LT1616 datasheet(13/28 Pages) LINER

13 / 28 page

LT1766/LT1766-5

1766fa

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 125mARMS. 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

the switching frequency. If the gain of the error amplifier

is high enough at the switching frequency, output ripple

voltage (although smaller for a ceramic output capacitor)

may still affect the proper operation of the regulator. A

filter capacitor CF in parallel with the RC/CC network is

suggested to control possible ripple at the VC pin. An “All

Ceramic” solution is possible for the LT1766 by choosing

the correct compensation components for the given

application.

Example: For VIN = 8V to 40V, VOUT = 3.3V at 1A, the

LT1766 can be stabilized, provide good transient re-

sponse and maintain very low output ripple voltage using

the following component values: (refer to the first page of

this data sheet for component references) C3 = 2.2

µF,

RC = 4.7k, CC = 15nF, CF = 220pF and C1 = 47µF. See

Application Note 19 for further detail on techniques for

proper loop compensation.

APPLICATIO S I FOR ATIO

its normal value. Below this point the oscillator frequency

decreases roughly linearly down to a limit of about 40kHz.

This lower oscillator frequency during short-circuit condi-

tions can then maintain control with the effective mini-

mum 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 LT1766 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 2 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

LT1616 Datasheet(PDF) 13 Page - Linear Technology

LT1616 Datasheet(HTML) 13 Page - Linear Technology

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28

Datasheet Download

Link URL