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LT8710

8710f

For more information www.linear.com/LT8710

8710 F08

GND

VIN

RIN1

RIN2

(OPTIONAL)

EN/FBIN

1.7V

1.3V

–

1.607V

51.5k

17.6µA

AT 1.607V

EA4

CHIP ENABLE

EN/FBN

LOGIC

ACTIVE MODE

SETTING THE OUTPUT VOLTAGE REGULATION

TheLT8710outputvoltageissetbyconnectinganexternal

resistor (RFBX) from the converter’s output, VOUT, to the

FBX pin. The equations below determines RFBX:

RFBX =

VOUT –1.213V

83.7µA

; Boost or SEPIC Converter

RFBX =

| VOUT |–9.6mV

83.1µA

; InvertingConverter

See the Electrical Characteristics for tolerances on the FBX

regulation voltage and current.

SETTING THE INPUT VOLTAGE REGULATION OR

UNDERVOLTAGE LOCKOUT

By connecting a resistor divider between VIN, EN/FBIN,

and GND, the EN/FBIN pin provides a mean to regulate

the input voltage or to create an undervoltage lockout

function. Referring to error amplifier EA4 in the block

diagram, when EN/FBIN is lower than the 1.607V refer-

ence, VC is pulled low. For example, if VIN is provided by

a relatively high impedance source (e.g. a solar panel) and

the current draw pulls VIN below a preset limit, VC will be

reduced,thusreducingcurrentdrawfromtheinputsupply

and limiting the input voltage drop. Note that using this

function in forced continuous mode (MODE pin low) can

result in current being drawn from the output and forced

into the input. If this behavior is not desired then set the

MODE pin high to prevent reverse current flow.

To set the minimum or regulated input voltage use:

VIN(MIN–REG) = 1.607V • 1+

RIN1

RIN2













+17.6µA •RIN1

RIN1=

VIN(MIN–REG)–1.607V

1.607V

RIN2







+17.6µA

where RIN1 and RIN2 are shown in Figure 8. For increased

accuracy, set RIN2 ≤ 10k. The resistor RIN2 is optional, but

it is recommended to be used to increase the accuracy of

the input voltage regulation by making the RIN1 current

much higher than the EN/FBIN pin current.

applicaTions inForMaTion

Figure 8. Configurable UVLO

Thissametechniquecanbeusedtocreateanundervoltage

lockout if the LT8710 is NOT in forced continuous mode.

When in discontinuous mode, forcing VC low will stop all

switching activity. Note that this does not reset the soft

start function, therefore resumption of switching activity

will not be accompanied by a soft-start.

Note that for very low input impedance supplies, a capaci-

tor from EN/FBIN to ground may be needed to prevent

oscillations from the input voltage regulation control loop.

At start-up, the minimum voltage on EN/FBIN must exceed

1.7V (typical) to begin a soft-start cycle. Afterwards, the

EN/FBIN voltage can drop below 1.7V and the input can

be regulated such that the EN/FBIN voltage is at ~1.607V.

So the equation below gives the start-up VIN for a desired

input regulation voltage:

VIN(START-UP)=

1.7V

1.607V

• VIN(MIN–REG)+0.78µA •RIN1

OUTPUT CURRENT MONITORING AND LIMITING

(RSENSE2 AND ISP-ISN AND IMON PINS)

The LT8710 has an output current monitor circuit that

can be used to monitor and/or limit the output current.

The current monitor circuit works as shown in Figure 9.

If it is not desirable to monitor and limit the output cur-

rent, simply connect the IMON pin to ground. Note that

the current sense resistor connected to the ISP and ISN

pins must still be used, and the value should follow the

guidelines in the next couple sections.

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