MP4700 – HIGH-BRIGHTNESS BCM, LOW SIDE BUCK WLED DRIVE WLED DRIVER
The MP4700 controls the buck converter in
current-boundary-conduction mode.
OPERATION
Internal Regulator
To improve zero current detection, add a 10pF
capacitor between the Q1 drain and source .
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.
Add a capacitor (Cout) in parallel to the LED
string to reduce the current ripple.
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.
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
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.
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.
LED Current Regulation and Valley Detection
For applications that do not need PWM
dimming control, apply a high voltage on the
PWM pin or leave the PWM pin open.
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 through a resistor (Rsense) to regulate the
current to:
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.
302mV
IL _PEAK
Rsense
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:
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.
302mV
ILED
2Rsense
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
to zero, the Q1 drain voltage (VSW) drops from
VBus to (VBus − VOUT) and oscillates thanks to the
inductor and the parasitic capacitors. When VSW
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.
Inductor design is critical to to ensure that the
switching frequency (fs) is within the 30kHz to
160kHz range.
(VBus Vout ) Vout
1
L
,
fs 2ILED
VBus
Where VBus is the input voltage of the Buck
converter, and Vout is the LED voltage.
MP4700 Rev. 1.0
10/29/2012
www.MonolithicPower.com
MPS Proprietary Information. Patent Protected. Unauthorized Photocopy and Duplication Prohibited.
© 2012 MPS. All Rights Reserved.
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