Datasheet

ADP5520
Rev. A | Page 35 of 40
Component Selection
The ADP5520 boost converter is designed for use with a 4.7 H
inductor. Choose an inductor with a sufficient current rating to
prevent it from going into saturation. The peak current limit of
the ADP5520 is 750 mA (maximum), so choose an inductor
with a greater saturation rating. To maximize efficiency, choose
an inductor with a low series resistance (DCR).
The ADP5520 is an asynchronous boost and, as such, requires
an external Schottky diode to conduct the inductor current to
the output capacitor and LED string when the power switch is
off. Ensure that the Schottky diode peak current rating is greater
than the maximum inductor current. Choose a Schottky diode
with an average current rating that is significantly larger than
the maximum LED current. To prevent thermal runaway, derate
the Schottky diode to ensure reliable operation at high junction
temperatures. To maximize efficiency, select a Schottky diode
with a low forward voltage. When the power switch is on, the
Schottky diode blocks the dc path from the output capacitor to
ground. Therefore, choose a Schottky diode with a reverse
breakdown greater than the maximum boost voltage. A 40 V,
1 A Schottky diode is recommended.
The input capacitor carries the input ripple current, allowing
the input power source to supply only the dc current. Use an
input capacitor with sufficient ripple current rating to handle
the inductor ripple. A 1 F X5R/X7R ceramic capacitor rated
for 16 V dc bias is recommended for the input capacitance.
The output capacitor maintains the output voltage when the
Schottky diode is not conducting. Due to the high levels of
boost voltage required, a 1 F X5R/X7R ceramic capacitor rated
for 50 V dc bias is recommended for output capacitance.
Note that dc bias characterization data is available from
capacitor manufacturers and should be taken into account
when selecting input and output capacitors.
PCB LAYOUT
Good PCB layout is important to maximize efficiency and to
minimize noise and electromagnetic interference (EMI). To
minimize large current loops, place the input capacitor, inductor,
Schottky diode, and output capacitor as close as possible to each
other and to the ADP5520 using wide tracks (use shapes where
possible). For thermal relief, the exposed pad of the LFCSP
package should be connected ground (GND).
PGND and GND should be connected to each other at the
bottom of the output capacitor.
Figure 52 shows an example PCB layout with the main power
components required for backlight driving.
07445-061
VBAT
GND
GND
GNDGND
CMP_IN
VDDIO
nRST
ILED
R3
PGND
SW
nINT
SDA
SCL
R2
R1 R0 C0 C1 C2 C3
BL_SNK
BST
VBAT
CAP_OUT
ADP5520
CONNECT EXPOSED
PAD TO GND
TOP OF LED STRING
BOTTOM OF LED STRING
SOLUTION SIZE
APPROXIMATELY 47mm
2
Figure 52. Example PCB Layout