LTC1265CS datasheet(12/16 Pages) LINER

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LTC1265/LTC1265-3.3/LTC1265-5

µH

0.125

Ω

VOUT

3.3V

0.8A

COUT

1000pF

100pF

3900pF

0.1

µF

CIN

PWR VIN

VIN

PGND

SENSE+

SENSE–

SGND

ITH

LTC1265 F08

SHDN

LTC1265-3.3

VIN

APPLICATIONS INFORMATION

Figure 8. Design Example Circuit

Figure 9. Design Example Efficiency Curve

Design Example

As a design example, assume VIN = 5V, VOUT = 3.3V, IMAX

= 0.8A and f = 250kHz. With this information we can easily

calculate all the important components.

From (1),

RSENSE = 100mV/0.8 = 0.125Ω

From (2) and assuming VD = 0.4V,

CT ≅ 100pF

Using (3), the value of the inductor is:

≥ 5.2(105)(0.125)(100pF)3.3V = 22µH

For the catch diode, a MBRS130LT3 or 1N5818 will be

sufficient in this application.

CIN will require an RMS current rating of at least 0.4A at

temperature, and COUT will require an ESR of (from 5):

ESRCOUT < 0.25Ω

The inductor ripple current is given by:

IRIPPLE

VOUT + VD

)

tOFF = 0.22A

At light loads the peak inductor current is at:

IPEAK = 25mV/0.125 = 0.2A

Therefore, at load current less than 0.1A the LTC1265 will

be in Burst Mode operation. Figure 8 shows the complete

circuit and Figure 9 shows the efficiency curve with the

above calculated component values.

LOAD CURRENT (mA)

0.01

100

0.1

1.0

1265 G11

L = DALE LPT4545-220 (22

µH)

VOUT = 3.3V

CT = 100pF

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