LTC3617_15 Datasheet, PDF (12/20 Pages) Linear Technology – 6A Monolithic Synchronous Step-Down Regulator for DDR Termination

LTC3617
Applications Information
Table 1. Representative Surface Mount Inductors
INDUCTANCE DCR SATURATION DIMENSIONS
(μH)
(mΩ) CURRENT (A)
(mm)
Vishay IHLP-2525CZ-01
0.10
1.5
60
6.5 × 6.9
0.15
1.9
52
6.5 × 6.9
0.20
2.4
41
6.5 × 6.9
0.22
2.5
40
6.5 × 6.9
0.33
3.5
30
6.5 × 6.9
0.47
4
26
6.5 × 6.9
Sumida CDMC6D28 Series
0.2
2.5
21.7
7.25 × 6.5
0.3
3.2
15.4
7.25 × 6.5
0.47
4.2
13.6
7.25 × 6.5
Cooper HCM0703 Series
0.22
2.8
40
6.8 × 7.1
0.47
4.2
26
6.8 × 7.1
0.68
5.5
25
6.8 × 7.1
Würth Electronik WE-HC744310 Series
0.24
1.8
40
7 × 6.9
0.52
3.7
20
7 × 6.9
Coilcraft SLC7530 Series
0.100
0.123
20
7.5 × 6.7
0.188
0.100
21
7.5 × 6.7
0.272
0.100
14
7.5 × 6.7
0.350
0.100
11
7.5 × 6.7
0.400
0.100
8
7.5 × 6.7
HEIGHT
(mm)
3
3
3
3
3
3
3
3
3
3.0
3.0
3.0
3.0
3.0
3
3
3
3
3
Output Capacitor (C_OUT ) Selection
The selection of C_OUT is typically driven by the required
ESR to minimize voltage ripple and load step transients
(low ESR ceramic capacitors are discussed in the next
section). Typically, once the ESR requirement is satisfied,
the capacitance is adequate for filtering. The output ripple
∆V_OUT is determined by:
∆V_OUT
≤
∆I_L
•


ESR
+
8
•
1
f_SW •
C_OUT


where f_OSC = operating frequency, C_OUT = output capaci-
tance and ∆I_L = ripple current in the inductor. The output
ripple is highest at maximum input voltage since ∆I_L
increases with input voltage.
In surface mount applications, multiple capacitors may
have to be paralleled to meet the capacitance, ESR or RMS
current handling requirement of the application. Aluminum
electrolytic, special polymer, ceramic and dry tantalum
capacitors are all available in surface mount packages.
Tantalum capacitors have the highest capacitance density,
but can have higher ESR and must be surge tested for
use in switching power supplies. Aluminum electrolytic
capacitors have significantly higher ESR, but can often
be used in extremely cost-sensitive applications provided
that consideration is given to ripple current ratings and
long-term reliability.
Ceramic Input and Output Capacitors
Ceramic capacitors have the lowest ESR and can be cost
effective, but also have the lowest capacitance density,
have high voltage and temperature coefficients, and exhibit
audible piezoelectric effects. In addition, the high Q of
ceramic capacitors along with trace inductance can lead
to significant ringing.
They are attractive for switching regulator use because of
their very low ESR, but care must be taken when using
only ceramic input and output capacitors.
Ceramic capacitors are prone to temperature effects
which require the designer to check loop stability over
the operating temperature range. To minimize their large
temperature and voltage coefficients, only X5R or X7R
ceramic capacitors should be used.
When a ceramic capacitor is used at the input and the power
is being supplied through long wires, such as from a wall
adapter, a load step at the output can induce ringing at the
V_IN pin. At best, this ringing can couple to the output and
be mistaken as loop instability. At worst, the ringing at the
input can be large enough to damage the part.
3617fa
12