SC1403ITSTRT View Datasheet(PDF) - Semtech Corporation
Part Name
Description
MFG CO.
'SC1403ITSTRT' PDF : 31 Pages View PDF
SC1403
POWER MANAGEMENT
Applications Information (Cont.)
(Phase node)
VLX
R1
L
CSH
C
CSL
R2
(Output)
RL
VO
PRELIMINARY
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:
PR1 =
Vo
⋅ (Vin
R1
- Vo ) =
3.3
⋅ (21 − 3.3
1 .3k Ω
)
=
45
mW
Choosing the Main Switching mosfet
The equation for the current sense signal, CSH - CSL, is given by:
V(CSH
−
CSL)
=
(VLX
−
Vo)
⋅
R2
+
R2
R1+ sC
⋅
R2
⋅
R1
(EQ 1)
where (VLX - 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.
(Phase node)
VLX
sL
CSH
CSL
RL
Rsns
(Output)
Vo
With resistive sensing, the current sense signal CSH-CSL can be
written in the complex s-domain as:
V (CSH
−
CSL)
=
(VLX
−
Vo)
⋅
Rsns
Rsns + RL
+
sL
(EQ 2)
where (VLX - 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:
R1
=
L ⋅ ILpk
C ⋅ 55mV
R2
=
R1 ⋅
(ILpk
55mV
⋅ RL − 55mV
)
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
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,
VDS _ MAX = VIN _ MAX = 21V
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:
I MIN
= I LOAD
−
∆ IL
2
I MAX
= ILOAD
+
∆IL
2
∆IL
=
Vo⋅ (1−D)
fs ⋅ L
D
=
Vo
Vin
( ) I RMS = D ⋅ I MIN 2 + I MIN ⋅ I MAX + I MAX 2
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.
PCONDUCTION = Rds(on) ⋅ IRMS2
The mosfet switching loss is estimated according to:
PSWITCHING = CRSS ⋅ VIN2 ⋅ fS ⋅ IOUT
IG
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:
2004 Semtech Corp.
12
United States Patent No. 6,377,032
www.semtech.com
Share Link: 
