LT3493E-3 查看數據表（PDF） - Linear Technology

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LT3493E-3

1.2A, 750kHz Step-Down Switching Regulator in 2mm × 3mm DFN

Linear Technology 

LT3493-3

APPLICATIO S I FOR ATIO

Table 2. Capacitor Vendors

Vendor

Phone

Panasonic (714) 373-7366

Kemet

Sanyo

(864) 963-6300

(408) 749-9714

Murata

AVX

(404) 436-1300

Taiyo Yuden (864) 963-6300

URL

www.panasonic.com

www.kemet.com

www.sanyovideo.com

www.murata.com

www.avxcorp.com

www.taiyo-yuden.com

Part Series

Ceramic,

Polymer,

Tantalum

Ceramic,

Tantalum

Ceramic,

Polymer,

Tantalum

Ceramic

Ceramic,

Tantalum

Ceramic

Comments

EEF Series

T494, T495

POSCAP

TPS Series

where COUT is in µF. Use X5R or X7R types and keep in

mind that a ceramic capacitor biased with VOUT will have

less than its nominal capacitance. This choice will provide

low output ripple and good transient response. Transient

performance can be improved with a high value capacitor,

but a phase lead capacitor across the feedback resistor R1

may be required to get the full benefit (see the Compen-

sation section).

For small size, the output capacitor can be chosen accord-

ing to:

COUT = 25/VOUT

where COUT is in µF. However, using an output capacitor

this small results in an increased loop crossover fre-

quency and increased sensitivity to noise. A 22pF capaci-

tor connected between VOUT and the FB pin is required to

filter noise at the FB pin and ensure stability.

High performance electrolytic capacitors can be used for

the output capacitor. Low ESR is important, so choose

one that is intended for use in switching regulators. The

ESR should be specified by the supplier and should be

0.1Ω or less. Such a capacitor will be larger than a

ceramic capacitor and will have a larger capacitance, be-

cause the capacitor must be large to achieve low ESR.

Table 2 lists several capacitor vendors.

Figure 4 shows the transient response of the LT3493-3

with several output capacitor choices. The output is 3.3V.

The load current is stepped from 250mA to 1A and back to

250mA, and the oscilloscope traces show the output

voltage. The upper photo shows the recommended value.

The second photo shows the improved response (less

voltage drop) resulting from a larger output capacitor

and a phase lead capacitor. The last photo shows the

response to a high performance electrolytic capacitor.

Transient performance is improved due to the large output

capacitance.

BOOST Pin Considerations

Capacitor C3 and diode D2 are used to generate a boost

voltage that is higher than the input voltage. In most cases

a 0.1µF capacitor and fast switching diode (such as the

1N4148 or 1N914) will work well. Figure 5 shows two

ways to arrange the boost circuit. The BOOST pin must be

at least 2.3V above the SW pin for best efficiency. For

outputs of 3.3V and above, the standard circuit (Figure 5a)

is best. For outputs between 3V and 3.3V, use a 0.22µF

capacitor. For outputs between 2.5V and 3V, use a 0.47µF

capacitor and a small Schottky diode (such as the BAT-54).

For lower output voltages the boost diode can be tied to the

input (Figure 5b). The circuit in Figure 5a is more efficient

because the BOOST pin current comes from a lower

voltage source. You must also be sure that the maximum

voltage rating of the BOOST pin is not exceeded.

The minimum operating voltage of an LT3493-3 applica-

tion is limited by the undervoltage lockout (6.8V) and by

the maximum duty cycle as outlined above. For proper

start-up, the minimum input voltage is also limited by the

boost circuit. If the input voltage is ramped slowly, or the

LT3493-3 is turned on with its SHDN pin when the output

3493-3f

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