MP24875 MPS, MP24875 Datasheet - Page 9

• Manufacturer Part Number: MP24875
• Description: 200khz, 55v Input, 1.5a High Power Led Driver
• Manufacturer: MPS

APPLICATION INFORMATION
COMPONENT SELECTION
Setting the LED Current
The LED current is set using a sensing resistor,
which is in series with the LEDs and connected
to GND. The voltage on the sensing resistor is
connected to FB pin.
For example, for a 700mA LED current, R
287mΩ.
Inductor L1
The inductor is required to supply constant
current to the output load while being driven by
the switched input voltage. A larger value
inductor will help to reduce the output filter
capacitance for the same LED current ripple.
However, the larger value inductor will have a
larger physical size, higher series resistance,
and/or lower saturation current.
A good rule for determining the inductance to use
is to allow the peak-to-peak ripple current in the
inductor to be approximately 30% to 40% of the
LED current. Also, make sure that the peak
inductor current is below the maximum switch
current limit. The inductance value can be
calculated by:
Where V
LEDs, V
frequency, V
drop, n is the numbers of LEDs in series, and ∆I
is the peak-to-peak inductor ripple current.
Choose an inductor that will not saturate under
the maximum inductor peak current. The peak
inductor current can be calculated by:
Where I
MP24875 Rev. 1.0
9/21/2010
LED
IN
OUT
I
LP
is the input voltage, f
is the LED current.
=
F
L1=
is the output voltage to drive the
I
is one LED diode forward voltage
LED
f
+
s
V
V
I
×
OUT
OUT
2 f
LED
∆I
× ×
MPS Proprietary Information. Unauthorized Photocopy and Duplication Prohibited.
V
L
OUT
s
= ×
=
×
n V
R
V
(1-
L1
FB
FB
×
V
F
V
(1
OUT
IN
S
−
is the switching
V
)
V
,
OUT
IN
)
© 2010 MPS. All Rights Reserved.
MP24875 – 1.5A, 200kHz, 55V HIGH-POWER LEDS DRIVER
FB
www.MonolithicPower.com
is
L
Output Rectifier Diode
The output rectifier diode supplies the current to
the inductor when the high-side switch is off. To
reduce losses due to the diode forward voltage
and recovery times, use a Schottky diode.
Choose a diode whose maximum reverse voltage
rating is greater than the maximum input voltage,
and whose current rating is greater than the
maximum load current. Table 2 lists example
Schottky diodes and manufacturers.
Input Capacitor C
The input current to the step-down converter is
discontinuous, therefore a capacitor is required to
supply the AC current to the step-down converter
while maintaining the DC input voltage. Use low
ESR capacitors for the best performance.
Ceramic capacitors are preferred, but tantalum or
low-ESR electrolytic capacitors may also suffice.
For simplification, choose the input capacitor with
RMS current rating greater than half of the
maximum load current. The input capacitor (C1)
can be electrolytic, tantalum or ceramic.
When using electrolytic or tantalum capacitors, a
small, high quality ceramic capacitor, i.e. 0.1µF,
should be placed as close to the IC as possible.
When using ceramic capacitors, make sure that
they have enough capacitance to provide
sufficient charge to prevent excessive voltage
ripple at input. The input voltage ripple caused by
capacitance can be estimated by:
B180-7-F
Diodes
B290-F
Table 1—Diode Selection Guide
∆
V
IN
=
f
s
I
×
LED
C
IN
IN
Voltage/
Current
80V, 1A
90V, 2A
×
Rating
V
V
OUT
IN
×
⎛
⎜
⎝
1
−
Manufacturer
Diodes Inc.
Diodes Inc.
V
V
OUT
IN
⎞
⎟
⎠
9