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SC4525FEVB Datasheet(PDF) 16 Page - Semtech Corporation

Part # SC4525FEVB
Description  18V, 3A, 350kHz Step-Down Switching Regulator
PDF  23 Pages
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Manufacturer  SEMTECH [Semtech Corporation]
Direct Link  http://www.semtech.com
Logo SEMTECH - Semtech Corporation

SC4525FEVB Datasheet(HTML) 16 Page - Semtech Corporation

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SC4525F
6
Applications Information (Cont.)
Place the compensator zero, F
Z, between 0% and
20% of the crossover frequency, F
C.
Use the compensator pole, F
P, to cancel the ESR zero,
F
Z.
Then, the parameters of the compensation network
can be calculated by the following equations.
m
7
g
10
R
20
C
A
=
7
Z1
5
R
F
π
2
1
C =
7
P1
8
R
F
π
2
1
C =
where gm=0.3mA/V is the EA gain of the SC4525F.
Example: Determine the voltage compensator for an
350kHz, 2V to 3.3V/3A converter with 47uF ceramic
output capacitor.
Choose a loop gain crossover frequency of 35kHz, and
place voltage compensator zero and pole at F
Z=7kHz
(20% of F
C), and FP= 677kHz. From the equation in step
2, the required compensator gain at F
C is shown by the
following equation.
Then the compensator parameters are
7.4k
10
3
0.
10
R
3
7
20
7
=
×
=
−
nF
1
.
3
10
4
.
7
10
7
2π
1
C
3
3
5
=
×
×
×
×
=
32pF
10
4
.
7
10
677
π
2
1
C
3
3
8
=
×
×
×
×
=
Select R
7=7.32k, C5=3.3nF, and C8= 33pF for the design.
Compensator parameters for various typical applications
are listed in Table 4.
3.
4.
5.
Thermal Considerations
For the power transistor inside the SC4525F, the
conduction loss P
C, the switching loss PSW, and bootstrap
circuit loss P
BST, can be estimated using the following.
O
CESAT
C
I
V
D
P
×
×
=
SW
O
IN
S
SW
F
I
V
t
2
1
P
×
×
×
×
=
40
I
V
D
P
O
BST
BST
×
×
=
whereV
BST is the BST supply voltage and tS is the equivalent
switching time of the NPN transistor (see Table 3).
Table 3 — Typical switching time
In addition, the quiescent current loss is
2mA
V
P
IN
Q
×
=
The total power loss of the SC4525F is therefore
Q
BST
SW
C
TOTAL
P
P
P
P
P
=
+
+
=
The temperature rise of the SC4525F is the product of
the total power dissipation (previous equation) and q
JA
(36oC/W), which is the thermal impedance from junction
to ambient for the SOIC-8 EDP package.
It is not recommended to operate the SC4525F above
25oC junction temperature.



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