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参数资料
| 型号： | SC1403ITSTRT |
| 元件分类： | 稳压器 |
| 英文描述： | 1 A DUAL SWITCHING CONTROLLER, 380 kHz SWITCHING FREQ-MAX, PDSO28 |
| 封装： | LEAD FREE, MO-153AE, TSSOP-28 |
| 文件页数： | 4/31页 |
| 文件大小： | 590K |
| 代理商： | SC1403ITSTRT |
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12
2004 Semtech Corp.
SC1403
PRELIMINARY
POWER MANAGEMENT
United States Patent No. 6,377,032
www.semtech.com
Applications Information (Cont.)
C
R1
R2
L
RL
CSH
CSL
VO
VLX
(Phase node)
(Output)
The equation for the current sense signal, CSH - CSL, is given by:
1)
(EQ
R1
R2
sC
R1
R2
)
V
(V
V
o
LX
CSL)
(CSH
+
=
where (V
LX - Vo) is the voltage across the inductor terminals. The
values for C, R2, and R1 can be found by comparing the above
circuit with resistive sensing, which is shown below.
Rsns
sL
VLX
RL
Vo
(Output)
(Phase node)
CSH
CSL
With resistive sensing, the current sense signal CSH-CSL can be
written in the complex s-domain as:
2)
(EQ
sL
RL
Rsns
V
o
LX
CSL
CSH
+
=
)
(
)
(
where (V
LX - Vo) is the voltage across the inductor terminals. Note
the similarity between EQ 1 and EQ 2. By choosing proper values
for C, R1, and R2, the current-sense voltage (CSH-CSL) will track
the inductor current.
The following equations determine C, R1, and R2:
55mV
C
ILpk
L
R1
=
()
55mV
RL
ILpk
55mV
R1
R2
=
The recommended value for C is 1.0uF. RL inductor resistance is
specified at 11.6 mohm typical. 55mV is the current sense thresh-
old. For the reference design, the values are set to C = 1uF and
R1 = R2 = 1.3K. This sets the current limit to approximately 10A.
Two guidelines must be used when selecting C, R1, and R2:
The values of R2 and R1 should not exceed approximately
1.5Kohm. The bias current from the CSH input flows through these
resistors and creates an error term.
The value for R1 should not be too small due to power consider-
ations. During a switching cycle, the voltage across R1 is either
(Vin - Vo) or (-Vo). This creates a power loss in R1: the power loss
can be determined by:
()
mW
45
k
1
R1
Vo
-
Vin
Vo
P 1
R
=
=
=
3
.
3
.
3
21
3
.
3
Choosing the Main Switching mosfet
The IRF7143 is used in the reference design. Before choosing the
main (high-side) mosfet, we need to check three parameters: volt-
age, power, and current rating.
The maximum drain to source voltage of the mosfet is mainly
determined by the switcher topology. Since this is a buck topology,
V
21
V
MAX
_
IN
MAX
_
DS
=
The IRF7413 is a 30V device, which allows for 70% derating at
21V operation.
The mosfet power dissipation has three components: conduction
losses, switching losses, and gate drive losses. The conduction
loss is determined using the RMS mosfet current; the equation is
shown below. The mosfet current is a trapezoid waveform with
values equal to:
2
L
LOAD
MIN
I
=
2
L
LOAD
MAX
I
+
=
L
fs
D
Vo
IL
=
)
1
(
Vin
Vo
D
=
()
2
MAX
MIN
RMS
I
D
I
+
+
=
As input voltage decreases, the duty cycle increases and the ripple
current decrease, and overall the RMS mosfet current will increase.
The conduction losses are then given by the formula below, where
Rds(on) is 18m-ohm for the IRF7413 at room temperature. Note
that Rds(on) increases with temperature.
2
)
(
RMS
on
ds
CONDUCTION
I
R
P
=
The mosfet switching loss is estimated according to:
G
OUT
S
IN
RSS
SWITCHING
I
f
V
C
P
=
2
Crss is the reverse transfer capacitance of the mosfet, which is
240pF for IRF7413. Ig is the gate driver current, which is 1A for
SC1403.
The mosfet gate drive loss is estimated from:
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