Datasheet

LT3844
15
3844fb
APPLICATIONS INFORMATION
V
SUPPLY(ON)
is the input voltage at which the undervoltage
lockout is disabled and the supply turns on.
Example: Select R
B
= 49.9kΩ, V
SUPPLY(ON)
= 14.5V (based
on a 15V minimum input voltage)
Rk
V
V
A
=
⎛
⎝
⎜
⎞
⎠
⎟
49 9
14 5
135
1.•
.
.
–
= 486.1k (499k resistor is selected)
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 LT3844 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
=
VV
R
IN
PU
–6
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 1M 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
At
V
SS
SS
=
2
1 231
μ •
.
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
CV
A
FAULT
SS
=
•.065
50μ
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 1ms and rise time less than 500ns. The
rising edge of the sync wave form triggers the discharge
of the internal oscillator capacitor.
Effi ciency Considerations
The effi ciency of a switching regulator is equal to the output
power divided by the input power times 100%. Express
percent effi ciency as:
% Effi ciency = 100% - (L1 + L2 + L3 + ...)
where L1, L2, etc. are individual loss terms as a percent-
age of input power.
Although all dissipative elements in the circuit produce
losses, four main contributors usually account for most
of the losses in LT3844 circuits:
1. LT3844 V
IN
and V
CC
current loss
2. I
2
R conduction losses
3. MOSFET transition loss
4. Schottky diode conduction loss
1. The V
IN
and V
CC
currents are the sum of the quiescent
currents of the LT3844 and the MOSFET drive currents.
The quiescent currents are in the LT3844 Electrical Char-
acteristics table. The MOSFET drive current is a result
of charging the gate capacitance of the power MOSFET
each cycle with a packet of charge, Q
G
. Q
G
is found in
the MOSFET data sheet. The average charging current is
calculated as Q
G
• f
SW
. The power loss term due to these
currents can be reduced by backdriving V
CC
with a lower
voltage than V
IN
such as V
OUT
.