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LT3840

3840fa

For more information www.linear.com/LT3840

higher than expected. Bias current error at the output can

be estimated as:

DVOUT(BIAS) = 5nA • R2

Great care should be taken to route the VFB line away from

noise sources, such as the inductor or the SW node.

Output Current Programming and Monitoring

The average current control loop of the LT3840 accurately

regulates the maximum output current of the switching

regulator. The default maximum differential sense volt-

age, VSENSE(MAX), is 50mV, but a voltage applied to the

ICTRL pin will program it lower. A 0V to 1V range on

the ICTRL pin corresponds to 0mV to 50mV differential

sense voltage. A way to provide the ICTRL programming

voltage is to connect a linear regulator or voltage divider

to the INTVCC pin.

Oncethemaximumdifferentialsensevoltageisdetermined

the RSENSE current sense resistor is calculated as follows:

RSENSE =

VSENSE(MAX)

IOUT(MAX)

Select a current sense resistor where the maximum power

dissipation rating is greater than the calculated power

dissipation:

PD(RSENSE) = RSENSE • IOUT(MAX)2

The average current control loop is compensated at

the ICOMP pin with a resistor and capacitor connected

to GND.

TheIMONpinisanaccurateoutputcurrentmonitorpin.The

LT3840 outputs a voltage that is 20 times the differential

sense voltage. A 0mV to 50mV differential sense voltage

corresponds to a 0V to 1V IMON voltage. A capacitor on

the pin filters the voltage ripple due to the inductor ripple

current. Typical capacitor values on the pin range from

1000pF to 0.1µF. The larger the capacitor value, the lower

the ripple. The capacitor does not affect the average cur-

rent control loop.

Output Short-Circuit Current Foldback

The LT3840 defaults to a straight line current limit where

the short-circuit current is the same as the drop out cur-

rent. For applications that require current foldback in a

applicaTions inForMaTion

Figure 7. Output Short-Circuit Foldback Current Circuit

3840 F07

ICTRL

VOUT

short-circuitloadcondition,adiodeandresistorconnected

fromVOUTtotheICTRLpinisrecommended(seeFigure7).

Thefoldbackcurrentinshort-circuit,IOUT(SC),iscalculated

as follows:

IOUT(SC) =

(VF(D1) + 7µA • R1)

20 • RSENSE

where VF is the forward voltage on the diode D1 at ~7uA

current.

Internal Power Supply

The internal auxiliary supply requires three external com-

ponents (CINTVCC, CAUXBST and LPWR) for operation, as

showninFigure3.CINTVCC,a4.7μF/10Vceramiccapacitor,

bypasses INTVCC. CAUXBST, a 1μF/10V ceramic capacitor,

connected between the AUXBST pin and the AUXSW1 pin,

provides bootstrapped drive to the internal switch.

A 33µH inductor with a saturation current greater than

0.6A is recommended for most applications. The Coilcraft

ME3220-333 is a good fit.

CBOOST Capacitor Selection

The recommended value of the BOOST capacitor, CBOOST,

is at least 100 times greater than the total gate capacitance

of the topside MOSFET. Typical values for most applica-

tions range from 0.1μF to 1μF.

Soft-Start and Voltage Tracking

The desired soft-start time (tSS) is programmed via the

CSS capacitor as follows:

CSS =

tSS • 9µA

1.75

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