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LT8710

8710f

For more information www.linear.com/LT8710

where:

RNTC(25) = Resistance of the NTC resistor at 25°C

= Material-specific constant of NTC resistor.

Specified at two temperatures such as

b25/85.

If more than two

bs are specified, use the most

appropriate for the application.

= Absolute temperature in Kelvin

T25 = Room temperature in Kelvin (298.15k)

To provide a desired load current for any given application,

RSENSE1 must be sized appropriately. The switch current

will be at its highest when the input voltage is at the lowest

of its range. The equation below calculates RSENSE1 for a

desired output current:

RSENSE1≤0.74• η•

VCSPN

IOUT

• 1–DCMAX

(

)• 1– iRIPPLE









where

= Converter efficiency (assume ~90%)

VCSPN = Max current limit voltage (see Max Current

Limit vs Duty Cycle (CSP-CSN) plot in the

Typical Performance Characteristics)

IOUT = Converter load current

DCMAX = Switching duty cycle at minimum VIN (see

Power Switch Duty Cycle in Appendix)

iRIPPLE = Peak-to-peakinductorripplecurrentpercent-

age at minimum VIN (recommended to use

25%)

REVERSE CURRENT APPLICATIONS (MODE PIN LOW)

When the forced continuous mode is selected (MODE pin

low), inductor current is allowed to reverse directions and

flow from the VOUT side to the VIN side. This can lead to

current sinking from the output and being forced into the

input. The reverse current is at a maximum magnitude

when VC is lowest. The graph of Max Current Limit vs Duty

Cycle (CSP – CSN) in the Typical Performance Character-

istics section can help to determine the maximum reverse

current capability.

The IMON pin voltage will indicate negative inductor cur-

rents. Refer to the equation for IMON in the Pin Functions.

Note that the IMON voltage is only accurate if the dynamic

voltage across RSENSE2 stays within –51.8mV to 500mV.

If the valley inductor current goes more negative than

–300mV as sensed by RSENSE2, the external PFET will turn

off, and the inductor current will start going more positive.

applicaTions inForMaTion

SWITCH CURRENT LIMIT (RSENSE1 AND CSP-CSN

PINS)

The external current sense resistor (RSENSE1) sets the

maximum peak current though the external NFET switch

(MN). The maximum voltage across RSENSE1 is 50mV

(typical) at very low switch duty cycles, and then slope

compensationdecreasesthecurrentlimitasthedutycycle

increases (see the Max Current Limit vs Duty Cycle (CSP-

CSN) plot in the Typical Performance Characteristics). The

equation below gives the switch current limit for a given

duty cycle and current sense resistor (find VCSPN at the

operating duty cycle in the plot mentioned).

ISW(LIMIT)=

VCSPN

RSENSE1

Figure 11. Temperature Dependent Output Using an NTC

Resistor Divider

8710 F11

GND

6.3V

RNTC

FBX

INTVCC

–

EA1

–

EA2

14.5k

1.213V

FROM SYSTEM

RFBX

VOUT

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