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LT3430IFE#PBF

P1-P3P4-P6P7-P9P10-P12P13-P15P16-P18P19-P21P22-P24P25-P27P28-P28
LT3430/LT3430-1
10
34301fa
inductor value to achieve a desirable output ripple volt-
age level. If output ripple voltage is of less importance,
the subsequent suggestions in Peak Inductor and Fault
Current and EMI will additionally help in the selection of
the inductor value.
Peak-to-peak output ripple voltage is the sum of a triwave
(created by peak-to-peak ripple current (I
LP-P
) times ESR)
and a square wave (created by parasitic inductance (ESL)
and ripple current slew rate). Capacitive reactance is as-
sumed to be small compared to ESR or ESL.
V I ESR ESL
dI
dt
RIPPLE LP P
=
()()
+
()
-
where:
ESR = equivalent series resistance of the output capaci-
tor
ESL = equivalent series inductance of the output capaci-
tor
dI/dt = slew rate of inductor ripple current = V
IN
/L
Peak-to-peak ripple current (I
LP-P
) through the inductor
and into the output capacitor is typically chosen to be
between 20% and 40% of the maximum load current. It
is approximated by:
I
VVV
VfL
LP P
OUT IN OUT
IN
-
=
()( )
()()()
–
Example: with V
IN
= 40V, V
OUT
= 5V, L = 22µH, ESR =
0.080Ω and ESL = 10nH, output ripple voltage can be
approximated as follows:
IA
dI
dt
VA
mV
RIPPLE
P-P
P-P
=
()
−
()
()
()()
=
==
=
()()
+
()()
()
=+=
−
−
−
540 5
40 22 10 200 10
099
40
22 10
10 1 8
0 99 0 08 10 10 10 1 8
0 079 0 018 97
63
6
6
96
••
.
•
•.
.. • .
..
To reduce output ripple voltage further requires an increase
in the inductor value with the trade-off being a physically
larger inductor with the possibility of increased component
height and cost.
Ceramic Output Capacitor
An alternative way to further reduce output ripple voltage
is to reduce the ESR of the output capacitor by using a
ceramic capacitor. Although this reduction of ESR removes
a useful zero in the overall loop response, this zero can
be replaced by inserting a resistor (R
C
) in series with the
V
C
pin and the compensation capacitor C
C
. (See Ceramic
Capacitors in Applications Information.)
Peak Inductor Current and Fault Current
To ensure that the inductor will not saturate, the peak
inductor current should be calculated knowing the
maximum load current. An appropriate inductor should
then be chosen. In addition, a decision should be made
whether or not the inductor must withstand continuous
fault conditions.
If maximum load current is 1A, for instance, a 1A induc-
tor may not survive a continuous 4A overload condition.
Dead shorts will actually be more gentle on the inductor
because the LT3430/LT3430-1 have frequency and current
limit foldback.
APPLICATIONS INFORMATION
Table 2
VENDOR/
PART NO.
VALUE
(µH)
I
DC
(Amps)
DCR
(Ohms)
HEIGHT
(mm)
Sumida
CDRH104R-150 15 3.6 0.050 4
CDRH104R-220 22 2.9 0.073 4
CDRH104R-330 33 2.3 0.093 4
CDRH124-220 22 2.9 0.066 4.5
CDRH124-330 33 2.7 0.097 4.5
CDRH127-330 33 3.0 0.065 8
CDRH127-470 47 2.5 0.100 8
CEI122-220 22 2.3 0.085 8
Coiltronics
UP3B-330 33 3 0.069 6.8
UP3B-470 47 2.4 0.108 6.8
UP4B-680 68 4.3 0.120 7.9
Coilcraft
DO3316P-153 15 3 0.046 5.2
DO5022p-683 68 3.5 0.130 7.1
LT3430/LT3430-1
11
34301fa
Peak switch and inductor 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 of operation, but errs only slightly on
the high side for discontinuous mode, so it can be used
for all conditions.
II
I
I
VVV
VfL
PEAK OUT
LP P
OUT
OUT IN OUT
IN
=+ =+
()( )
()( )()()
-
22
–
EMI
Decide if the design can tolerate an “open” core geometry
like a rod or barrel, which have high magnetic fi eld radiation,
or whether it needs a closed core like a toroid to prevent
EMI problems. This is a tough decision because the rods
or barrels are temptingly cheap and small and there are
no helpful guidelines to calculate when the magnetic fi eld
radiation will be a problem.
Additional Considerations
After making an initial choice, consider additional factors
such as core losses and second sourcing, etc. Use the
experts in Linear Technology’s Applications department
if you feel uncertain about the fi nal choice. They have ex-
perience with a wide range of inductor types and can tell
you about the latest developments in low profi le, surface
mounting, etc.
Maximum Output Load Current
Maximum load current for a buck converter is limited by
the maximum switch current rating (I
P
). The current rating
for the LT3430/LT3430-1 is 3A. Unlike most current mode
converters, the LT3430/LT3430-1 maximum switch current
limit does not fall off at high duty cycles. Most current
mode converters suffer a drop off of peak switch current
for duty cycles above 50%. This is due to the effects of
slope compensation required to prevent subharmonic
oscillations in current mode converters. (For detailed
analysis, see Application Note 19.)
