SC4502 View Datasheet(PDF) - Semtech Corporation
Part Name
Description
MFG CO.
'SC4502' PDF : 19 Pages View PDF
SC4502/SC4502H
POWER MANAGEMENT
Applications Information
D
=
1−
1−
5
12 + 0.5
0.3
12 + 0.5
=
0.62
Therefore
ωp2
≤
2
(10Ω)⋅ (10µF)
=
20Krads−1
⇒
3.18KHz
and
ωz2
≥
10Ω ⋅ (1− 0.62)2
5.3µH
= 272Krads−1 ⇒
43.4KHz
The spacing between p2 and z2 is the closest when the
converter is delivering the maximum output current from
the lowest V . This represents the worst-case
IN
compensation condition. Ignoring C5 and C6 for the
moment, C4 forms a low frequency pole with the
equivalent output resistance R of the error amplifier:
O
ωp1
=
1
ROC4
=
1
4.7MΩ • 560pF
= 380 rads−1 ⇒ 60Hz
C4 and R3 also forms a zero with angular frequency:
1
1
ωz1 = R3C4 = 59KΩ • 560pF
= 30.3 Krads−1 ⇒ 4.8KHz
The poles p1, p2 and the RHP zero z2 all increase phase
shift in the loop response. For stable operation, the overall
loop gain should cross 0dB with -20dB/decade slope. Due
to the presence of the RHP zero, the crossover frequency
should not be higher than
z2
3
. Placing z1 near p2 nulls its
effect and maximizes loop bandwidth. Thus
R3C4
≈
VOUT ⋅ C2
2 ⋅ IOUT(MAX)
(15)
RO
=
Amplifier Open Loop Gain
Transconductance
=
49dB
60µΩ−1
=
4.7MΩ
R3 determines the mid-band loop gain of the converter.
Increasing R3 increases the mid-band gain and the
crossover frequency. However it reduces the phase
margin. The values of R3 and C4 can be determined
GND
R3
C4
C6
R2
SHDN
R1 C5
VOUT
C3 R4
U1
C2
C1
D1
L1
VIN
Figure 8. Suggested PCB Layout for the SC4502/SC4502H. Notice that there is no
via directly under the device. All vias are 12mil in diameter.
2005 Semtech Corp.
13
www.semtech.com
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