LT1578 Linear Technology, LT1578 Datasheet - Page 17

• Manufacturer Part Number: LT1578
• Description: 1.5A/ 200kHz Step-Down Switching Regulator
• Manufacturer: Linear Technology

APPLICATIONS
PARASITIC RESONANCE
Resonance or “ringing” may sometimes be seen on the
switch node (see Figure 7). Very high frequency ringing
following the switch voltage rise time is caused by switch/
diode/input capacitance lead inductance and diode ca-
pacitance. Schottky diodes have very high “Q” junction
capacitance that can ring for many cycles when excited at
high frequency. If 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.
50mA/DIV
5V/DIV
5V/DIV
Figure 8. Discontinuous Mode Ringing
Figure 7. Switch Node Response
U
50ns/DIV
1 s/DIV
INFORMATION
U
W
1578 F07
1578 F08
INDUCTOR
CURRENT
RISE AND FALL
WAVEFORMS ARE
SUPERIMPOSED
(PULSE WIDTH IS
NOT 350ns)
SWITCH NODE
VOLTAGE
U
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 LT1578 has special circuitry
inside which mitigates this problem, but negative voltages
over 1V 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
10 MHz. This ringing is not harmful to the regulator and it
has not been shown to contribute significantly to EMI. Any
attempt to damp it with an RC snubber will slightly degrade
efficiency.
INPUT BYPASSING AND VOLTAGE RANGE
Input Bypass Capacitor
Step-down converters draw current from the input supply
in pulses. The average height of these pulses is equal to
load current, and the duty cycle is equal to V
and fall times of the current are very fast. A local bypass
capacitor across the input supply is necessary to ensure
proper operation of the regulator and minimize the ripple
current fed back into the input supply. The capacitor also
forces switching current to flow in a tight local loop,
minimizing EMI .
Do not cheat on the ripple current rating of the input
bypass capacitor, but also do not be overly concerned with
the value in microfarads . The input capacitor is intended
to absorb all the switching current ripple, which can have
an RMS value as high as one half of the load current. Ripple
current ratings on the capacitor must be observed to
ensure reliable operation. In many cases it is necessary to
parallel two capacitors to obtain the required ripple rating.
Both capacitors must be of the same value and manufac-
turer to guarantee power sharing. The actual value of the
capacitor in microfarads is not particularly important
LT1578/LT1578-2.5
OUT
/ V
17
IN
. Rise