LT1374-5 데이터 시트보기 (PDF) - Linear Technology

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LT1374-5

4.5A, 500kHz Step-Down Switching Regulator

Linear Technology 

LT1374

APPLICATIONS INFORMATION

Output Capacitor

The output capacitor is normally chosen by its Effective

Series Resistance (ESR), because this is what determines

output ripple voltage. At 500kHz, any polarized capacitor

is essentially resistive. To get low ESR takes volume, so

physically smaller capacitors have high ESR. The ESR

range for typical LT1374 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. 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.

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

AVX TAJ

0.7 to 0.9

0.4

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 200mARMS. The formula

to calculate this is:

Output Capacitor Ripple Current (RMS):

( ( )())(( ) ) ( ) IRIPPLE RMS

= 0.29 VOUT

Lf

VIN − VOUT

VIN

Ceramic Capacitors

Higher value, lower cost ceramic capacitors are now

becoming available in smaller case sizes. These are tempt-

ing for switching regulator use because of their very low

ESR. Unfortunately, the ESR is so low that it can cause

loop stability problems. Solid tantalum capacitor’s ESR

generates a loop “zero” at 5kHz to 50kHz that is instrumen-

tal in giving acceptable loop phase margin. Ceramic

capacitors remain capacitive to beyond 300kHz and usu-

ally resonate with their ESL before ESR becomes effective.

They are appropriate for input bypassing because of their

high ripple current ratings and tolerance of turn-on surges.

Linear Technology plans to issue a design note on the use

of ceramic capacitors in the near future.

OUTPUT RIPPLE VOLTAGE

Figure 3 shows a typical output ripple voltage waveform

for the LT1374. Ripple voltage is determined by the high

frequency impedance of the output capacitor, and ripple

current through the inductor. Peak-to-peak ripple current

through the inductor into the output capacitor is:

( ( )( )( )( ) ) IP-P =

VOUT VIN − VOUT

VIN L f

For high frequency switchers, the sum of ripple current

slew rates may also be relevant and can be calculated

from:

Σ dI = VIN

dt L

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