LT3506/LT3506A
8
3506afc
FB Resistor Network
The output voltage is programmed with a resistor divider
between the output and the FB pin. Choose the 1% resis-
tors according to:
R1 = R2(V
OUT
/0.8 – 1)
The parallel combination of R1 and R2 should be 10k or
less to avoid bias current errors. Reference designators
refer to the Block Diagram in Figure 2.
Input Voltage Range
The minimum input voltage is determined by either the
LT3506’s minimum operating voltage of ~3.6V, or by its
maximum duty cycle. The duty cycle is the fraction of
time that the internal switch is on and is determined by
the input and output voltages:
DC = (V
OUT
+ V
D
)/(V
IN
– V
SW
+ V
D
)
where V
D
is the forward voltage drop of the catch diode
(~0.4V) and V
SW
is the voltage drop of the internal switch
(~0.3V at maximum load). This leads to a minimum input
voltage of:
V
IN(MIN)
= (V
OUT
+ V
D
)/DC
MAX
– V
D
+ V
SW
with DC
MAX
= 0.89 (0.78 for the LT3506A).
A more detailed analysis includes inductor loss and the
dependence of the diode and switch drop on operating
current. A common application where the maximum duty
cycle limits the input voltage range is the conversion of 5V
to 3.3V. The maximum load current that the LT3506 can
deliver at 3.3V depends on the accuracy of the 5V input
supply. With a low loss inductor (DCR less than 80mΩ),
the LT3506 can deliver 1.2A for V
IN
> 4.7V and 1.6A for
V
IN
> 4.85V. The maximum input voltage is determined
by the absolute maximum ratings of the V
IN
and BOOST
pins and by the minimum duty cycle DC
MIN
= 0.08 (0.15
for the LT3506A):
V
IN(MAX)
= (V
OUT
+ V
D
)/DC
MIN
– V
D
+ V
SW
.
This limits the maximum input voltage to ~21V with V
OUT
= 1.2V and ~15V with V
OUT
= 0.8V. For the LT3506A the
maximum input voltage is ~8V with V
OUT
= 0.8V. Note
that this is a restriction on the operating input voltage;
the circuit will tolerate transient inputs up to the absolute
maximum rating.
Inductor Selection and Maximum Output Current
A good fi rst choice for the inductor value is:
L = 2 • (V
OUT
+ V
D
) for the LT3506
L = (V
OUT
+ V
D
) for the LT3506A
where V
D
is the voltage drop of the catch diode (~0.4V)
and L is in H. With this value the maximum load current
will be ~1.6A, independent of input voltage. The inductor’s
RMS current rating must be greater than your maximum
load current and its saturation current should be about 30%
higher. To keep effi ciency high, the series resistance (DCR)
should be less than 0.1Ω. Table 1 lists several vendors and
types that are suitable. Of course, such a simple design
guide will not always result in the optimum inductor for
your application. A larger value provides a slightly higher
maximum load current, and will reduce the output volt-
age ripple. If your load is lower than 1.6A, then you can
decrease the value of the inductor and operate with higher
ripple current. This allows you to use a physically smaller
inductor, or one with a lower DCR resulting in higher ef-
fi ciency. Be aware that if the inductance differs from the
simple rule above, then the maximum load current will
depend on input voltage. There are several graphs in the
Typical Performance Characteristics section of this data
sheet that show the maximum load current as a function
of input voltage and inductor value for several popular
output voltages. Also, low inductance may result in dis-
continuous mode operation, which may be acceptable,
but further reduces maximum load current. For details of
maximum output current and discontinuous mode opera-
tion, see Linear Technology Application Note 44. Finally,
for duty cycles greater than 50%(V
OUT
/V
IN
< 0.5), there
is a minimum inductance required to avoid subharmonic
oscillations. See Application Note 19 for detailed informa-
tion on subharmonic oscillations. The following discussion
assumes continuous inductor current.
APPLICATIONS INFORMATION
