LT1376CS8 datasheet(17/28 Pages) LINER

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LT1375/LT1376

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.

Schottky diode capacitance of 100pF will create a reso-

nance at 100MHz. This ringing is not harmful to the

LT1376 and can normally be ignored.

Overshoot or ringing following switch fall time is created

by switch capacitance rather than diode capacitance. This

ringing per se is not harmful, but the overshoot can cause

problems if the amplitude becomes too high. The negative

voltage can forward bias parasitic junctions on the IC chip

and cause erratic switching. The LT1376 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. Again, 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. 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

The term inside the radical has a maximum value of 0.5

when input voltage is twice output, and stays near 0.5 for

a relatively wide range of input voltages. It is common

5V/DIV

Figure 7. Switch Node Resonance

20ns/DIV

1375/76 F07

20ns/DIV

1375/76 F11

0.5

µs/DIV

1375/76 F08

Figure 8. Discontinuous Mode Ringing

5V/DIV

100mA/DIV

RISE AND FALL

WAVEFORMS ARE

SUPERIMPOSED

(PULSE WIDTH IS

NOT 120ns)

SWITCH NODE

VOLTAGE

INDUCTOR

CURRENT

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