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LT1507IN8-3.3#PBF

P1-P3P4-P6P7-P9P10-P12P13-P15P16-P18P19-P20
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LT1507
APPLICATIONS INFORMATION
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be due to a radiated magnetic field coupling into PC
board traces. But why were some boards bad and
others good? In a moment of desperation (or divine
inspiration) I unsoldered a “bad” inductor, rotated it
180° and resoldered it. Problem fixed!!
It turns out that the inductor was symmetrical in all
regards except that the polarity of the magnetic field
reversed when the unit was rotated 180° because
current flowed in the opposite direction in the coil. In
one direction, the magnetically induced ripple in the
board traces
added
to output ripple. Rotating the induc-
tor caused the induced field to
reduce
output ripple.
Unfortunately the inductor had no physical package
assymmetry to indicate rotation, including part mark-
ing, so we had to visually examine the winding in each
unit before soldering it to the boards. This little horror
story should not preclude the use of open core induc-
tors, but it emphasizes the need to carefully check the
effect these seductively small, low cost inductors may
have on regulator or system performances.
4. Look for an inductor (see Table 1) which meets the
requirements of core shape, peak current (to avoid
saturation), average current (to limit heat) and fault
current (if the inductor gets too hot, wire insulation will
melt and cause turn-to-turn shorts). Keep in mind that
all good things like high efficiency, surface mounting,
low profile and high temperature operation will increase
cost, sometimes dramatically.
5. After making an initial choice, consider secondary things
like output voltage ripple, second sourcing, etc. Use the
experts in the Linear Technology Applications Depart-
ment if you feel uncertain about the final choice. They
have experience with a wide range of inductor types and
can tell you about the latest developments in low profile,
surface mounting, etc.
Table 1. Representative Surface Mount Units
VALUE DC CORE SERIES HEIGHT
MANUFACTURER (µH) (A) TYPE (Ω) CORE (mm)
Coiltronics
CTX5-1 5 2.3 Tor 0.027 KMµ 4.2
CTX10-1 10 1.9 Tor 0.039 KMµ 4.2
CTX5-1P 5 1.8 Tor 0.021 52 4.2
CTX10-1P 10 1.6 Tor 0.030 52 4.2
Sumida
CDRH64 10 1.7 SC 0.084 Fer 4.5
CDRH73 10 1.7 SC 0.055 Fer 3.4
CD73 10 1.4 Open 0.062 Fer 3.5
CD104 10 2.4 Open 0.041 Fer 4.0
Gowanda
SM20-102K 10 1.3 Open 0.038 Fer 7
Dale
IHSM-4825 10 3.1 Open 0.071 Fer 5.6
IHSM-5832 10 4.3 Open 0.053 Fer 7.1
SC = Semi-closed geometry
Fer = Ferrite core material
52 = Type 52 powdered iron core material
KMµ = Kool Mµ
OUTPUT CAPACITOR
The output capacitor is normally chosen by its effective
series resistance (ESR), because that is what determines
output ripple voltage. At 500kHz any polarized capacitor is
essentially resistive. To get low ESR takes
volume
; physi-
cally larger capacitors have lower ESR. The ESR range
needed for typical LT1507 applications is 0.05Ω to 0.5Ω.
A typical output capacitor is an AVX type TPS, 100µF at
10V, with a guaranteed ESR less than 0.1Ω. This is a “D”
size surface mount solid tantalum capacitor. TPS capaci-
tors are specially constructed and tested for low ESR so
they give the lowest ESR for a given volume. The value in
microfarads is not particularly critical and values from
22µF to greater than 500µF work well, but you cannot
cheat mother nature on ESR. If you find a tiny 22µF solid
tantalum capacitor, it will have high ESR and output ripple
voltage will be terrible. The chart in Table 2 shows some
typical solid tantalum surface mount capacitors.
11
LT1507
APPLICATIONS INFORMATION
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appropriate for input bypassing because of their high
ripple current ratings and tolerance of turn-on surges.
OUTPUT RIPPLE VOLTAGE
Ripple voltage is determined by the high frequency imped-
ance of the output capacitor and ripple current through the
inductor. Ripple current is triangular (continuous mode)
with a peak-to-peak value of:
I
VVV
VLf
OUT IN OUT
IN
P-P
=
−()( )
()()()
Output ripple voltage is also triangular with peak-to-peak
amplitude of:
V
RIPPLE
= (I
P–P
)(ESR) (peak-to-peak)
Example: with V
IN
= 5V, V
OUT
= 3.3V, L = 5µH, ESR = 0.1Ω;
I
VAmV
RIPPLE
P-P P-P
P-P
=
−
=
=Ω=
−
(.)( .)
.
(. )(. )
33 5 33
5 5 10 500 10
045
045 01 45
63
MAXIMUM OUTPUT LOAD CURRENT
Maximum load current will be less than the 1.5A rating of
the LT1507, especially with lower inductor values. Induc-
tor ripple current must be taken into account as well as
reduced switch current at high duty cycles. Maximum
switch current
rating (I
P
) of the LT1507 is 1.5A up to 50%
duty cycle (DC), decreasing to 1.35A at 80% duty cycle,
shown graphically in Typical Performance Characteristics
and as a formula below. Current rating decreases with
duty cycle because the LT1507 has internal slope com-
pensation to prevent current mode subharmonic switch-
ing. For more details on subharmonic oscillation read
Application Note 19. Peak guaranteed switch current (I
P
)
is found from:
IA
V
IA
V
V
V
P
IN
P
OUT
IN
IN
=≤
=− ≥
15 05
175
05
05
..
.
