SC4210A 데이터 시트보기 (PDF) - Semtech Corporation
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SC4210A Datasheet PDF : 11 Pages
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SC4210A
POWER MANAGEMENT
Applications Information (Cont.)
The pass transistor does add an additional pole to the
transfer function. Generally speaking, this parasitic non-
dominant pole is at frequencies well above the unity gain
frequency but should be considered when various types
of N-MOSFETs are available.
The purpose of the RBLEED is to improve the transient
response, reduce overshoot, and to remove an unwanted
output ripple voltage if no load is applied to the output.
In a practical sense, it is chosen to bleed (drain) about
100µA - 150µA. The optimum value depends on the
input/output voltage ratio and the constraints on the
output ripple voltage. Simulation analysis and real life
circuit testing have shown very close correlation.
Figure 8.
An output capacitor is not required for stability. If one is
used, compensation is required. Assuming there is no
output cap present, only Ccomp will be used. Under this
condition, the non-dominant pole and both zero’s are
pushed well above the unity gain frequency. Using only a
Ccomp = 100pF without an output capacitor, the system
yields a PM of 78.5º with a unity gain frequency of 387kHz;
see Figure 9.
Following the procedure described above yields a stable
operation and excellent transient response over a wide
range of output capacitors: extra low-ESR “ceramics” and
“organics”, mid-ESR “polymers” and “tantalums”, low-
cost aluminum capacitors. Below in Table 1 are the
summarized results of choosing RCOMP and CCOMP values
for the typical application circuit shown on Page 1.
VOUT_RIPPLE
(mVp-p)
COUT (µF)
ESR (Ω )
RCOMP (K) CCOMP (pF)
32
0.1 0.003 - 0.005 2.7
33
53
1
0.002 - 0.003 2.7
150
50
10 0.001 - 0.002 24
100
45
10
1-5
24
100
46
10
10 - 20
24
100
46
22
5 - 10
24
100
41
33 0.001 - 0.002 82
150
55
0
-
24
100
55
0
-
0
100
Table 1
Figure 9.
Test Conditions:
VIN = 3.3V, VOUT = 1.8V, IOUT = 6A/0.12A, Sr = 0.3A/µs.
2004 Semtech Corp.
9
www.semtech.com
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