Datasheet

LT3800
15
3800fc
APPLICATIONS INFORMATION
Some magnetics vendors specify a volt-second product
in their data sheet. If they do not, consult the vendor to
make sure the specifi cation is not being exceeded by your
design. The required volt-second product is calculated as
follows:
Volt-Second
V
OUT
f
O
•1–
V
OUT
V
IN






Magnetics vendors specify either the saturation current, the
RMS current, or both. When selecting an inductor based
on inductor saturation current, the peak current through
the inductor, I
OUT(MAX)
+ (∆I/2), is used. When selecting
an inductor based on RMS current the maximum load
current, I
OUT(MAX)
, is used.
The requirement for avoiding current mode instability is
keeping the rising slope of sensed inductor ripple current
(S1) greater than the falling slope (S2). During continuous-
current switcher operation, the rising slope of the current
waveform in the switched inductor is less than the falling
slope when operating at duty cycles (DC) greater than 50%.
To avoid the instability condition during this operation, a
false signal is added to the sensed current, increasing the
perceived rising slope. To prevent current mode instability,
the slope of this false signal (Sx) must be suffi cient such
that the sensed rising slope exceeds the falling slope, or
S1 + Sx ≥ S2. This leads to the following relations:
Sx ≥ S2 (2DC – 1)/DC
where:
S2 ~ V
OUT
/L
Solving for L yields a relation for the minimum inductance
that will satisfy slope compensation requirements:
L
MIN
= V
OUT
•
2DC – 1
DC • Sx
The LT3800 maximizes available dynamic range using a
slope compensation generator that continuously increases
the additional signal slope as duty cycle increases. The
slope compensation waveform is calibrated at an 80%
duty cycle, to generate an equivalent slope of at least
1E
5
• I
LIMIT
A/sec, where I
LIMIT
is the programmed con-
verter current limit. Current limit is programmed by using
a sense resistor (R
S
) such that I
LIMIT
= 150mV/R
S
, so the
equation for the minimum inductance to meet the current
mode instability criterion can be reduced to:
L
MIN
= (5E
–5
)(V
OUT
)(R
S
)
For example, with V
OUT
= 5V and R
S
= 20m:
L
MIN
= (5E
–5
)(5)(0.02) = 5µH
After calculating the minimum inductance value, the volt-
second product, the saturation current and the RMS current
for your design, an off the shelf inductor can be selected
from a magnetics vendor. A list of magnetics vendors
can be found at http://www.linear.com/ezone/vlinks or by
contacting the Linear Technology Applications department.
Output Voltage Programming
Output voltage is programmed through a resistor feedback
network to V
FB
(Pin 6) on the LT3800. This pin is the inverting
input of the error amplifi er, which is internally referenced
to 1.231V. The divider is ratioed to provide 1.231V at the
V
FB
pin when the output is at its desired value. The output
voltage is thus set following the relation:
R2 = R1•
V
OUT
1.231
–1






when an external resistor divider is connected to the
output as shown.
Programming LT3800 Output Voltage
3800 AI03
LT3800
V
OUT
R2
R1
6
17
V
FB
SGND