sc2441a Semtech Corporation, sc2441a Datasheet - Page 35

• Manufacturer Part Number: sc2441a
• Description: Sc2441a 1.8v To 20v Input 2-phase Synchronous Step-down Controllers With Step-up Converter
• Manufacturer: Semtech Corporation

20 log G vc f ( ) C f ( )
arg G vc f ( ) C f ( )
The resulting crossover frequency is about 49.2kHz with
phase margin 90
If the circuit noise makes the converter jitter, a larger C
than the calculated value can be used. Effectively the
converter bandwidth is reduced to reject high frequency
noises. The final circuit should be checked for stability
under load transients at different line voltages. The load
transient also needs to be measured to ensure that the
output voltage is within the specification window.
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POWER MANAGEMENT
Applications Information
(
2006 Semtech Corp.
(
Figure 25. Bode plots of the loop response.
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-
-
73.323
17.588
90.001
94.713
)
×
180
p
)
100
50
50
90
92
94
96
0
10
10
10
10
o
.
100
100
1 . 10
1 . 10
3
3
f
f
f
1 . 10
1 . 10
4
4
1 . 10
1 . 10
5
5
3 10
3 10
´
1 . 10
´
1 . 10
5
5
6
6
3
35
PC Board Layout Issues
Circuit board layout is very important for the proper
operation of high frequency switching power converters.
A power ground plane is required to reduce ground
bounces. The followings are suggested for proper layout.
Power Stage
1) Separate the power ground from the signal ground. In
SC2441A the power ground PGND1 should be tied to
the source terminal of lower MOSFETs. The signal ground
AGND should be tied to the negative terminal of the
output capacitor (output return terminal).
2) Minimize the size of pulse current loop. Place the top
MOSFET, the bottom MOSFET and the input capacitors
close to each other with short and wide traces. In addition
to the aluminum energy storage capacitors, add multi-
layer ceramic (MLC) capacitors from the input to the power
ground to improve high frequency bypass.
3) Reduce high frequency voltage ringing. Widen and
shorten the drain and source traces of the MOSFETs to
reduce stray inductances. Add a small RC snubber if
necessary to reduce the high frequency ringing at the
phase node. Sometimes slowing down the gate drive
signal also helps in reducing the high frequency ringing at
the phase node.
4) Shorten the gate driver path. Integrity of the gate drive
(voltage level, leading and falling edges) is important for
circuit operation and efficiency. Short and wide gate drive
traces reduce trace inductances. Bond wire inductance
is about 2~3nH. If the length of the PCB trace from the
gate driver to the MOSFET gate is 1 inch, the trace
inductance will be about 25nH. If the gate drive current
is 2A with 10ns rise and falling times, the voltage drops
across the bond wire and the PCB trace will be 0.6V and
5V respectively. This may slow down the switching
transient of the MOSFETs. These inductances may also
ring with the gate capacitance.
5) Put the decoupling capacitor for the gate drive power
supplies (BST and VCC) close to the IC and power ground.
SC2441A
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