sc1470 Semtech Corporation, sc1470 Datasheet - Page 13

• Manufacturer Part Number: sc1470
• Description: Synchronous Buck Pseudo Fixed Frequency Power Supply Controller
• Manufacturer: Semtech Corporation

660µF may be used. Alternatively, one 15m
220µF, 330µF or 470µF capacitor may be used (with
the appropriate change to the calculation for C
depending upon 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 sufficient ripple
current rating for this RMS current, for example a 10µF,
1210 size, 25V ceramic capacitor can handle
approximately 3A
sheets.
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
Thermal Considerations
The junction temperature of the device may be
calculated as follows:
I
I
R
POWER MANAGEMENT
T
IN(RMS)
VALLEY
IN
VALLEY
DS(ON)
J
ILIM
2005 Semtech Corp.
(
RMS
T
)
= 5.13A, R
= 2.14A
A
at V
I
VALLEY
I
OUT
P
V
GS
D
OUT
= 4.5V for purposes of this calculation:
I
RMS
RIPPLE
1
JA
2 .
DS(ON)
V
BAT
RMS
_
2
VBAT
C
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
or 12m
TOP
),
13
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 = switching frequency (kHz)
VBST = boost pin voltage during t
D = duty cycle
Inserting the following values for VBAT
this is the worst case condition for power dissipation in
the controller) as an example (VOUT = 1.2V):
T
VCCA = VDDP = 5V
I
I
V
Q
f = 266kHz
VBAT
VBST
D
gives us:
and:
P
P
T
VCCA
VDDP
VCCA
VDDP
A
A
D
JA
g
JA
g
D
g
g
(MIN)
D
J
5
. 0
V
= gate drive voltage, typically 5V (V)
= 85°C
= 5V
= ambient temperature (°C)
= power dissipation in (W)
= FET gate charge, from the FET datasheet (C)
= 60nC
= thermal impedance junction to ambient from
= 100°C/W
g
088
= operating current (A)
= 1100µA (data sheet maximum)
= gate drive operating current (A)
= 150µA (data sheet maximum)
(MIN)
85
(MIN)
5
VCCA
60
= 1.2/8 = 0.15
Q
1100
g
= 8V
= VBAT
10
. 0
W
f
088
9
I
VCCA
VBST
10
266
(MIN)
100
6
+VDDP = 13V
VDDP
5
10
1
mA
93
3
150
8 .
13
I
D
VDDP
10
C
1
W
ON
6
10
(V)
(MIN)
3
condition (since
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