lt3430-1 Linear Technology Corporation, lt3430-1 Datasheet - Page 18

• Manufacturer Part Number: lt3430-1
• Description: High Voltage, 3a, 100khz Step-down Switching Regulator
• Manufacturer: Linear Technology Corporation

LT3430/LT3430-1
APPLICATIONS INFORMATION
The V
possible from the switch and boost nodes. The LT3430/
LT3430-1 pinout has been designed to aid in this. The
ground for these components should be separated from
the switch current path. Failure to do so will result in poor
stability or subharmonic like oscillation.
Board layout also has a signifi cant effect on thermal
resistance. Pins 1, 8, 9 and 16, GND, should be soldered
to a continuous copper ground plane under the LT3430/
LT3430-1 die. The FE package has an exposed pad (Pin
17) which is the best thermal path for heat out of the
package. Soldering the exposed pad to the copper ground
plane under the device will reduce die temperature and
increase the power capability of the LT3430/LT3430-1. Add-
ing multiple solder fi lled feedthroughs under and around
the four corner pins to the ground plane will also help.
Similar treatment to the catch diode and coil terminations
will reduce any additional heating effects.
PARASITIC RESONANCE
Resonance or “ringing” may sometimes be seen on the
switch node (see Figure 7). Very high frequency ringing
following switch rise time is caused by switch/diode/input
capacitor lead inductance and diode capacitance. Schottky
diodes have very high “Q” junction capacitance that can
ring for many cycles when excited at high frequency. If
18
C
and FB components should be kept as far away as
2V/DIV
Figure 7. Switch Node Resonance
SW RISE
50ns/DIV
SW FALL
3430 F07
total lead length for the input capacitor, diode and switch
path is 1 inch, the inductance will be approximately 25nH.
At switch off, this will produce a spike across the NPN
output device in addition to the input voltage. At higher
currents this spike can be in the order of 10V to 20V
or higher with a poor layout, potentially exceeding the
absolute max switch voltage. The path around switch,
catch diode and input capacitor must be kept as short as
possible to ensure reliable operation. When looking at this,
a >100MHz oscilloscope must be used, and waveforms
should be observed on the leads of the package. This
switch off spike will also cause the SW node to go below
ground. The LT3430/LT3430-1 have special circuitry inside
which mitigates this problem, but negative voltages over
0.8V lasting longer than 10ns should be avoided. Note that
100MHz oscilloscopes are barely fast enough to see the
details of the falling edge overshoot in Figure 7.
A second, much lower frequency ringing is seen during
switch off time if load current is low enough to allow the
inductor current to fall to zero during part of the switch off
time (see Figure 8). Switch and diode capacitance reso-
nate with the inductor to form damped ringing at 1MHz
to 10MHz. This ringing is not harmful to the regulator
and it has not been shown to contribute signifi cantly to
EMI. Any attempt to damp it with a resistive snubber will
degrade effi ciency.
10mV/DIV
0.2A/DIV
V
V
L = 22µH
LT3430
IN
OUT
Figure 8. Discontinuous Mode Ringing
= 40V
= 5V
1µs/DIV
3430 F08
SWITCH NODE
VOLTAGE
INDUCTOR
CURRENT AT
I
OUT
= 0.1A
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