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LT8613

8613f

applicaTions inForMaTion

Enable Pin

The LT8613 is in shutdown when the EN pin is low and

active when the pin is high. The rising threshold of the EN

comparator is 1.0V, with 40mV of hysteresis. The EN pin

can be tied to VIN if the shutdown feature is not used, or

tied to a logic level if shutdown control is required.

Adding a resistor divider from VIN to EN programs the

LT8613 to regulate the output only when VIN is above a

desired voltage (see the Block Diagram). Typically, this

threshold, VIN(EN), is used in situations where the input

supply is current limited, or has a relatively high source

resistance. A switching regulator draws constant power

from the source, so source current increases as source

voltage drops. This looks like a negative resistance load

to the source and can cause the source to current limit or

latch low under low source voltage conditions. The VIN(EN)

threshold prevents the regulator from operating at source

voltages where the problems might occur. This threshold

can be adjusted by setting the values R3 and R4 such that

they satisfy the following equation:

VIN(EN) =











 • 1.0V

(10)

where the LT8613 will remain off until VINisaboveVIN(EN).

Duetothecomparator’shysteresis,switchingwillnotstop

until the input falls slightly below VIN(EN).

When operating in Burst Mode operation for light load

currents, the current through the VIN(EN) resistor network

can easily be greater than the supply current consumed

by the LT8613. Therefore, the VIN(EN) resistors should be

large to minimize their effect on efficiency at low loads.

Current Control Loop

In addition to regulating the output voltage the LT8613

includes a current regulation loop for setting the average

input or output current limit as shown in the Typical Ap-

plications section.

The LT8613 measures voltage drop across an external

current sense resistor using the ISP and ISN pins. This

resistor may be connected between the inductor and the

output capacitor to sense the output current or may be

placed between the VIN bypass capacitor and the input

power source to sense input current. The current loop

modulates the internal cycle-by-cycle switch current limit

such that the average voltage across ISP-ISN pins does

not exceed 50mV.

Care must be taken and filters should be used to assure

the signal applied to the ISN and ISP pins has a peak-to-

peak ripple of less than 30mV for accurate operation. In

addition to high crest factor current waveforms such as

the input current of DC/DC regulators, another cause of

high ripple voltage across the sense resistor is excessive

resistor ESL. Typically the problem is solved by using a

small ceramic capacitor across the sense resistor or using

a filter network between the ISP and ISN pins.

The ICTRL pin allows the ISP-ISN set point to be linearly

controlled from 50mV to 0mV as the ICTRL pin is ramped

from1Vdownto0V,respectivelyandasshowninFigure 3.

When this functionality is unused the ICTRL pin may be

tiedtoINTVCCorfloated.InadditiontheICTRLpinincludes

a 2µA pull-up source such that a capacitor may be added

for soft-start functionality.

TheIMONpinisavoltageoutputproportionaltothevoltage

across the current sense resistor such that VIMON = 20 •

(ISP-ISN) as shown in Figure 4. This output can be used

to monitor the input or output current of the LT8613 or

may be an input to an ADC for further processing.

Figure 3. LT8613 Sense Voltage vs ICTRL Voltage

ICTRL VOLTAGE (mV)

500

1000

1500

2000

8613 F03

LT8613 Datasheet(PDF) 16 Page - Linear Technology

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