LTC3853_15 datasheet(30/36 Pages) LINER

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LTC3853

3853fc

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For channel 1, the DCRSENSE filter/divider values are:

R1||R2

1100

Ω

0.23

≅ 4.75k;

R1• RD

−RD

4.75k • 0.23

− 0.23

≅ 1430Ω

The power loss in R1 at the maximum input voltage is:

PLOSSR1=

(VIN(MAX) − VOUT)• VOUT

(20V

− 5V) • 5V

4.75k

= 15.8mW

The respective values for Channel 2 are R1 = 4.75k, R2 =

1430Ω; and PLOSSR1 = 11.6mW. And for Channel 3 are

R1 = 2.21k, R2 = 1.82k; and PLOSSR1 = 10.2mW.

BurstModeoperationischosenforhighlightloadefficiency

(Figure 14) by floating the MODE/PLLIN pin. Power loss

due to the DCR sensing network is slightly higher at light

loads than would have been the case with a suitable sense

resistor (9mΩ). At heavier loads, DCR sensing provides

higher efficiency.

ThepowerdissipationonthetopsideMOSFETcanbeeasily

estimated. Choosing a Siliconix Si4816BDY dual MOSFET

results in: RDS(ON) = 0.023Ω/0.016Ω, CMILLER @ 100pF.

At maximum input voltage with T(estimated) = 50°C:

PMAIN =

20V

( )2 1+(0.005)(50°C – 25°C)

[

]•

0.023

Ω

(

)+ 20V

( )2 5A









Ω

( ) 100pF

(

)•

5 – 2.3

2.3









500kHz

(

) = 243mW

A short-circuit to ground will result in a folded back cur-

rent of:

ISC =

1/ 3

( )75mV

0.009

Ω

–

90ns(20V)

3.3

µH









= 2.5A

with a typical value of RDS(ON) and d = (0.005/°C)(25)

= 0.125. The resulting power dissipated in the bottom

MOSFET is:

PSYNC =

20V – 5V

20V

2.5A

(

)2 1.125

(

) 0.016Ω

(

)

= 84mW

which is less than under full-load conditions.

CIN is chosen for an RMS current rating of at least 2A at

temperature assuming only one channel is on. COUT is

chosen with an ESR of 0.02Ω for low output ripple. The

output ripple in continuous mode will be highest at the

maximum input voltage. The output voltage ripple due to

ESR is approximately:

VORIPPLE = RESR (DIL) = 0.02Ω(1.5A) = 30mVP-P

Figure 14. Design Example Efficiency vs Load

APPLICATIONS INFORMATION

LOAD CURRENT (A)

0.01

0.1

100

0.1

0.01

3853 F14

EFFICIENCY

POWER LOSS

DCR

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