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LT1507

APPLICATIONS INFORMATION

LT1507 • F04

VIN

HIGH

FREQUENCY

CIRCULATING

PATH

LOAD

SWITCH NODE

INPUT BYPASSING AND VOLTAGE RANGE

Input Bypass Capacitor

Stepdown 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 essential resistive. Ripple

current rating is the critical parameter. RMS ripple current

can be calculated from:

RMS

RIPPLE

OUT

()

(–

)

high speed circulating current path shown in Figure 4 and

to make connections to the output capacitor in a manner

that minimizes output ripple and noise. For more details,

see Applications Information section in the LT1376 data

sheet.

Figure 4. High Speed Switching Path

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

practice, therefore, to simply use the worst-case value and

assume that RMS ripple current is one half of load current.

At maximum output current of 1.5A for the LT1507, the

input bypass capacitor should be rated at 0.75A ripple

current. Note however, that there are many secondary

considerations in choosing the final ripple current rating.

These include ambient temperature, average versus peak

load current, equipment operating schedule and required

product lifetime. For more details see Application Notes 19

and 46.

Input Capacitor Type

Some caution must be used when selecting the type of

capacitor used at the input of regulators. Aluminum

electrolytics are lowest cost, but are physically large to

achieve adequate ripple current rating, and size con-

straints (especially height) may preclude their use.

Ceramic capacitors are now available in larger values and

their high ripple current and voltage rating make them

ideal for input bypassing. Cost is slightly higher and

footprint may also be somewhat larger. Solid tantalum

capacitors are a good choice except that they have a

history of occasional spectacular failures when they are

subjected to very large current surges during power-up.

The capacitors can short and then burn with a brilliant

white light and lots of nasty smoke. This phenomenon

occurs in only a small percentage of units, but it has led

some OEM companies to forbid their use in high surge

applications. The input bypass capacitor of regulators can

see such high surges when a battery or high capacitance

source is connected.

Several manufacturers have developed a line of solid

tantalum capacitors specially tested for surge capability

(AVX TPS series for instance, see Table 2). Even these

units may fail if the input current surge exceeds a value

equal to the voltage rating of the capacitor divided by 1

Ω

(10A for a 10V capacitor). For this reason, AVX recom-

mends using the highest voltage rating possible for the

input capacitor.

For equal case size, this means that lower

values of capacitance must be used. As stated above, this

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