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LT1374

1374fb

APPLICATIONS INFORMATION

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. Schot-

tky 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 abso-

lute 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 LT1374 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 a resistive snubber will 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 time of the current is 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 don’t get hung up on 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 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 manufacturer to

guarantee power sharing. The actual value of the capacitor

in microfarads is not particularly important because at

500kHz, any value above 5

µF is essentially resistive. RMS

ripple current rating is the critical parameter. Actual RMS

current can be calculated from:

RIPPLE RMS

OUT

( ) =−

()/ 2

5V/DIV

Figure 7. Switch Node Resonance

20ns/DIV

1374 F07

INDUCTOR

CURRENT

20ns/DIV

1375/76 F11

0.5

µs/DIV

1374 F08

Figure 8. Discontinuous Mode Ringing

5V/DIV

100mA/DIV

SWITCH NODE

VOLTAGE

RISE AND FALL

WAVEFORMS ARE

SUPERIMPOSED

(PULSE WIDTH IS

NOT 120ns)

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