LT3570
APPLICATIONS INFORMATION
to 1.2A at DC2 = 0.8. The maximum output current is a
function of the chosen inductor value:
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.
ΔIL2
2
IOUT2(MAX) =ILIM2
–
ΔIL2
2
=1.5• 1– 0.25•DC2 –
(
)
V
VOUT1 – V
IN1
IOUT1 • VOUT1
(
)
IN1
IPEAK1
=
+
V
2• f •L • VOUT1
Choosing an inductor value so that the ripple current is
smallwillallowamaximumoutputcurrentneartheswitch
current limit.
IN1
Make sure that I
LIM1
is less than the switch current I
.
LIM1
PEAK1
I
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:
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-
I
= 1.5 • (1 – 0.25 • DC1)
LIM1
where DC1 is the duty cycle and is defined as:
V
VOUT1
IN1
DC1=1–
ous. Discontinuous operation occurs when I
is less
OUT2
than ΔI /2.
L2
Rememberalsothatinductancecandropsignificantlywith
DC current and manufacturing tolerance. Consideration
should also be given to the DC resistance of the inductor
as this contributes directly to the efficiency losses in the
overall converter. Table 1 lists several inductor vendors
and types that are suitable.
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.
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-
agerippleandgoodtransientresponse.Forlowervoltages,
10μF is adequate for ripple requirements but increasing
Assume that the average inductor current for a boost
converter is equal to the load current times V
/V
OUT1 IN1
C
OUT
will improve transient performance. Other types and
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
values will also work; the following discusses tradeoffs in
output ripple and transient performance.
Theoutputcapacitorfilterstheinductorcurrenttogenerate
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
Calculate peak inductor current at full load current to en-
sure that the inductor will not saturate. Peak current can
be significantly higher than output current, especially with
smallerinductorsandlighterloads,sodon’tomitthisstep.
3570fa
11
Linear [ Linear ]
