1765EFE-3.3 Ver la hoja de datos (PDF) - Linear Technology

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componentes Descripción

Fabricante

1765EFE-3.3

Monolithic 3A, 1.25MHz Step-Down Switching Regulator

Linear Technology 

LT1765/LT1765-1.8/LT1765-2.5/

LT1765-3.3/LT1765-5

APPLICATIONS INFORMATION

Table 2. 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)

Figure 3 shows a comparison of output ripple for a ceramic

and tantalum capacitor at 200mA ripple current.

VOUT USING 47μF, 0.1Ω

TANTALUM CAPACITOR

(10mV/DIV)

VOUT USING 2.2μF

CERAMIC CAPACITOR

(10mV/DIV)

VSW

(5V/DIV)

0.2μs/DIV

1765 F03

Figure 3. Output Ripple Voltage Waveform

( ) IOUT MAX =

Continuous Mode

IP

–

(VOUT)(VIN – VOUT)

2(L)(f)(VIN)

For VIN = 8V, VOUT = 5V and L = 3.3μH,

( )( ) IOUT(MAX) = 3−

2

(5)(8− 5)

3.3 • 10−6 1.25 • 106

(8 )

= 3 − 0.23 = 2.77A

Note that the worst case (minimum output current avail-

able) condition is at the maximum input voltage. For the

same circuit at 15V, maximum output current would be

only 2.6A.

Inductor Selection

The output inductor should have a saturation current rating

greater than the peak inductor current set by the current

comparator of the LT1765. The peak inductor current will

depend on the output current, input and output voltages

and the inductor value:

( ) ( ) IPEAK

=

IOUT

+

VOUT VIN − VOUT

2(L)(f) VIN

INDUCTOR CHOICE AND MAXIMUM OUTPUT

CURRENT

Maximum output current for an LT1765 buck converter is

equal to the maximum switch rating (IP) minus one half

peak to peak inductor ripple current. The LT1765 main-

tains a constant switch current rating at all duty cycles.

(Patent Pending)

For most applications, the output inductor will be in the

1μH to 10μH range. Lower values are chosen to reduce

the physical size of the inductor, higher values allow higher

output currents due to reduced peak to peak ripple current.

The following formula gives maximum output current for

continuous mode operation, implying that the peak to peak

ripple (2x the term on the right) is less than the maximum

switch current.

VIN = Maximum input voltage

f = Switching frequency, 1.25MHz

If an inductor with a peak current lower than the maximum

switch current of the LT1765 is chosen a soft-start circuit

in Figure 10 should be used. Also, short-circuit conditions

should not be allowed because the inductor may saturate

resulting in excessive power dissipation.

Also, consideration should be given to the resistance

of the inductor. Inductor conduction loses are directly

proportional to the DC resistance of inductor. Sometime,

the manufacturers will also provide maximum current

rating based on the allowable losses in the inductor. Care

should be taken, however. At high input voltages and low

DCR, excessive switch current could flow during shorted

output condition.

Suitable inductors are available from Coilcraft, Coiltronics,

Dale, Sumida, Toko, Murata, Panasonic and other

manufacturers.

1765fd

9

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