Datasheet
Application information STOD1317B
14/22 DocID022607 Rev 2
7 Application information
7.1 External passive components
7.1.1 Inductor selection
The inductor is the key passive component for switching converters.
For the step-up converter an inductance between 3.3 µH and 6.8 µH is recommended.
It is very important to select the right inductor according to the maximum current the inductor
can handle in order to avoid saturation. The peak current for the step-up can be calculated
as:
Equation 1
where
V
MID
: step-up output voltage, it is fixed internally to V
OUT
+ 0.38 V;
I
OUT
: output current;
V
IN
: input voltage of the STOD1317B;
fs: switching frequency. Use the minimum value of 1 MHz for worst case;
: efficiency of the step-up converter (0.80 at maximum load).
7.1.2 Input and output capacitor selection
It is recommended to use ceramic capacitors with low ESR as input and output capacitors in
order to filter any disturbance present in the input line and to obtain stable operation of the
step-up converter and LDO. A minimum real capacitance value of 3 µF must be guaranteed
for C
MID
and C
OUT
in all conditions.
7.2 Recommended PCB layout
The STOD1317B is a high frequency power switching device so it requires a proper PCB
layout in order to obtain the necessary stability and optimize line/load regulation and output
voltage ripple.
The input capacitor must be as close as possible to the V
IN
pin.
In order to minimize the ground noise, a common ground node for power ground (PGND)
and a different one for analog ground (GND) must be used. The exposed pad is connected
to PGND through vias.
Grounding is fundamental to the operation of DC-DC converters; run separate ground paths
for critical parts of the circuit (GND and Power GND), each connected back to a single
ground point.
Separate ground lines prevent the current and noise of one component from interfering with
other components. If using a ground plane, utilize “split” plane techniques to give effective
grounding. Use multiple vias to decrease the trace impedance to ground.
LfsV2
)VINV(VIN
VIN
IV
I
MID
MINMIDMIN
MIN
OUTMID
BOOSTPEAK
-
η