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LT1375/LT1376

APPLICATIONS INFORMATION

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 LT1376, 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, 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. Ce-

ramic 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. The highest voltage rating is 50V, so 25V

may be a practical upper limit when using solid tantalum

capacitors for input bypassing.

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.

Minimum Input Voltage (After Start-Up)

Minimum input voltage to make the LT1376 “run” cor-

rectly is typically 5V, but to regulate the output, a buck

converter input voltage must always be higher than the

output voltage. To calculate minimum operating input

voltage, switch voltage loss and maximum duty cycle

must be taken into account. With the LT1376, there is the

additional consideration of proper operation of the boost

circuit. The boost circuit allows the power switch to

saturate for high efficiency, but it also sometimes results

in a start-up or operating voltage that is several volts

higher than the standard running voltage, especially at

light loads. An approximate formula to calculate minimum

running voltage at load currents above 100mA is:

IN MIN

OUT

( ) =

()( )

088

Ω

Minimum Start-Up Voltage and Operation at

Light Loads

The boost capacitor supplies current to the BOOST pin

during switch on time. This capacitor is recharged only

during switch off time. Under certain conditions of light

load and low input voltage, the capacitor may not be

recharged fully during the relatively short off time. This

causes the boost voltage to collapse and minimum input

voltage is increased. Start-up voltage at light loads is

higher than normal running voltage for the same reasons.

The graph in Figure 9 shows minimum input voltage for a

5V output, both for start-up and for normal operation.

The circuit in Figure 10 will allow operation at light load

with low input voltages. It uses a small PNP to charge the

boost capacitor C2, and an extra diode D3 to complete the

power path from VSW to the boost capacitor.

LT1376CS Datasheet(PDF) 18 Page - Linear Technology

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