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LTC3774

3774fc

For more information www.linear.com/LTC3774

APPLICATIONS INFORMATION

Toensurethattheloadcurrentwillbedeliveredoverthefull

operatingtemperaturerange,thetemperaturecoefficientof

DCR resistance, approximately 0.4%/°C, should be taken

into account. The LTC3774 features a DCR temperature

compensationcircuitthatusesanNTCtemperaturesensing

resistor for this purpose. See the Inductor DCR Sensing

Temperature Compensation section for details.

Typically, C1 and C2 are selected in the range of 0.047µF

to 0.47µF. If C1 and C2 are chosen to be 220nF, and an

inductor of 330nH with 0.32mΩ DCR is selected, R1 and

R2 will be 4.7k and 942Ω respectively. The bias current at

SNSD+ and SNSA+ is about 30nA and 500nA respectively,

and it causes some small error to the sense signal.

There will be some power loss in R1 and R2 that relates to

the duty cycle, and will be the most in continuous mode

at the maximum input voltage:

PLOSS R

( ) =

VIN(MAX) – VOUT

(

)• VOUT

Ensure that R1 and R2 have a power rating higher than this

value. However, DCR sensing eliminates the conduction

loss of a sense resistor; it will provide a better efficiency

at heavy loads. To maintain a good signal-to-noise ratio

for the current sense signal, using a minimum ∆VSENSE of

2mV for duty cycles less than 40% is desirable. The actual

ripplevoltagewillbedeterminedbythefollowingequation:

∆VSENSE =

VOUT

VIN

•

VIN – VOUT

R1• C1• fOSC

Inductor DCR Sensing Temperature Compensation

and the ITEMP Pin

Inductor DCR current sensing provides a lossless method

of sensing the instantaneous current. Therefore, it can

provide higher efficiency for applications of high output

currents. However, the DCR of the inductor, which is the

small amount of DC winding resistance of the copper,

typically has a positive temperature coefficient. As the

temperature of the inductor rises, its DCR value increases.

The current limit of the controller is therefore reduced.

The LTC3774 offers a method to counter this inaccuracy

by allowing the user to place an NTC temperature sensing

resistor near the inductor to actively correct this error. The

ITEMP pin, when left floating, is at a voltage around 5V and

DCR temperature compensation is disabled. The ITEMP

pin has a constant 30µA precision current flowing out the

pin. By connecting an NTC resistor from the ITEMP pin

to SGND, the maximum current sense threshold can be

varied over temperature according the following equation:

VSENSEMAX(ADJ) = VSENSE(MAX) •

2–

VITEMP

2.8

1.5

where:

VSENSEMAX(ADJ)isthemaximumadjustedcurrentsense

threshold.

VSENSE(MAX) is the maximum current sense threshold

specified in the Electrical Characteristics table. It is typi-

cally 30mV, 25mV, 20mV, 15mV or 10mV depending

on the setting ILIM pins.

VITEMP is the voltage of the ITEMP pin.

ThevalidvoltagerangeforDCRtemperaturecompensation

on the ITEMP pin is 1.4V to 0.6V, with 1.4V or above being

no DCR temperature correction and 0.6V the maximum

correction.However,ifthedutycycleofthecontrollerisless

than 25%, the ITEMP range is extended from 1.4V to 0V.

The NTC resistor has a negative temperature coefficient,

meaning its value decreases as temperature rises. The

VITEMP voltage, therefore, decreases as temperature

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