SC1403ITSTRT View Datasheet(PDF) - Semtech Corporation
Part Name
Description
MFG CO.
'SC1403ITSTRT' PDF : 31 Pages View PDF
SC1403
POWER MANAGEMENT
Applications Information (Cont.)
Designing the Loop
There are two aspects concerning the loop design. One is the
power train design and the other is the external compensation
design. A good loop design is a combination of the two. In the
SC1403, the control-to-output/power train response is dominated
by the load impedance, the effective current sense resistor, out-
put capacitance, and the ESR of the output caps. The low fre-
quency gain is dominated by the output load impedance and the
effective current sense resistor. Inherent to Virtual Current SenseTM,
there is one additional low frequency pole sitting between 100Hz
and 1kHz and a zero between 15kHz and 25kHz. To compensate
for the SC1403 is easy since the output of error amplifier COMP
pin is available for external compensation. A traditional pole-zero-
pole compensation is not necessary in the design using SC1403.
To ensure high phase margin at crossover frequency while mini-
mizing the component count, a simple high frequency pole is usu-
ally sufficient. In the reference design below, single-pole compen-
sation method is demonstrated. And the loop measurement re-
sults are compared to those obtained from the simulation model.
Transient response is also done to validate the model. Also, to
help speed up the design process, a list of recommended output
caps vs compensation caps value is given in table I.
Single-Pole Compensation Method
Given parameters:
Vin = 19V, Vout = 3.3V @ 2.2A,
Output impedance, Ro = 3.3V/2.2A = 1.5 Ω ,
Panasonic SP cap, Co = 180uF, Resr = 15 mΩ ,
Output inductor, Lo = 4.7uH
Switching frequency, Fs = 300kHz
PRELIMINARY
200
150
100
50
0
-50
-100
-150
-200
100
1000
f (Hz)
10000
100000
Measured Control-to-Output gain & phase response (up to
100kHz) is plotted below.
50
40
30
20
10
0
-10
-20
-30
-40
-50
100
1000
10000
f (Hz)
100000
Simulated Control-to-Output gain & phase response (up to
100kHz) is plotted below.
200
150
50
100
40
50
30
20
0
10
-50
0
-100
-10
-150
-20
-30
-200
100
1000
10000
100000
-40
f (Hz)
-50
100
1000
10000
100000
f (Hz)
Single-pole compensation of the error amplifier is achieved by
connecting a 100pF capacitor from the COMP pin of the SC1403
to ground. The simulated feedback gain & phase response (up to
100kHz) is plotted below.
2004 Semtech Corp.
14
United States Patent No. 6,377,032
www.semtech.com
Share Link: 
