LT3570
11
3570fb
to 1.2A at DC2 = 0.8. The maximum output current is a
function of the chosen inductor value:
I
OUT2(MAX)
=I
LIM2
–
ΔI
L2
2
=1.5 • 1– 0.25 •DC2
()
–
ΔI
L2
2
Choosing an inductor value so that the ripple current is
small will allow a maximum output current near the switch
current limit.
One approach to choosing the inductor is to start with the
simple rule given above, look at the available inductors
and choose one to meet cost or space goals. Then use
these equations to check that the LT3570 will be able to
deliver the required output current. Note again that these
equations assume that the inductor current is continu-
ous. Discontinuous operation occurs when I
OUT2
is less
than ΔI
L2
/2.
Boost Inductor Selection
For most applications the inductor will fall in the range
of 2.2µH to 22µH. Lower values are chosen to reduce
physical size of the inductor. Higher values allow more
output current because they reduce peak current seen by
the power switch, which has a 1.5A current limit. Higher
values also reduce input ripple voltage and reduce core
loss. The following procedure is suggested as a way of
choosing a more optimum inductor.
Assume that the average inductor current for a boost
converter is equal to the load current times V
OUT1
/V
IN1
and decide whether or not the inductor must withstand
continuous overload conditions. If average inductor cur-
rent at maximum load current is 0.5A, for instance, a 0.5A
inductor may not survive a continuous 1.5A overload
condition. Also be aware that boost converters are not
short-circuit protected, and that under short conditions,
inductor current is limited only by the available current
of the input supply.
Calculate peak inductor current at full load current to en-
sure that the inductor will not saturate. Peak current can
be signifi cantly higher than output current, especially with
smaller inductors and lighter loads, so don’t omit this step.
Powdered iron cores are forgiving because they saturate
softly, whereas ferrite cores saturate abruptly. Other
core materials fall somewhere in between. The following
formula assumes continuous mode operation but it errs
only slightly on the high side for discontinuous mode, so
it can be used for all conditions.
I
PEAK1
=
I
OUT1
•V
OUT1
V
IN1
+
V
IN1
V
OUT1
–V
IN1
()
2•f•L•V
OUT1
Make sure that I
PEAK1
is less than the switch current I
LIM1
.
I
LIM1
is at least 1.5A at low duty cycles and decreases
linearly to 1.2A at DC1 = 0.8. The maximum switch current
limit can be calculated by the following formula:
I
LIM1
= 1.5 • (1 – 0.25 • DC1)
where DC1 is the duty cycle and is defi ned as:
DC1= 1–
V
IN1
V
OUT1
Remember also that inductance can drop signifi cantly with
DC current and manufacturing tolerance. Consideration
should also be given to the DC resistance of the inductor
as this contributes directly to the effi ciency losses in the
overall converter. Table 1 lists several inductor vendors
and types that are suitable.
Buck Output Capacitor Selection
For 5V and 3.3V outputs, a 10µF, 6.3V ceramic capacitor
(X5R or X7R) at the output results in very low output volt-
age ripple and good transient response. For lower voltages,
10µF is adequate for ripple requirements but increasing
C
OUT
will improve transient performance. Other types and
values will also work; the following discusses tradeoffs in
output ripple and transient performance.
The output capacitor fi lters the inductor current to gener ate
an output with low voltage ripple. It also stores energy in
order to satisfy transient loads and stabilize the LT3570’s
control loop. Because the LT3570 operates at a high
frequency, minimal output capacitance is necessary. In
addition, the control loop operates well with or without
the presence of output capacitor series resistance (ESR).
Ceramic capacitors, which achieve very low output ripple
APPLICATIONS INFORMATION
