LT3840
applicaTions inForMaTion
higher than expected. Bias current error at the output can
be estimated as:
short-circuitloadcondition,adiodeandresistorconnected
fromV totheICTRLpinisrecommended(seeFigure7).
OUT
DV
= 5nA • R2
V
OUT
OUT(BIAS)
Great care should be taken to route the V line away from
R1
D1
FB
noise sources, such as the inductor or the SW node.
Output Current Programming and Monitoring
ICTRL 3840 F07
The average current control loop of the LT3840 accurately
regulates the maximum output current of the switching
regulator. The default maximum differential sense volt-
Figure 7. Output Short-Circuit Foldback Current Circuit
Thefoldbackcurrentinshort-circuit,I
as follows:
,iscalculated
OUT(SC)
age, V
, is 50mV, but a voltage applied to the
SENSE(MAX)
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
(VF(D1) + 7µA •R1)
IOUT(SC)
=
20 •RSENSE
to the INTV pin.
where V is the forward voltage on the diode D1 at ~7uA
CC
F
current.
Oncethemaximumdifferentialsensevoltageisdetermined
the R
current sense resistor is calculated as follows:
SENSE
Internal Power Supply
VSENSE(MAX)
The internal auxiliary supply requires three external com-
RSENSE
=
IOUT(MAX)
ponents (C
, C
and L
) for operation, as
INTVCC AUXBST
PWR
showninFigure3.C
,a4.7μF/10Vceramiccapacitor,
CC AUXBST
INTVCC
Selectacurrentsenseresistorwherethemaximumpower
dissipation rating is greater than the calculated power
dissipation:
bypasses INTV . C
, a 1μF/10V ceramic capacitor,
connected between the AUXBST pin and the AUXSW1 pin,
provides bootstrapped drive to the internal switch.
2
P
= R
• I
D(RSENSE)
SENSE OUT(MAX)
A 33µH inductor with a saturation current greater than
0.6A is recommended for most applications. The Coilcraft
ME3220-333 is a good fit.
The average current control loop is compensated at
the ICOMP pin with a resistor and capacitor connected
to GND.
C
Capacitor Selection
BOOST
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.
The recommended value of the BOOST capacitor, C
,
BOOST
is atleast100 times greaterthan the totalgate 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 (t ) is programmed via the
SS
C
SS
capacitor as follows:
Output Short-Circuit Current Foldback
tSS • 9µA
CSS =
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
3840fa
For more information www.linear.com/LT3840
17
Linear [ Linear ]
