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

MP4700 Rev. 1.0

www.MonolithicPower.com

10

10/29/2012

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© 2012 MPS. All Rights Reserved.

OPERATION

Internal Regulator

The MP4700 uses a low dropout (LDO)

regulator to supply the IC. Use a DC voltage in

the range of 8V to 18V to power the IC.

The internal LDO regulator maintains the VCC

voltage at 8.4V when the input voltage is less

than 12V. The VCC pin requires a ceramic

bypass capacitor.

When the input voltage exceeds 12V, the LDO

regulator switches to switch-controlled current

source mode. The VCC voltage charges to its

peak voltage (9.8V) and then the current source

stops. After the VCC voltage decreases to its

valley voltage (9V), the current source turns on

again. This switch-controlled current source

mode reduces the LDO power consumption and

improves efficiency.

LED Current Regulation and Valley Detection

In floating-buck-converter configuration, as

shown in the typical application circuit, the

MP4700 controls the MOSFET (Q1) using peak

current control. The CS pin senses the peak

current to:

L _ PEAK

sense

302mV

I

R



In normal operation, the MP4700 turns on Q1

when the current in the freewheeling diode

goes to zero. As a result, the average LED

current is:

LED

sense

302mV

I

2R



The zero-current detection is realized at the

DRV pin by sensing the MOSFET drain dv/dt

current through the Q1’s miller capacitor. When

the current through the freewheeling diode goes

drops to the minimum value, the dv/dt current

through the miller capacitor rises from negative

to zero. At this point, the MP4700 turns on Q1

as the inductor current goes to zero and the Q1

drain voltage is at its minimum.

The MP4700 controls the buck converter in

current-boundary-conduction mode.

To improve zero current detection, add a 10pF

capacitor between the Q1 drain and source .

string to reduce the current ripple.

Boundary operation mode minimizes the Q1

turn-on loss and eliminates the freewheeling

diode’s reverse recovery loss to reduce passive

components’

size

requirements

at

high

switching

frequencies.

Furthermore,

the

required inductance value is already small,

further reducing the inductor size.

Brightness Dimming Control

The MP4700 employs PWM dimming to control

the LED current. Use a 100Hz-to-2kHz PWM

signal. PWM input high triggers IC switching.

PWM input low turns off the IC.

For applications that do not need PWM

dimming control, apply a high voltage on the

PWM pin or leave the PWM pin open.

Frequency Setting and Inductor Design

In case the zero-current detection circuit fails—

which can happens at start-up during an output

short condition with a large output capacitor—

applying a maximum off time of about 2.8ms

ensures that the MP4700 continues to operate

and the prevents short current runaway.

The MP4700 has a maximum switching

frequency of 160kHz to avoid extreme circuit

losses and ensure better EMI performance. If

the converter reaches the maximum frequency,

it

will

operate

in

discontinuous

current

conduction mode. Avoid this operation mode

since the LED current is out of regulation.

Inductor design is critical to to ensure that the

160kHz range.

Bus

out

out

s

LED

Bus

(V

V ) V

1

L

f2 I

V







,