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LT3430/LT3430-1

34301fa

where:

f = switching frequency

tON = switch minimum on time

VF = diode forward voltage

VIN = Input voltage

I • R = inductor I • R voltage drop

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/LT3430-1 clock

frequencies of 200KHz/100kHz, a VIN of 40V and a (VF +

I • R) of say 0.7V, the maximum tON to maintain control

would be approximately 90ns for the LT3430 and 180ns

for the LT3430-1, unacceptably short times.

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

lator 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

frequency decreases roughly linearly down to a limit of

about 40kHz. (30kHz for LT3430-1) This lower oscillator

frequency during short-circuit 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 for the LT3430 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 0.05Ω to 0.2Ω.

A typical output capacitor is an AVX type TPS, 100µF at

10V, with a guaranteed ESR less than 0.1Ω (The LT3430-1

will typically use two of these capacitors in parallel). 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 22µF to greater than 500µF work well, but you cannot

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

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 current 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

APPLICATIONS INFORMATION

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)

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