Datasheet

LT3800
14
3800fc
APPLICATIONS INFORMATION
The UVLO voltage, V
IN(UVLO)
, is set using the following
relation:
R
A
= R
B
•
V
IN(UVLO)
– 1.35V
1.35V
If additional hysteresis is desired for the enable function,
an external positive feedback resistor can be used from
the LT3800 regulator output.
The shutdown function can be disabled by connecting the
SHDN pin to 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 pull the SHDN pin to the
6V clamp voltage, 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.
Inductor Selection
The primary criterion for inductor value selection in LT3800
applications is ripple current created in that inductor. Basic
design considerations for ripple current are output voltage
ripple, and the ability of the internal slope compensation
waveform to prevent current mode instability. Once the
value is determined, an inductor must also have a satu-
ration current equal to or exceeding the maximum peak
current in the inductor.
Ripple current (∆I
L
) in an inductor for a given value (L)
can be approximated using the relation:
I
L
= 1–
V
OUT
V
IN






•
V
OUT
f
O
•L
The typical range of values for ∆I is 20% to 40% of
I
OUT(MAX)
, where I
OUT(MAX)
is the maximum converter
output load current. Ripple currents in this range typically
yield a good design compromise between inductor perfor-
mance versus inductor size and cost, and values in this
range are generally a good starting point. A starting point
inductor value can thus be determined using the relation:
L = V
OUT
•
1–
V
OUT
V
IN






f
O
• 0.3 •I
OUT(MAX)
Use of smaller inductors increase output ripple currents,
requiring more capacitance on the converter output. Also,
with converter operation with duty cycles greater than
50%, the slope compensation criterion, described later,
must be met. Designing for smaller ripple currents requires
larger inductor values, which can increase converter cost
and/or footprint.
Programming LT3800 V
IN
UVLO
3800 AI02
LT3800
V
IN
R
B
R
A
3
17
SHDN
SGND