The LT3430/LT3430-1 are able to maintain peak switch
current limit over the full duty cycle range by using patented
circuitry* to cancel the effects of slope compensation
on peak switch current without affecting the frequency
compensation it provides.
Maximum load current would be equal to maximum switch
current for an infi nitely large inductor, but with fi nite
inductor size, maximum load current is reduced by one-
half peak-to-peak inductor current (I
LP-P
). The following
formula assumes continuous mode operation, implying
that the term on the right is less than one-half of IP.
I
OUT(MAX)
=
Continuous Mode
I–
I
2
=I
P
LP-P
P
−
+
()
−
()
()()
VVVVV
LfV
OUT F IN OUT F
–
2
IIN
()
For V
OUT
= 5V, V
IN
= 12V, V
F(D1)
= 0.52V, f = 200kHz and
L = 15µH:
I
A
OUT MAX
()
−
=−
+
()
−
()
()()
()
=− =
3
5 0 52 12 5 0 52
2 15 10 200 10 12
30525
63
.–.
••
..
Note that there is less load current available at the higher
input voltage because inductor ripple current increases.
At V
IN
= 24V, duty cycle is 23% and for the same set of
conditions:
I
A
OUT MAX()
.–.
••
..
=−
+
()
−
()
()()
()
=− =
−
3
5 0 52 24 5 0 52
2 15 10 200 10 24
3 0 71 2 29
63
To calculate actual peak switch current with a given set
of conditions, use:
II
I
VVVVV
LfV
SW PEAK
OUT
P
OUT
OUT F IN OUT F
IN
()
=+
=+
+−
()
()()( )
I
2
L-P
() –
2
APPLICATIONS INFORMATION
*US Patent # 6,498,466
LT3430/LT3430-1
12
34301fa
Reduced Inductor Value and Discontinuous Mode
If the smallest inductor value is of most importance to a
converter design, in order to reduce inductor size/cost,
discontinuous mode may yield the smallest inductor solu-
tion. The maximum output load current in discontinuous
mode, however, must be calculated and is defi ned later
in this section.
Discontinuous mode is entered when the output load
current is less than one-half of the inductor ripple current
(I
LP-P
). In this mode, inductor current falls to zero before
the next switch turn on (see Figure 8). Buck converters
will be in discontinuous mode for output load current
given by:
I
OUT
Discontinuous Mode
<
+()(––)
()( )()()
VVVVV
VfL
OUT F IN OUT F
IN
2
The inductor value in a buck converter is usually chosen
large enough to keep inductor ripple current (I
LP-P
) low;
this is done to minimize output ripple voltage and maximize
output load current. In the case of large inductor values,
as seen in the equation above, discontinuous mode will
be associated with “light loads.”
When choosing small inductor values, however, discontinu-
ous mode will occur at much higher output load currents.
The limit to the smallest inductor value that can be chosen
is set by the LT3430/LT3430-1 peak switch current (I
P
) and
the maximum output load current required, given by:
I
OUT(MAX)
Discontinuous Mode
=<
I
LP-P
2
=
=
()
+
I
IfLV
VVVVV
P
PIN
OUT F IN OUT F
2
2
2
2
()( )
••
()(––)
I
LP-P
Example: For V
IN
= 15V, V
OUT
= 5V, V
F
= 0.52V, f = 200kHz
and L = 4.7µH.
I
OUT(MAX)
Discontinuous
Mode
=
+
−
3 200 10 4 7 10 15
25052155052
236
•( • )( . • )( )
(.)(––.)
I
OUT(MAX)
= 1.21A
Discontinuous Mode
What has been shown here is that if high inductor ripple
current and discontinuous mode operation can be tolerated,
small inductor values can be used. If a higher output load
current is required, the inductor value must be increased.
If I
OUT(MAX)
no longer meets the discontinuous mode
criteria, use the I
OUT(MAX)
equation for continuous mode;
the LT3430/LT3430-1 are designed to operate well in both
modes of operation, allowing a large range of inductor
values to be used.
Short-Circuit Considerations
The LT3430/LT3430-1 are current mode controllers. They
use the V
C
node voltage as an input to a current compara-
tor which turns off the output switch on a cycle-by-cycle
basis as this peak current is reached. The internal clamp on
the V
C
node, nominally 2V, then acts as an output switch
peak current limit. This action becomes the switch current
limit specifi cation. The maximum available output power
is then determined by the switch current limit.
A potential controllability problem could occur under
short-circuit conditions. If the power supply output is
short circuited, the feedback amplifi er responds to the
low output voltage by raising the control voltage, V
C
, to its
peak current limit value. Ideally, the output switch would
be turned on, and then turned off as its current exceeded
the value indicated by V
C
. However, there is fi nite response
time involved in both the current comparator and turnoff
of the output switch. These result in a minimum on time
t
ON(MIN)
. When combined with the large ratio of V
IN
to
(V
F
+ I • R), the diode forward voltage plus inductor I • R
voltage drop, the potential exists for a loss of control.
Expressed mathematically the requirement to maintain
control is:
ft
VIR
V
ON
F
IN
•
•
≤
+
APPLICATIONS INFORMATION
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LT3430IFE#PBF

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Manufacturer:
Analog Devices / Linear Technology
Description:
Switching Voltage Regulators Hi V, 3A, 200kHz Buck Sw Reg
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