LTC4449 datasheet(16/38 Pages) LINER

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LTC3774

3774fc

For more information www.linear.com/LTC3774

APPLICATIONS INFORMATION

The LTC3774 could also be used like any typical current

mode controller by disabling the SNSD+ pin, shorting it

to ground. An RSENSE resistor or a RC filter can be used

to sense the output inductor signal and connects to the

SNSA+ pin. If the RC filter is used, its time constant,

R • C, is equaled to L/DCR of the output inductor. In these

applications, the current limit, VSENSE (MAX), will be five

times larger for the specified ILIM, and the operating

voltage range of SNSA+ and SNS– is from 0V to 5.25V.

Inductor DCR Sensing

The LTC3774 is specifically designed for high load current

applications requiring the highest possible efficiency; it is

capable of sensing the signal of an inductor DCR in the

sub milliohm range (Figure 4). The DCR is the DC winding

resistanceoftheinductor’scopper,whichisoftenlessthan

1mΩ for high current inductors. In high current and low

output voltage applications, a conduction loss of a high

DCR or a sense resistor will cause a significant reduction

in power efficiency. For a specific output requirement,

chose the inductor with the DCR that satisfies the maxi-

mum desirable sense voltage, and uses the relationship

of the sense pin filters to output inductor characteristics

as depicted below.

DCR =

VSENSE(MAX)

IMAX +

∆IL

L/DCR = R1• C1 = 5 • R2 • C2

where:

VSENSE(MAX): Maximum sense voltage for a given ILIM

threshold

IMAX: Maximum load current

∆IL: Inductor ripple current

L, DCR: Output inductor characteristics

R1•C1: Filter time constant of the SNSD+ pin

R2•C2: Filter time constant of the SNSA+ pin

Figure 4. Inductor DCR Current Sensing

VIN

PWM

ITEMP

RNTC

100k

INDUCTOR

DCR

SNSD+

SNSA+

SNS–

GND

LTC3774

VOUT

3774 F04

PLACE C1, C2 NEXT TO IC

PLACE R1, R2 NEXT TO INDUCTOR

R1C1 = 5 • R2C2

20k

RITEMP

43.2k

LTC4449

VLOGIC

VCC

BOOST

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