3509 Ver la hoja de datos (PDF) - Linear Technology
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3509 Datasheet PDF : 24 Pages
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APPLICATIONS INFORMATION
Table 2. Recommended Inductors
MANUFACTURER/
PART NUMBER
VALUE
ISAT
(μH)
(A)
Coilcraft
LPS4018-222ML
2.2
2.8
LPS5030-332ML
3.3
2.5
LPS5030-472ML
4.7
2.5
LPS6225-682ML
6.8
2.7
LPS6225-103ML
10
2.1
Sumida
CDRH4D22/HP-2R2N
2.2
3.2
CDRH4D22/HP-3R5N
3.5
2.5
CDRH4D22/HP-4R7N
4.7
2.2
CDRH5D28/HP-6R8N
6.8
3.1
CDRH5D28/HP-8R2N
8.2
2.7
CDRH5D28R/HP-100N 10
2.45
Cooper
SD52-2R2-R
2.2
2.30
SD52-3R5-R
3.5
1.82
SD52-4R7-R
4.7
1.64
SD6030-5R8-R
5.8
1.8
SD7030-8R0-R
8.0
1.85
SD7030-100-R
10.0
1.7
Toko
A997AS-2R2N
2.2
1.6
A997AS-3R3N
3.3
1.2
A997AS-4R7M
4.7
1.07
Würth
7447745022
2.2
3.5
7447745033
3.3
3.0
7447745047
4.7
2.4
7447745076
7.6
1.8
7447445100
10
1.6
DCR HEIGHT
(Ω)
(mm)
0.07
1.7
0.066
2.9
0.083
2.9
0.095
2.4
0.105
2.4
0.0035
2.4
0.052
2.4
0.066
2.4
0.049
3.0
0.071
3.0
0.074
3.0
0.0385
2.0
0.0503
2.0
0.0568
2.0
0.045
3.0
0.058
3.0
0.065
3.0
0.06
1.8
0.07
1.8
0.1
1.8
0.036
2.0
0.045
2.0
0.057
2.0
0.095
2.0
0.12
2.0
LT3509
The prior analysis is valid for continuous mode operation
(IOUT > ΔILIM / 2). For details of maximum output current
in discontinuous mode operation, see Linear Technology’s
Application Note 44. Finally, for duty cycles greater than
50% (VOUT/VIN > 0.5), a minimum inductance is required
to avoid subharmonic oscillations. This minimum induc-
tance is
LMIN
=
(VOUT
+
VF
)
•
1.4
fSW
where fSW is in MHz and LMIN is in μH.
If using external synchronization, calculate LMIN using the
RT frequency and not the SYNC frequency.
Frequency Compensation
The LT3509 uses current mode control to regulate the
output, which simplifies loop compensation and allows
the necessary filter components to be integrated. The fixed
internal compensation network has been chosen to give
stable operation over a wide range of operating conditions
but assumes a minimum load capacitance. The LT3509
does not depend on the ESR of the output capacitor for
stability so the designer is free to use ceramic capacitors
to achieve low output ripple and small PCB footprint.
Figure 10 shows an equivalent circuit for the LT3509 control
loop. The error amp is a transconductance amplifier with
finite output impedance. The power section, consisting of
the modulator, power switch and inductor is modeled as a
transconductance amplifier generating an output current
proportional to the voltage at the COMP-NODE. The gain
of the power stage (gmp) is 1.1S. Note that the output
capacitor integrates this current and that the internal
capacitor integrates the error amplifier output current,
resulting in two poles in the loop. In most cases, a zero is
required and comes either from the output capacitor ESR
3509fc
15
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