ADP1655 View Datasheet(PDF) - Analog Devices
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ADP1655 Datasheet PDF : 24 Pages
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ADP1655
APPLICATIONS INFORMATION
EXTERNAL COMPONENT SELECTION
Selecting the Inductor
The ADP1655 boost converter increases the battery voltage
to allow driving of one or two LEDs, whose combined voltage
drop is higher than the battery voltage plus the current source
headroom voltage. This allows the converter to regulate the
LED current over the entire battery voltage range and with a
wide variation of LED forward voltage.
The inductor saturation current should be greater than the sum
of the dc input current and half the inductor ripple current. A
reduction in the effective inductance due to saturation increases
the inductor current ripple. Suggested inductors are shown in
Table 15.
Table 15. Suggested Inductors
Value
DCR ISAT Dimensions
Vendor (μH) Part No. (mΩ) (A) L × W × H (mm)
Toko
2.2 FDSE0312 160 3.1 3 × 3 × 1.2
Toko
2.0 DE2812C 67 1.8 3.0 × 3.2 × 1.22
Coilcraft 2.2 LPS3010 220 1.4 3 × 3 × 1.0
Coilcraft 2.2 LPS3314 100 1.5 3 × 3 × 1.4
Selecting the Input Capacitor
The ADP1655 requires an input bypass capacitor to supply
transient currents while maintaining constant input and output
voltages. The input capacitor carries the input ripple current,
allowing the input power source to supply only the dc current.
Use an input capacitor with a sufficient ripple current rating to
handle the inductor ripple. Increased input capacitance reduces
the amplitude of the switching frequency ripple on the battery.
Because of the dc bias characteristics of ceramic capacitors, a
0603, 6.3 V X5R/X7R, 10 μF ceramic capacitor is preferable.
Higher value input capacitors help to reduce the input voltage
ripple and improve transient response. Maximum input
capacitor current is calculated using the following equation:
ICIN ≥ I LOAD(MAX)
VOUT (VIN − VOUT )
VIN
To minimize supply noise, place the input capacitor as close to
the VIN pin of the ADP1655 as possible. As with the output
capacitor, a low ESR capacitor is suggested. A list of suggested
input capacitors is shown in Table 16.
Table 16. Suggested Input Capacitors
Vendor
Murata
TDK
Tayio
Yuden
Value
10 μF, 6.3 V
10 μF, 6.3 V
10 μF, 6.3 V
Part No.
GRM188R60J106ME47
C1608JB0J106K
JMK107BJ106MA
Dimensions
L × W × H (mm)
1.6 × 0.8 × 0.8
1.6 × 0.8 × 0.8
1.6 × 0.8 × 0.8
Selecting the Output Capacitor
The output capacitor maintains the output voltage and supplies
the LED current during NFET power switch on period. It also
stabilizes the loop. A 10.0 μF, 16 V X5R/X7R ceramic capacitor
is suggested.
Note that dc bias characterization data is available from capa-
citor manufacturers and should be taken into account when
selecting input and output capacitors. 16 V capacitors are
recommended for most two-LED designs. Designs with 1 mm
height restrictions can also use 0603 case size, 16 V capacitors
in parallel. A list of suggested output capacitors is shown in
Table 17.
Table 17. Suggested Output Capacitors
Vendor
Murata
Murata
Tayio
Yuden
Value
10.0 μF, 10 V
10.0 μF, 16 V
10.0 μF, 16 V
Part No.
GRM21BR71A106KE51
GRM31CR61C106KA88
EMK212BJ106KG
Dimensions
L × W × H (mm)
2 × 1.25 × 1.25
3.2 × 1.6 × 1.6
2 × 1.25 × 1.25
Higher output capacitor values reduce the output voltage ripple
and improve load transient response. When choosing this value,
it is also important to account for the loss of capacitance due to
output voltage dc bias.
Ceramic capacitors are manufactured with a variety of dielec-
trics, each with different behavior over temperature and applied
voltage. Capacitors must have a dielectric that ensures the
minimum capacitance over the necessary temperature range
and dc bias conditions. X5R or X7R dielectrics with a voltage
rating of 10.0 V or 16 V are suggested for best performance.
Y5V and Z5U dielectrics are not suggested for use with any
dc-to-dc converter because of their poor temperature and dc
bias characteristics.
Rev. 0 | Page 20 of 24
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