LT3790 datasheet(22/30 Pages) LINER

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LT8390A

8390afa

For more information www.linear.com/LT8390A

APPLICATIONS INFORMATION

Programming Input or Output Current Limit

The input or output current limit can be programmed by

placing an appropriate value current sense resistor, RIS, in

the input or output power path. The voltage drop across

RIS is (Kelvin) sensed by the ISP and ISN pins. The CTRL

pin should be tied to a voltage higher than 1.35V to get

the full-scale 100mV (typical) threshold across the sense

resistor. The CTRL pin can be used to reduce the current

threshold to zero, although relative accuracy decreases

with the decreasing sense threshold. When the CTRL pin

voltage is between 0.3V and 1.15V, the current limit is:

IIS(MAX) =

VCTRL −0.25V

10 •RIS

When VCTRL is between 1.15V and 1.35V the current limit

varies with VCTRL, but departs from the equation above

by an increasing amount as VCTRL increases. Ultimately,

when VCTRL is larger than 1.35V, the current limit no

longer varies. The typical V(ISP-ISN) threshold vs VCTRL is

listed in Table 2.

Table 2. V(ISP-ISN) Threshold vs VCTRL

VCTRL (V)

V(ISP-ISN) (mV)

1.15

1.20

94.5

1.25

1.30

99.5

1.35

100

When VCTRL is larger than 1.35V, the current threshold

is regulated to:

IIS(MAX) =

100mV

RIS

The CTRL pin should not be left open (tie to VREF if not

used). The CTRL pin can also be used in conjunction with

a thermistor to provide overtemperature protection for

the output load, or with a resistor divider to VIN to reduce

output power and switching current when VIN is low.

The presence of a time varying differential voltage ripple

signal across the ISP and ISN pins at the switching

Figure 13. Programming Input Current Limit

LT8390A

ISP

8390a F13a

ISN

RIS

TO DRAIN OF

SWITCH A

FROM POWER

INPUT

LT8390A

ISP

8390a F13b

ISN

RIS

TO DRAIN OF

SWITCH A

FROM POWER

INPUT

(13a)

(13b)

frequency is expected. If the current sense resistor RIS

is placed between power input and input bulk capacitor

(Figure 13a), or between output bulk capacitor and system

output (Figure 14a), a filter is typically not necessary. If

the RIS is placed between input bulk capacitor and input

decoupling capacitor (Figure 13b), or between output

decoupling capacitor and output bulk capacitor (Figure

14b), a low pass filter formed by RF and CF is recom-

mended to reduce the current ripple and stabilize the

current loop. Since the bias currents of the ISP and ISN

pins are matched, no offset is introduced by RF. If input

or output current limit is not used, the ISP and ISN pins

should be shorted to VIN, VOUT, or ground.

ISMON Current Monitor

The ISMON pin provides a buffered monitor output of the

currentflowingthroughtheISP/ISNcurrentsenseresistor,

RIS. The VISMON voltage is calculated as V(ISP-ISN) • 10 +

0.25V. Since the ISMON pin has the same 0.25V offset

as the CTRL pin, the master LT8390A ISMON pin can

be directly tied to the slave LT8390A CTRL pin for equal

current sharing in parallel applications.

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