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LTC3855

3855f

Sensing the Output Voltage with a Differential

Amplifier

The LTC3855 includes a low offset, unity gain, high band-

widthdifferentialamplifierforapplicationsthatrequiretrue

remote sensing. Sensing the load across the load capaci-

torsdirectlygreatlybenefitsregulationinhighcurrent,low

voltage applications, where board interconnection losses

can be a significant portion of the total error budget.

TheLTC3855differentialamplifierhasatypicaloutputslew

rate of 2V/μs. The amplifier is configured for unity gain,

meaning that the difference between DIFFP and DIFFN is

translated to DIFFOUT, relative to SGND.

Care should be taken to route the DIFFP and DIFFN PCB

traces parallel to each other all the way to the terminals

of the output capacitor or remote sensing points on the

board. In addition, avoid routing these sensitive traces

near any high speed switching nodes in the circuit. Ideally,

the DIFFP and DIFFN traces should be shielded by a low

impedance ground plane to maintain signal integrity.

Inductor DCR Sensing Temperature Compensation and

the ITEMP Pins

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 a copper inductor 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.

LTC3855 offers a method to counter this inaccuracy by

allowing the user to place an NTC temperature sensing

resistor near the inductor. ITEMP pin, when left floating, is

at a voltage around 5V and DCR temperature compensa-

tion is disabled. ITEMP pin has a constant 10µA precision

current flowing out the pin. By connecting an NTC resistor

from ITEMP pin to SGND, the maximum current sense

threshold can be varied over temperature according the

following equation:

SENSEMAX ADJ

SENSE MAX

ITEMP

(

)

(

) •

. –

1 8

1 3

Where:

VSENSEMAX(ADJ)isthemaximumadjustedcurrentsense

threshold.

VSENSE(MAX) is the maximum current sense threshold

specified in the electrical characteristics table. It is typi-

cally 75mV, 50mV, or 30mV depending on the setting

ILIM pins.

VITEMP is the voltage of ITEMP pin.

The valid voltage range for DCR temperature compensa-

tion on the ITEMP pin is between 0.5V to 0.2V, with 0.5V

or above being no DCR temperature correction and 0.2V

the maximum correction. However, if the duty cycle of the

controller is less than 25%, the ITEMP range is extended

from 0.5V to 0V.

An NTC resistor has a negative temperature coefficient,

that means that its value decreases as temperature rises.

The VITEMP voltage, therefore, decreases as temperature

increases and in turn the VSENSEMAX(ADJ) will increase to

compensate the DCR temperature coefficient. The NTC

resistor, however, is non-linear and user can linearize its

value by building a resistor network with regular resis-

tors. Consult the NTC manufacture datasheets for detailed

information.

Another use for the ITEMP pins, in addition to NTC com-

pensated DCR sensing, is adjusting VSENSE(MAX) to values

between the nominal values of 30mV, 50mV and 75mV for

a more precise current limit. This is done by applying a

voltage less than 0.5V to the ITEMP pin. VSENSE(MAX) will

be varied per the above equation and the same duty cycle

limitationswillapply.Thecurrentlimitcanbeadjustedusing

this method either with a sense resistor or DCR sensing.

FormoreinformationseetheNTCCompensatedDCRSens-

ing paragraph in the Applications Information section.

Frequency Selection and Phase-Locked Loop

(FREQ and MODE/PLLIN Pins)

Theselectionofswitchingfrequencyisatrade-offbetween

efficiency and component size. Low frequency opera-

tion increases efficiency by reducing MOSFET switching

losses, but requires larger inductance and/or capacitance

to maintain low output ripple voltage. The switching

operaTion

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