1766 查看數據表（PDF） - Linear Technology

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1766

5.5V to 60V 1.5A, 200kHz Step-Down Switching Regulator

Linear Technology 

LT1766/LT1766-5

APPLICATIONS INFORMATION

frequency decreases roughly linearly down to a limit

of about 40kHz. 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 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 0.05Ω 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 capaci-

tors 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 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 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 125mARMS. The formula to calculate

this is:

Output capacitor ripple current (RMS):

IRIPPLE(RMS)

=

0.29(VOUT)(VIN −

(L)(f)(VIN)

VOUT

)

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 choos-

ing 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 response

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 = 2.2μF,

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

Application Note 19 for further detail on techniques for

proper loop compensation.

1766fc

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