Datasheet

0.47mF
0.47mF
0.47mF
TO “C”
FROM “GATE”
Z = 600W,
0.1W, 2A
TO “A”
CASE 1
0.47mF 0.47mF
0.47mF
TO “C”
FROM “GATE”
TO “A”
CASE 2
1W
TPS2419
www.ti.com SLVS998B –FEBRUARY 2010–REVISED SEPTEMBER 2011
Input Filtering
Voltage transients, converter switching noise and ripple, and ringing due to current interruptions can potentially
cause undesired on-off cycling, especially at very light loads. This includes voltage gradients (especially at MHz
frequencies) across the ground plane effecting the apparent V
(A)
and V
(C)
. The effects of these unwanted signals
can be reduced by providing input filtering as shown in Figure 16 and Figure 13. There are two potential
problems that the filter might have to help with, 1) internally generated switching noise, and 2) fast ringing
transients caused by nearby power system events. Case 2 (in Figure 16) filtering is better at suppressing internal
switching noise and Case 1 is better for large bus transients in the megahertz range. The "Z" element in CASE 1
is a high-impedance ferrite bead with low resistance to limit the dc voltage error. The L-C filter limits the apparent
V
(A)
voltage swings during high-speed transients. The L-C in series with A also causes a phase delay in sensed
steady-state switching noise, creating an apparent additional V
(AC)
.
The filter capacitors should be located close to the TPS2419's GND pin and be connected to GND by a solid
plane. The A-C capacitor should be located directly across the TPS2419 pins. These values were empirically
chosen in a particular test setup and may have to be tuned for different systems.
The waveform of Figure 17 shows turnon in the presence of 135 mVpp ripple by the circuit of Figure 13. The
ORing circuit was loaded with 10 kΩ parallel to 0.1 µF, and had only a -4.5 mV turnoff threshold. This condition is
often difficult to turn on into due to the V
(A-C)
difference that occurs when the MOSFET diode peak charges the
output. The output voltage was monitored with the oscilloscope probe ac-coupled, causing visual artifacts due to
the probe settling time. The increase in output ripple is evident as the dynamic impedance of the MOSFET diode
is shorted by the channel resistance.
Selection of the A and C sense points can also play a role in limiting unwanted turnoff events. Sensing voltages
at bus bypass capacitors may benefit operation by limiting the apparent switching and transient noise.
The TPS2419 uses C as both a voltage sense and power pin. Placing resistance in this lead will cause a
reduction in V
(C)
due to IxR voltage drop, changing the apparent turnon and turnoff thresholds.
Figure 16. Input Filtering Configurations
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