SC1403(2002) データシートの表示（PDF） - Semtech Corporation
部品番号
コンポーネント説明
メーカー
SC1403 Datasheet PDF : 30 Pages
| |||
SC1403
POWER MANAGEMENT
Applications Information
PRELIMINARY
where Vin_nom=15V, Vo=3.3V, D=Vo/Vin_nom, Fs=300KHz,
Ts=3.33uS and ∆Io=2.2 A. Lo is subsequently calculated to be
3.9uH. For the interest of this design, Lo is chosen to be 4.7uH.
Vendor P/N
VDS (V)
ID (A)
Rds(0n) Package
@ 4.5V
(ohm)
Choosing Current Sense Resistor
Since the SC1403 implements Virtual Current SenseTM, the exter-
nal current sense resistor is not required for the control loop. How-
ever, it is required for cycle-by-cycle current limit. Cycle-by-cycle
current limit is enabled when the voltage across the current sense
resistor exceeds 50mV nominal. Depending on the system re-
quirement, this current limit can vary, it is usually 10 to 30% higher
than the maximum load. Taking into consideration of the +/-20%
variation on the 50mV, the value of the current sense resistor can
be calculated using the following equation:
RSENSE
=
40mV(min)
IPK _ OC
For a DC OC trip point between 6.3A to 9.8A, Rsense is chosen to
be 5.5m Ω . Considering the maximum power dissipation, two
Vishay WSL2010 11m Ω 1% resistors are used.
Choosing the Main Switching MOSFET
Before choosing the main switch MOSFET, we need to know two
critical parameters: voltage and current rating. In order to mini-
mize the conduction loss, we recommend using the lowest Rds(on)
for the same voltage and current rating. The maximum drain to
source voltage of the switch MOSFET is mainly decided by the
topology of the switcher. Since this is a buck topology,
VDS _ MAX = VIN _ MAX = 21V
Applying a derating of 70%, a 30V MOSFET is used in the design.
The peak current of the MOSFET is determined by
IPEAK
=
60mV
5.5mΩ
=
11A
According to the calculated voltage and current rating, Si4886DY,
IRF7413, FDS9412 or STS12NF30L meets the requirement. The
specs for these MOSFETs are listed in the table below. For the
purpose of this exercise, STS12NF30L is chosen. Next step is to
determine its power handling capability. Based on 85 °C ambient
temperature, 150 °C junction temperature and 50 °C /W ther-
mal resistance, its power handling is calculated as follows:
Si4886DY
30
IRF7413
30
FDS9412
30
STS12NF30L
30
13 0.0135 SO-8
13
0.011
SO-8
7.9 0.036
SO-8
12 0.0085 SO-8
The following calculations are done to verify that the power dissi-
pation of the main switch MOSFET is well within 1.86W, which is
the maximum allowable power dissipation for the package.
PTOTAL _ DISS = PCONDUCTION + PSWITCHING + PGATE
PCONDUCTION = Rds(on) ⋅ IRMS2 ⋅ Dnom
where Rds(on) = 0.01 Ω @Tj=25 °C and Vgs = 4.5V. In order to
find Rds(on)@ Tj=100 °C , use 1.40*Rds(on)@25 °C . Therefore,
Rds(on) @ Tj = 100 °C is equal to 0.014 Ω .
IRMS =
where
(I1 2 + I1 ⋅I2 + I22 )
3
I1
=
IMAX
+
∆IO _ MAX
2
=
7.1A,
I2 = IMAX −
∆IO _ MAX = 4.9A
2
and
Dnom
=
VOUT
VIN _ NOM
The worst case conduction loss is calculated to be 112mW. And
the switching loss of the MOSFET is given by,
PSWITCHING
=
CRSS ⋅ VIN2
IG
⋅ fS ⋅IOUT
where Crss is the reverse transfer capacitance of the MOSFET; it
is equal to 200pF for STS12NF30L, Ig is the gate driver current; it
is equal to 1A for SC1403. And Vin_nom = 15V, fs = 300KHz. The
switching loss is calculated to 81mW. And the gate loss is given by,
PGATE
=
1
2
⋅ CG
⋅
V2
⋅
fS
TJ = 150°C;
TA = 85°C;
θ
=
JA
50°C/W
where Cg=11nF, V=5V and fs=300KHz. The gate loss is calcu-
lated to be 41mW.
PT
=
TJ − TA
θ JA
=
150 −
50
85
= 1.30W
2002 Semtech Corp.
So the total power dissipation is calculated to be 234mW and is
well within the maximum power dissipation allowance of the
MOSFET. No special heating sinking is required when laying out
the MOSFET.
12
www.semtech.com
Share Link: 