MP4688 – 80V INPUT, 1A HIGH POWER LED DRIVER
APPLICATION INFORMATION
COMPONENT SELECTION
Setting the LED Current
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 especially under high switching
frequency applications.
The LED current ILED is set using a sensing
resistor RFB, which is in series with the LEDs
and connected to GND. The voltage on the
sensing resistor RFB is connected to FB pin.
VFB
ILED
=
RFB
The RMS current through the input capacitor is
about:
For example, for a 700mA LED current, RFB is
287mꢀ.
VOUT
ID = ILED
⋅
’
V
Inductor Selection and Frequency Setting
IN
The inductor LO is required to supply a constant
current to the LED. The inductor value is related
to the switching frequency fs setting:
With low ESR capacitors, the input voltage
ripple can be estimated by:
⎛
⎞
⎟
⎠
ILED
VOUT
VOUT
VOUT ⋅ V − V
ΔV =
×
× 1−
⎜
(
IN
OUT
IN
fs =
fs ×CIN
V
V
IN
IN
⎝
V ⋅ 8⋅10−8 ⋅ V +L⋅15%⋅I
(
)
IN
IN
LED
Choose the input capacitor with enough RMS
current rating and enough capacitance for small
input voltage ripple.
Where, VIN is the input voltage, VOUT is the
output voltage to drive the LEDs. A larger value
inductor will result in smaller switching
frequency.
When electrolytic or tantalum capacitors are
applied, a small, high quality ceramic capacitor,
i.e. 0.1μF, should be placed as close to the IC
as possible.
The peak inductor current is about:
−8
⎛
⎜
⎝
⎞
⎟
⎠
4⋅10 ⋅ V
IN
ILP = ILED
⋅
+1.075
Output Capacitor COUT
L⋅ILED
The output capacitor (COUT) is not necessary for
MP4688. The LED current ripple (peak-to-peak
value) is about 15% of the LED DC current.
Output capacitor can be used to further reduce
the LED current ripple. Low ESR capacitors are
preferred to keep the output voltage ripple low
so that the AC ripple current through the LEDs
is small. The output voltage ripple can be
estimated by:
Choose an inductor that will not saturate under
the maximum inductor peak current.
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.
The RMS current through the diode is about:
⎛
⎞ ⎛
⎞
⎟
⎠
VOUT
VOUT
V
1
ΔVOUT
=
× 1−
× R
⎟ ⎜
+
⎜
ESR
fS ×LO
8× fS × COUT
VOUT
⎝
IN ⎠ ⎝
ID = ILED ⋅ 1−
V
IN
Choose a diode whose maximum reverse
voltage rating is greater than the maximum
input voltage, and whose current rating is
greater than the maximum diode current.
Input Capacitor CIN
MP4688 Rev. 1.12
3/28/2012
www.MonolithicPower.com
MPS Proprietary Information. Patent Protected. Unauthorized Photocopy and Duplication Prohibited.
© 2012 MPS. All Rights Reserved.
8
MPS [ MONOLITHIC POWER SYSTEMS ]
