LT1506CS8 datasheet(17/24 Pages) LINER

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LT1506

APPLICATIONS INFORMATION

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 4.5A for the LT1506, the

input bypass capacitor should be rated at 2.25A 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, and Design Note 95.

Input Capacitor Type

Some caution must be used when selecting the type of

capacitor used at the input to 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 fairly high and footprint

may also be somewhat large. Solid tantalum capacitors

would be a good choice, except that they have a history of

occasional spectacular failures when they are subjected to

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 these 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 3), but even these

units may fail if the input voltage surge approaches the

maximum voltage rating of the capacitor. AVX recom-

mends derating capacitor voltage by 2:1 for high surge

applications.

Larger capacitors may be necessary when the input volt-

age is very close to the minimum specified on the data

sheet. Small voltage dips during switch on time are not

normally a problem, but at very low input voltage they may

cause erratic operation because the input voltage drops

below the minimum specification. Problems can also

occur if the input-to-output voltage differential is near

minimum. The amplitude of these dips is normally a

function of capacitor ESR and ESL because the capacitive

reactance is small compared to these terms. ESR tends to

be the dominate term and is inversely related to physical

capacitor size within a given capacitor type.

SYNCHRONIZING (-SYNC Option for DD Package)

The SYNC pin, is used to synchronize the internal oscilla-

tor to an external signal. The SYNC input must pass from

a logic level low, through the maximum synchronization

threshold with a duty cycle between 10% and 90%. The

input can be driven directly from a logic level output. The

synchronizing range is equal to

initial operating frequency

up to 1MHz. This means that

minimum practical sync

frequency is equal to the worst-case

high self-oscillating

frequency (560kHz), not the typical operating frequency of

500kHz. Caution should be used when synchronizing

above 700kHz because at higher sync frequencies the

amplitude of the internal slope compensation used to

prevent subharmonic switching is reduced. This type of

subharmonic switching only occurs at input voltages less

than twice output voltage. Higher inductor values will tend

to eliminate this problem. See Frequency Compensation

section for a discussion of an entirely different cause of

subharmonic switching before assuming that the cause is

insufficient slope compensation. Application Note 19 has

more details on the theory of slope compensation.

At power-up, when VC is being clamped by the FB pin (see

Figure 2, Q2), the sync function is disabled. This allows the

frequency foldback to operate in the shorted output con-

dition. During normal operation, switching frequency is

controlled by the internal oscillator until the FB pin reaches

1.5V, after which the SYNC pin becomes operational.

THERMAL CALCULATIONS

Power dissipation in the LT1506 chip comes from four

sources: switch DC loss, switch AC loss, boost circuit

current, and input quiescent current. The following

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