LT3509 LINER [Linear Technology], LT3509 Datasheet - Page 15

• Manufacturer Part Number: LT3509
• Description: Dual 36V, 700mA Step-Down Regulator
• Manufacturer: LINER [Linear Technology]

APPLICATIONS INFORMATION
Table 2. Recommended Inductors
MANUFACTURER/
PART NUMBER
Coilcraft
LPS4018-222ML
LPS5030-332ML
LPS5030-472ML
LPS6225-682ML
LPS6225-103ML
Sumida
CDRH4D22/HP-2R2N
CDRH4D22/HP-3R5N
CDRH4D22/HP-4R7N
CDRH5D28/HP-6R8N
CDRH5D28/HP-8R2N
CDRH5D28R/HP-100N
Cooper
SD52-2R2-R
SD52-3R5-R
SD52-4R7-R
SD6030-5R8-R
SD7030-8R0-R
SD7030-100-R
Toko
A997AS-2R2N
A997AS-3R3N
A997AS-4R7M
Würth
7447745022
7447745033
7447745047
7447745076
7447445100
VALUE
(μH)
10.0
2.2
3.3
4.7
6.8
2.2
3.5
4.7
6.8
8.2
2.2
3.5
4.7
5.8
8.0
2.2
3.3
4.7
2.2
3.3
4.7
7.6
10
10
10
ISAT
2.45
2.30
1.82
1.64
1.85
1.07
(A)
2.8
2.5
2.5
2.7
2.1
3.2
2.5
2.2
3.1
2.7
1.8
1.7
1.6
1.2
3.5
3.0
2.4
1.8
1.6
0.0035
0.0385
0.0503
0.0568
0.066
0.083
0.095
0.105
0.052
0.066
0.049
0.071
0.074
0.045
0.058
0.065
0.036
0.045
0.057
0.095
DCR
0.07
0.06
0.07
0.12
(Ω)
0.1
HEIGHT
(mm)
1.7
2.9
2.9
2.4
2.4
2.4
2.4
2.4
3.0
3.0
3.0
2.0
2.0
2.0
3.0
3.0
3.0
1.8
1.8
1.8
2.0
2.0
2.0
2.0
2.0
The prior analysis is valid for continuous mode operation
(I
in discontinuous mode operation, see Linear Technology’s
Application Note AN44. Finally, for duty cycles greater
than 50% (V
required to avoid subharmonic oscillations. This minimum
inductance is
where f
If using external synchronization, calculate L
R
Frequency Compensation
The LT3509 uses current mode control to regulate the
output, which simplifi es loop compensation and allows
the necessary fi lter components to be integrated. The fi xed
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 amplifi er with
fi nite output impedance. The power section, consisting of
the modulator, power switch and inductor is modeled as a
transconductance amplifi er 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 amplifi er output current,
resulting in two poles in the loop. In most cases, a zero is
required and comes either from the output capacitor ESR
T
OUT
L
frequency and not the SYNC frequency.
MIN
> ΔI
SW
=
LIM
(
is in MHz and L
V
OUT
/ 2). For details of maximum output current
OUT
+
/V
V
IN
F
)•
> 0.5), a minimum inductance is
f
. 1 4
SW
MIN
is in μH.
LT3509
MIN
using the
15
3509f