MP2370 – 1.2A, 24V, 1.4MHz STEP-DOWN WHITE LED DRIVER

STEP-DOWN WHITE LED DRIVER

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.