MP4700 – HIGH-BRIGHTNESS BCM, LOW SIDE BUCK WLED DRIVE WLED DRIVER

MP4700 Rev. 1.0

www.MonolithicPower.com

14

10/29/2012

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Select the Inductor

The MP4700 operates at BCM. Select a proper

inductor to ensure that the operating frequency

is within a desired range.

Assuming the 280V bus voltage has -10% to

+20% variation, and the LED voltage has a

±10%

variation.

The

maximum

system

operating frequency at the maximum input bus

voltage and the minimum output LED voltage

should not exceed the MP4700’s 160kHz

frequency limit.

Bus _ max

LED _ min

LED _ min

s_ max

LED

Bus_ max

(V

V

) V

1

L

f2 I

V







Choosing a maximum operating frequency at

150kHz, the inductor should exceed 1.1mH.

The peak inductor current is around 2 times the

LED current when the MP4700 operates in

BCM. An inductor rated at 1.3mH/0.6A would fit

this application.

Select the Power MOSFET

The voltage stress of the power MOSFET

should exceed the bus voltage. A MOSFET with

400V voltage rating will suffice for this

application. The peak current through the

MOSFET equals the peak inductor current, or 2

times the LED current (250mA). The maximum

RMS current through the MOSFET is:

LED _ max

RMS _ MOS _ max

LED

bus _ min

V

I

2I

*

0.27A

3V



Choose a MOSFET with a current rating around

2A to 3A.

Select the Diode

The voltage stress of the diode should exceed

the bus voltage—400V will suffice for this

application. The maximum average current

through the diode is:

LED _ min

Avg _ Diode _ max

LED

bus _ max

V

I2I

* (1

) 0.21A

V





This allows for a current rating within 1A to 2A.

Select the Current Sense Resistor

The MP4700 operates in BCM and the current

sense resistor is:

sense

LED

302mV

R600m

2I







Select the Output Capacitor

The output capacitor should be large enough to

filter the output ripple current and limit the LED

current ripple to within a desired range (usually

below ±5%).

Use the equivalent dynamic resistance of the

LED string to design the output capacitor.

Figure 5 defines of the equivalent dynamic

LED

LED

LED

I

V

R







Figure 5: LED Equivalent Dynamic Resistance

Calculate the value of the ceramic output

capacitor as:

L _ PEAK

OUT

s _ min

LED

LED _ ripple _ pk _ pk

I

C

8f

R

I





Where

is

the

minimum

operating

frequency, which occurs at the minimum input

voltage and maximum output voltage condition

for this application.