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LT3430

sn3430 3430is

APPLICATIO S I FOR ATIO

If this condition is not observed, the current will not be

limited at IPK, but will cycle-by-cycle ratchet up to some

higher value. Using the nominal LT3430 clock frequency

of 200KHz, a VIN of 40V and a (VF + I • R) of say 0.7V, the

maximum tON to maintain control would be approximately

90ns, an unacceptably short time.

The solution to this dilemma is to slow down the oscilla-

tor when the FB pin voltage is abnormally low thereby

indicating some sort of short-circuit condition. Oscillator

frequency is unaffected until FB voltage drops to about 2/3

of its normal value. Below this point the oscillator fre-

quency decreases roughly linearly down to a limit of about

40kHz. This lower oscillator frequency during short-cir-

cuit conditions can then maintain control with the effective

minimum on time.

It is recommended that for [VIN/(VOUT + VF)] ratios > 10,

a soft-start circuit should be used to control the output

capacitor charge rate during start-up or during recovery

from an output short circuit, thereby adding additional

control over peak inductor current. See Buck Converter

with Adjustable Soft-Start later in this data sheet.

OUTPUT CAPACITOR

The output capacitor is normally chosen by its effective

series resistance (ESR), because this is what determines

output ripple voltage. To get low ESR takes

volume, so

physically smaller capacitors have high ESR. The ESR

range for typical LT3430 applications is 0.05

Ω to 0.2Ω. A

typical output capacitor is an AVX type TPS, 100

µF at 10V,

with a guaranteed ESR less than 0.1

Ω. This is a “D” size

surface mount solid tantalum capacitor. TPS capacitors

are specially constructed and tested for low ESR, so they

give the lowest ESR for a given volume. The value in

microfarads is not particularly critical, and values from

µF to greater than 500µF work well, but you cannot

cheat mother nature on ESR. If you find a tiny 22

µF solid

tantalum capacitor, it will have high ESR, and output ripple

voltage will be terrible. Table 3 shows some typical solid

tantalum surface mount capacitors.

Table 3. Surface Mount Solid Tantalum Capacitor ESR

and Ripple Current

E Case Size

ESR (Max.,

Ω)

Ripple Current (A)

AVX TPS, Sprague 593D

0.1 to 0.3

0.7 to 1.1

D Case Size

AVX TPS, Sprague 593D

0.1 to 0.3

0.7 to 1.1

C Case Size

AVX TPS

0.2 (typ)

0.5 (typ)

Many engineers have heard that solid tantalum capacitors

are prone to failure if they undergo high surge currents.

This is historically true, and type TPS capacitors are

specially tested for surge capability, but surge ruggedness

is not a critical issue with the

output capacitor. Solid

tantalum capacitors fail during very high

turn-on surges,

which do not occur at the output of regulators. High

discharge surges, such as when the regulator output is

dead shorted, do not harm the capacitors.

Unlike the input capacitor, RMS ripple current in the

output capacitor is normally low enough that ripple cur-

rent rating is not an issue. The current waveform is

triangular with a typical value of 250mARMS. The formula

to calculate this is:

Output capacitor ripple current (RMS):

Lf V

RIPPLE RMS

OUT

() =

() −

()

()( )( )

029

Ceramic Capacitors

Higher value, lower cost ceramic capacitors are now

becoming available. They are generally chosen for their

good high frequency operation, small size and very low

ESR (effective series resistance). Their low ESR reduces

output ripple voltage but also removes a useful zero in the

loop frequency response, common to tantalum capaci-

tors. To compensate for this, a resistor RC can be placed

in series with the VC compensation capacitor CC. Care

must be taken however, since this resistor sets the high

frequency gain of the error amplifier, including the gain at

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