Datasheet

LT3845
18
3845fd
If low supply current in standby mode is required, select
a higher value of R
B
.
The supply turn off voltage is 9% below turn on. In the
example the V
SUPPLY(OFF)
would be 13.2V.
If additional hysteresis is desired for the enable function,
an external positive feedback resistor can be used from
the LT3845 regulator output.
The shutdown function can be disabled by connecting the
SHDN pin to the V
IN
through a large value pull-up resistor.
This pin contains a low impedance clamp at 6V, so the SHDN
pin will sink current from the pull-up resistor(R
PU
):
I
SHDN
=
V
IN
–6V
R
PU
Because this arrangement will clamp the SHDN pin to the
6V, it will violate the 5V absolute maximum voltage rating of
the pin. This is permitted, however, as long as the absolute
maximum input current rating of 1mA is not exceeded.
Input SHDN pin currents of <100μA are recommended: a
1MΩ or greater pull-up resistor is typically used for this
confi guration.
Soft-Start
The desired soft-start time (t
SS
) is programmed via the
C
SS
capacitor as follows:
C
SS
=
2µA • t
SS
1.231V
The amount of time in which the power supply can withstand
a V
IN
, V
CC
or V
SHDN
UVLO fault condition (t
FAULT
) before
the C
SS
pin voltage enters its active region is approximated
by the following formula:
t
FAULT
=
C
SS
• 0.65V
50µA
Oscillator SYNC
The oscillator can be synchronized to an external clock.
Set the R
SET
resistor at least 10% below the desired sync
frequency.
It is recommended that the SYNC pin be driven with a
square wave that has amplitude greater than 2V, pulse
width greater than 1μs and rise time less than 500ns. The
rising edge of the sync wave form triggers the discharge
of the internal oscillator capacitor. The SYNC pin is not
available in the N-Package.
Minimum On-Time Considerations (Buck Mode)
Minimum on-time t
ON(MIN)
is the smallest amount of time
that the LT3845 is capable of turning the top MOSFET on and
off again. It is determined by internal timing delays and the
amount of gate charge required turning on the top MOSFET.
Low duty cycle applications may approach this minimum
on-time limit and care should be taken to ensure that:
t
ON
=
V
OUT
V
IN
•f
SW
> t
ON(MIN)
where t
ON(MIN)
is 400ns worst case.
If the duty cycle falls below what can be accommodated by
the minimum on-time, the LT3845 will begin to skip cycles.
The output will be regulated, but the ripple current and
ripple voltage will increase. If lower frequency operation
is acceptable, the on-time can be increased above t
ON(MIN)
for the same step-down ratio.
Layout Considerations
The LT3845 is typically used in DC/DC converter designs
that involve substantial switching transients. The switch
drivers on the IC are designed to drive large capacitances
and, as such, generate signifi cant transient currents
themselves. Careful consideration must be made regard-
ing supply bypass capacitor locations to avoid corrupting
the ground reference used by IC.
Typically, high current paths and transients from the
input supply and any local drive supplies must be kept
isolated from SGND, to which sensitive circuits such as
the error amp reference and the current sense circuits
are referred.
Effective grounding can be achieved by considering switch
current in the ground plane, and the return current paths of
each respective bypass capacitor. The V
IN
bypass return,
V
CC
bypass return, and the source of the synchronous
APPLICATIONS INFORMATION