
1
2
A
A
D
JA
g
JA
g
D
gx
x
gx
1(MIN)
2(MIN)
J
= switching frequency (kHz)
V
V
= 250kHz
= 300kHz
= gate drive voltage, typically 5V (V)
= 85°C
= 5V
= ambient temperature (°C)
= power dissipation in (W)
= duty cycle
= thermal impedance junction to ambient from
= 84°C/W
= FET gate charge, from the FET datasheet (C)
= 60nC
g
g
= operating current (A)
= 1100µA (data sheet maximum)
= gate drive operating current (A)
= 150µA (data sheet maximum)
(MIN)
T
2
(MIN)
A
= 1.5/8 = 0.1875
= 1.2/8 = 0.15
Q
Q
g
g
= 8V
VCCA
1
= VBAT
2
P
D
f
f
1
2
JA
VBST
VBST
(MIN)
I
VCCA
+VDDP = 13V
C
1
1
VDDP
mA
mA
D
D
1
2
I
VDDP
ON
W
(V)
(MIN)
condition (since
www.semtech.com
SC483