LTC3403EDD View Datasheet(PDF) - Linear Technology
Part Name
Description
MFG CO.
LTC3403EDD Datasheet PDF : 16 Pages
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LTC3403
APPLICATIO S I FOR ATIO
Table 1. Representative Surface Mount Inductors
Part
Number
Value
(µH)
DCR
MAX DC
Size
(ΩMAX) Current (A) WxLxH (mm3)
Sumida
CDRH2D11
1.5
0.068
2.2
0.098
3.3
0.123
0.90
3.2 x 3.2 x 1.2
0.78
0.60
Sumida
2.2
CDRH2D18/LD 3.3
4.7
0.041
0.054
0.078
0.85
3.2 x 3.2 x 2.0
0.75
0.63
Sumida
CMD4D06
2.2
0.116
3.3
0.174
4.7
0.216
0.95
3.5 x 4.1 x 0.8
0.77
0.75
Murata
LQH32C
1.0
0.060
2.2
0.097
4.7
0.150
1.00
2.5 x 3.2 x 2.0
0.79
0.65
Taiyo Yuden
1.0
0.080
LQLBC2518
1.5
0.110
2.2
0.130
0.78
1.8 x 2.5 x 1.8
0.66
0.60
Toko
D412F
2.2
0.14
3.3
0.20
4.7
0.22
1.14
4.6 x 4.6 x 1.2
0.90
0.80
CIN and COUT Selection
In continuous mode, the source current of the top MOSFET
is a square wave of duty cycle VOUT/VIN. To prevent large
voltage transients, a low ESR input capacitor sized for the
maximum RMS current must be used. The maximum
RMS capacitor current is given by:
CIN
required IRMS
≅
IOMAX
[VOUT (VIN – VOUT
VIN
)]1/ 2
This formula has a maximum at VIN = 2VOUT, where IRMS
= IOUT/2. This simple worst-case condition is commonly
used for design because even significant deviations do not
offer much relief. Note that the capacitor manufacturer’s
ripple current ratings are often based on 2000 hours of life.
This makes it advisable to further derate the capacitor, or
choose a capacitor rated at a higher temperature than
required. Always consult the manufacturer if there is any
question.
The selection of COUT is driven by the required effective
series resistance (ESR). Typically, once the ESR require-
ment for COUT has been met, the RMS current rating
generally far exceeds the IRIPPLE(P-P) requirement. The
output ripple VOUT is determined by:
∆VOUT
≅
∆IL ESR
+
8f
1
C OUT
where f = operating frequency, COUT = output capacitance
and IL = ripple current in the inductor. For a fixed output
voltage, the output ripple is highest at maximum input
voltage since IL increases with input voltage.
Aluminum electrolytic and dry tantalum capacitors are
both available in surface mount configurations. In the case
of tantalum, it is critical that the capacitors are surge tested
for use in switching power supplies. An excellent choice is
the AVX TPS series of surface mount tantalum. 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.
The bulk capacitance values in Figure 1(a) (CIN = 10µF,
COUT = 4.7µF) are tailored to mobile phone applications, in
which the output voltage is expected to slew quickly
according to the needs of the power amplifier. Holding the
output capacitor to 4.7µF facilitates rapid charging and
discharging. When the output voltage descends quickly in
forced continuous mode, the LTC3403 will actually pull
current from the output until the command from VREF is
satisfied. On alternate half cyles, this current actually exits
the VIN terminal, potentially causing a rise in VIN and
forcing current into the battery. To prevent deterioration
of the battery, use sufficient bulk capacitance with low
ESR; at least 10µF is recommended.
3403f
10
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