LTC3542 Datasheet, PDF (9/16 Pages) Linear Technology – 500mA, 2.25MHz Synchronous Step-Down DC/DC Converter

APPLICATIONS INFORMATION
Table 1. Representative Surface Mount Inductors
MAX DC
PART NUMBER VALUE CURRENT DCR
MANUFACTURER
(μH)
(A)
(Ω)
SIZE (mm³)
Sumida
CDRH2D11-2RM 2.2 0.780 0.098 3.2 × 3.2 × 1.2
CDRH3D16 2.2
1.2 0.075 3.8 × 3.8 × 1.8
CMD4D11
2.2
0.95 0.116 4.4 × 5.8 × 1.2
CDH2D09B
3.3
0.85 0.15 2.8 × 3 × 1
CLS4D09
4.7
0.75 0.15 4.9 × 4.9 × 1
Murata
LQH32CN
2.2
0.79 0.097 2.5 × 3.2 × 1.55
LQH43CN
4.7
0.75 0.15 4.5 × 3.2 × 2.6
TDK
IVLC453232 2.2
0.85 0.18 4.8 × 3.4 × 3.4
VLF3010AT- 2.2
2R2M1R0
1.0 0.12 2.8 × 2.6 × 1
LTC3542
design. An additional 0.1μF to 1μF ceramic capacitor is
also recommended on V_IN for high frequency decoupling,
when not using an all ceramic capacitor solution.
Output Capacitor (C_OUT) Selection
The selection of C_OUT is driven by the required ESR to
minimize voltage ripple and load step transients. Typically,
once the ESR requirement is satisfied, the RMS current
rating generally far exceeds the I_RIPPLE(P-P) requirement,
except for an all ceramic solution. The output ripple (ΔV_OUT)
is determined by:
ΔV_OUT
≈
ΔI_L
⎛
⎝⎜
ESR
+
8
•
ƒ_O
1
•
C_OUT
⎞
⎠⎟
where f_O is the switching frequency, C_OUT is the output
capacitance and ΔI_L is the inductor ripple current. For a fixed
output voltage, the output ripple is highest at maximum
input voltage since ΔI_L increases with input voltage.
If tantalum capacitors are used, it is critical that the capaci-
tors are surge tested for use in switching power supplies.
An excellent choice is the AVX TPS series of surface mount
tantalums, available in case heights ranging from 2mm to
4mm. These are specially constructed and tested for low
ESR so they give the lowest ESR for a given volume. Other
capacitor types include Sanyo POSCAP, Kemet T510 and
T495 series, and Sprague 593D and 595D series. Consult
the manufacturer for other specific recommendations.
Ceramic Input and Output Capacitors
Higher value, lower cost ceramic capacitors are now
becoming available in smaller case sizes. Their high
ripple current rating, high voltage rating and low ESR
are tempting for switching regulator use. However, the
ESR is so low that it can cause loop stability problems.
Since the LTC3542’s control loop does not depend on
the output capacitor’s ESR for stable operation, ceramic
capacitors can be used to achieve very low output ripple
and small circuit size. X5R or X7R ceramic capacitors are
recommended because these dielectrics have the best
temperature and voltage characteristics of all the ceramics
for a given value and size.
Great care must be taken when using only ceramic input
and output capacitors. 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. For more information, see Application
Note 88. The recommended capacitance value to use is
10μF for both input and output capacitors.
3542fa
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