.( )
.
for
V
for
V
OUT
OUT
Table 2. Surface Mount Solid Tantalum Capacitor ESR
and Ripple Current
E CASE SIZE ESR (MAX Ω) RIPPLE CURRENT (A)
AVX TPS, Sprague 593D 0.1 to 0.3 0.7 to 1.1
AVX TAJ 0.7 to 0.9 0.4
D CASE SIZE
AVX TPS, Sprague 593D 0.1 to 0.3 0.7 to 1.1
AVX TAJ 0.9 to 2.0 0.36 to 0.24
C CASE SIZE
AVX TPS 0.2 (Typ) 0.5 (Typ)
AVX TAJ 1.8 to 3.0 0.22 to 0.17
Many engineers have heard that solid tantalum capacitors
are prone to failure if they undergo high surge currents.
This is historically true, and type TPS capacitors are
specially tested for surge capability, but surge rugged-
ness is not a critical issue with the
output
capacitor. Solid
tantalum capacitors fail during very high
turn-on
surges
which do not occur at the output of regulators. High
discharge
surges, such as when the regulator output is
dead shorted, do not harm the capacitors.
Unlike the input capacitor, RMS ripple current in the
output capacitor is normally low enough that ripple cur-
rent rating is not an issue. The current waveform is
triangular with a typical value of 200mA RMS. The formula
to calculate this is:
Output Capacitor Ripple Current (RMS)
I RMS
VVV
LfV
RIPPLE
OUT IN OUT
IN
()
.( )( – )
()()( )
=
029
Ceramic Capacitors
Higher value, lower cost ceramic capacitors are now
becoming available in smaller case sizes. These are tempt-
ing for switching regulator use because of their very low
ESR. Unfortunately, the ESR is so low that it can cause
loop stability problems when ceramic is used for the
output capacitor. Solid tantalum capacitor ESR generates
a loop “zero” at 5kHz to 50kHz that is instrumental in
giving acceptable loop phase margin. Ceramic capacitors
remain capacitive to beyond 300kHz and usually resonate
with their ESL before ESR becomes effective. They are
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LT1507
APPLICATIONS INFORMATION
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Example: with V
OUT
= 3.3V, V
IN
= 5V;
V
OUT
/V
IN
= 3.3/5 = 0.67
I
P
= 1.75 – (0.5)(0.66) = 1.42A
Maximum load current would be equal to maximum
switch current
for an infinitely large inductor,
but with
finite inductor size, maximum load current is reduced by
one half peak-to-peak inductor current. The following
formula assumes continuous mode operation; the term on
the right must be less than one half of I
P
.
Continuous mode:
II
VVV
LfV
OUT MAX P
OUT IN OUT
IN
()
–
()(– )
()()( )
=
2
For the conditions above, with L = 5µH and f = 500kHz;
I
A
OUT MAX()
.–
( . )( – . )
.–. .
=
()
()
==
−
142
33 5 33
2 5 10 500 10 5
142 022 12
63
At V
IN
= 8V, V
OUT
/V
IN
= 0.41, so I
P
is equal to 1.5A and
I
OUT(MAX)
is equal to;
15
33 8 33
2 5 10 500 10 8
15 039 111
63
.–
( . )( – . )
.–. .
−
()
()
==A
Note that there is less load current available at the higher
input voltage because inductor ripple current increases.
This is not always the case. Certain combinations of
inductor value and input voltage range may yield lower
available load current at the lowest input voltage due to
reduced peak switch current at high duty cycles. If load
current is close to the maximum available, please check
maximum available current at both input voltage
extremes. To calculate actual peak switch current with a
given set of conditions, use:
II
VVV
LfV
SWITCH PEAK OUT
OUT IN OUT
IN
()
(– )
()()( )
=+
2
For lighter loads where discontinuous mode operation can
be used, maximum load current is equal to:
Discontinuous mode:
I
IfLV
VVV
OUT MAX
PIN
OUT IN OUT
()
()()()( )
()(–)
=
2
2
Example: with L = 2µH, V
OUT
= 5V and V
IN(MAX)
= 15V;
I
m
A
OUT MAX()
(.)
()( – )
=
()
()
=
−
1 5 500 10 2 10 15
2 5 15 5
338
23 6
The main reason for using such a tiny inductor is that it is
physically very small, but keep in mind that peak-to-peak
inductor current will be very high. This will increase output
ripple voltage. If the output capacitor has to be made larger
to reduce ripple voltage, the overall circuit could actually
be larger.
CATCH DIODE
The suggested catch diode (D1) is a 1N5818 Schottky or
its Motorola equivalent, MBR130. It is rated at 1A average
forward current and 30V reverse voltage. Typical forward
voltage is 0.42V at 1A. The diode conducts current only
during switch OFF time. Peak reverse voltage is equal to
regulator input voltage. Average forward current in normal
operation can be calculated from:
I
IVV
V
D AVG
OUT IN OUT
IN
()
(– )
=
This formula will not yield values higher than 1A with
maximum load current of 1.25A unless the ratio of input to
output voltage exceeds 5:1. The only reason to consider a
larger diode is the worst-case condition of a high input
voltage and
overloaded
(not shorted) output. Under short-
circuit conditions, foldback current limit will reduce diode
current to less than 1A, but if the output is overloaded and
does not fall to less than 1/3 of nominal output voltage,
foldback will not take effect. With the overloaded condi-
tion, output current will increase to a typical value of 1.8A,
determined by peak switch current limit of 2A. With V
IN
=
10V, V
OUT
= 2V (3.3V overloaded) and I
OUT
= 1.8A:
IA
D AVG()
.( –)
.==
1 8 10 2
10
144
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LT1507IN8-3.3#PBF

Mfr. #:
Buy LT1507IN8-3.3#PBF
Manufacturer:
Analog Devices / Linear Technology
Description:
Switching Voltage Regulators 500kHz Mono Buck Mode Sw Reg
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