SLUU131A – September 2002 – Revised February 2003
9
TPS40001 Based Converter Delivers 10-A Output
Figure 2 is presents the measured loop gain and phase characteristics. At the loop crossover frequency of
20 kHz the phase margin is approximately 50 degrees.
Gain – db
GAIN AND PHASE MARGIN
vs
FREQUENCY
Phase
– degrees
–40
–30
–20
–10
0
10
20
30
40
50
30
–200
–150
–100
–50
0
50
100
150
200
100
1000
10000
100000
1000000
Phase
Gain
Frequency – Hz
Figure 2.
4.8
Snubber Component Selection
The switch node where Q1 and L1 come together is very noisy. An R-C network fitted between this node and
ground can help reduce ringing and voltage overshoot on Q2. This ringing noise should be minimized to prevent
it from confusing the control circuitry which is monitoring this node for current limit, Predictive Gate Drive
t
, and
DCM control functions.
As a starting point, the snubber capacitor, C12, is generally chosen to be 5 to 8 times larger than the parasitic
capacitance at the node, which is primarily C
OS
of Q2. Since C
OS
is around 1600 pF for Q2 at 5 V, C12 is chosen
to be 10 nF. R3 is empirically determined to be 2.2
Ω
, which minimizes the ringing and overshoot at the switch
node. With the relatively low input voltage of 5 V, the power loss,
½
CV
2
f, is relatively small at 37 mW.
