Setting LED Current
The current sense resistor is inserted between
the anode of WLED and GND. The current
sense resistor value is calculated as:
R1 = 0.15V/Iled
For 1A WLED current, choose R1 = 150mΩ
Selecting the Inductor
A 1µH to 10µH inductor with a DC current rating
of at least 25% percent higher than the
maximum load current is recommended for
most applications. For highest efficiency, the
inductor’s DC resistance should be less than
200mΩ. Refer to Table 2 for suggested surface
mount inductors. For most designs, the required
inductance value can be derived from the
following equation.
L= ((Vout)x(Vin - Vout))/(Vin x Il X fsw)
Where ΔIL is the inductor ripple current.
Choose the inductor ripple current to be 30% of
the maximum load current. The maximum
inductor peak current is calculated from:
Il(Max) =Iload + (Il/2)
Under light load conditions below 100mA, a
larger inductance is recommended to improve
efficiency. See Table 2 for suggested inductors.
Also note that the maximum recommended load
current is 1A if the duty cycle exceeds 35%.
Selecting the Input Capacitor
The input capacitor reduces the surge current
drawn from the input supply and the switching
noise from the device. The input capacitor
impedance at the switching frequency should be
less than the output impedance of the input source
to prevent high frequency switching current from
passing through the input. Ceramic capacitors with
X5R or X7R dielectrics are highly recommended
because of their low ESR and small temperature
coefficients. For most applications, a 4.7µF
capacitor is sufficient.
Selecting the Output Capacitor
The output capacitor keeps the output current
ripple small and ensures feedback loop stability.
The output capacitor impedance should be low
at the switching frequency. Ceramic capacitors
with X5R or X7R dielectrics are recommended
for their low ESR characteristics. For most
applications, a 2.2µF ceramic capacitor will be
sufficient.
The MP2370 can also work properly without
output capacitor by adding a small capacitor in
the feedback loop (see Figure 2). The small
capacitor is recommended 680pF.
PC Board Layout
The high current paths (GND, IN and SW) should
be placed very close to the device with short,
direct and wide traces. The input capacitor needs
to be as close as possible to the IN and GND pins.
The external feedback resistors should be placed
next to the FB pin. Keep the switch node traces
short and away from the feedback network.
