LT1578CS8 datasheet(17/28 Pages) LINER

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LT1578/LT1578-2.5

APPLICATIONS INFORMATION

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.

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 VOUT/VIN. Rise

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

Figure 7. Switch Node Response

Figure 8. Discontinuous Mode Ringing

5V/DIV

50mA/DIV

50ns/DIV

1578 F07

µs/DIV

1578 F08

INDUCTOR

CURRENT

SWITCH NODE

VOLTAGE

RISE AND FALL

WAVEFORMS ARE

SUPERIMPOSED

(PULSE WIDTH IS

NOT 350ns)

5V/DIV

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