LT3435 查看數據表（PDF） - Linear Technology

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LT3435

High Voltage 3A, 500kHz Step-Down Switching Regulator with 100µA Quiescent Current

Linear Technology 

LT3435

APPLICATIO S I FOR ATIO

taken to ensure the ripple and surge ratings are not

exceeded. The AVX TPS and Kemet T495 series are surge

rated AVX recommends derating capacitor operating volt-

age by 2:1 for high surge applications.

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 higher ESR. The ESR

range for typical LT3435 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 could be unacceptable. Table 2 shows some

typical solid tantalum surface mount capacitors.

Table 2. Surface Mount Solid Tantalum Capacitor ESR

and Ripple Current

E CASE SIZE

ESR MAX (Ω)

RIPPLE CURRENT (A)

AVX TPS

0.1 to 0.3

0.7 to 1.1

D CASE SIZE

AVX TPS

0.1 to 0.3

0.7 to 1.1

C CASE SIZE

AVX TPS

0.2

0.5

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)

( )( ) 0.29 VOUT

IRIPPLE(RMS) =

VIN – VOUT

= IP-P

( )( )( ) L f VIN

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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). Low ESR reduces output

ripple voltage but also removes a useful zero in the loop

frequency response, common to tantalum capacitors. To

compensate for this a resistor RC can be placed in series

with the VC compensation capacitor CC (Figure 10). 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, along

with a small feedforward capacitor CFB, is suggested to

control possible ripple at the VC pin.

OUTPUT RIPPLE VOLTAGE

Figure 3 shows a typical output ripple voltage waveform

for the LT3435. Ripple voltage is determined by the

impedance of the output capacitor and ripple current

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

the inductor into the output capacitor is:

( ) VOUT VIN – VOUT

IP-P = (VIN)(L)(f)

3435f

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