MP4700
High-Brightness,
BCM, Low Side Buck White LED Driver
The Future of Analog IC Technology
DESCRIPTION
FEATURES
The MP4700 is a high-efficiency step-down
converter designed to drive high-brightness
light emitting diodes.
The MP4700 drives an external MOSFET in
boundary conduction mode, which features no
reverse recovery loss in the freewheeling diode
and soft turn-on with zero-current and valley
voltage for the power MOSFET that improves
efficiency and minimizes the inductor value and
size. The boundary conduction control mode
regulates the LED current by sensing the
MOSFET peak current through an external
resistor. Its low 300mV feedback voltage
reduces power loss and improves efficiency.
The MP4700 implements PWM dimming to the
LED current.
Protection features include output short
protection, under-voltage lockout for the IC
input voltage and bus input voltage, a limited
maximum switching frequency, and thermal
shut down.
8V-to-18V Input Voltage
Constant-Current LED Driver
Power MOSFET Zero-Current Turn-On
No Freewheeling Diode Reverse Recovery
Issues
High Efficiency and Reliability in Boundary
Conduction Mode
Low 1mA Operation Current
PWM Dimming Control
Hiccup Short Circuit Protection
UVLO for Bus Input Voltage
Input UVLO, Thermal Shutdown
Maximum Frequency Limited to 160kHz
Available in SOIC8 Package
APPLICATIONS
LED Backlighting for TV and Monitor
DC/DC or AC/DC LED Driver applications
General Illumination
Industrial Lighting
Automotive/ Decorative LED Lighting
All MPS parts are lead-free and adhere to the RoHS directive. For MPS green
status, please visit MPS website under Quality Assurance. “MPS” and “The
Future of Analog IC Technology” are Registered Trademarks of Monolithic
Power Systems, Inc.
TYPICAL APPLICATION
MP4700 Rev. 1.0
10/29/2012
www.MonolithicPower.com
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© 2012 MPS. All Rights Reserved.
1
MP4700 – HIGH-BRIGHTNESS BCM, LOW SIDE BUCK WLED DRIVE WLED DRIVER
ORDERING INFORMATION
Part Number*
MP4700GS
Package
SOIC8
Top Marking
MP4700
*For Tape & Reel, add suffix –Z (eg. MP4700GS-Z)
PACKAGE REFERENCE
ABSOLUTE MAXIMUM RATINGS (1)
Recommended Operating Conditions
VIN .................................................. -0.3V to 20V
VCC, DRV ....................................... -0.3V to 11V
PWM, INUV, CS ............................ -0.3V to 6.5V
Continuous Power Dissipation ....... (TA=25°C) (2)
SOIC8………………………………………..1.3W
Junction Temperature ...............................150°C
Lead Temperature ....................................260°C
Storage Temperature ............... -55°C to +150°C
VIN ...................................................... 8V to 18V
VCC, DRV………………………. -0.3V to 10.5V
Operating Junct. Temp(TJ) ...... –40°C to +125°C
MP4700 Rev. 1.0
10/29/2012
Thermal Resistance
(4)
θJA
(3)
θJC
SOIC8 ..................................... 96 ...... 45 ... °C/W
Notes:
1) Exceeding these ratings may damage the device.
2) The maximum allowable power dissipation is a function of the
maximum junction temperature TJ (MAX), the junction-toambient thermal resistance θJA, and the ambient temperature
TA. The maximum allowable continuous power dissipation at
any ambient temperature is calculated by PD (MAX) = (TJ (MAX)TA)/θJA. Exceeding the maximum allowable power dissipation
will cause excessive die temperature, and the regulator will go
into thermal shutdown. Internal thermal shutdown circuitry
protects the device from permanent damage.
3) The device is not guaranteed to function outside of its operating
conditions.
4) Measured on JESD51-7, 4-layer PCB.
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2
MP4700 – HIGH-BRIGHTNESS BCM, LOW SIDE BUCK WLED DRIVE WLED DRIVER
ELECTRICAL CHARACTERISTICS
VIN =12VDC, VPWM= 5V, no load on pin DRV, TA = +25°C, unless otherwise noted.
Parameters
Symbol
Input Voltage
VIN
Input Supply Current
IINQS
(Quiescent)
Input Supply Current
IINRUN
(Operation)
VCC Voltage
VCC
VCC peak voltage
VCCH
VCC valley voltage
VCCL
VCC UVLO
VCCUVLO
VCC UVLO Hysteresis
VCCHys
PWM Input High Threshold
VPWMH
PWM Input Low Threshold
VPWML
PWM Pull-Up Current
IPWM Pull UP
Bus Input Voltage UVLO
VINUV_th_rising
Threshold
Bus Input Voltage UVLO
VINUV_hys
Hysteresis
PWM Dimming ON Propagation
τPWMon_PD
Delay
PWM Dimming OFF
τPWMoff_PD
Propagation Delay
CS Pin Reference Voltage
VREF
CS Pin Reference Voltage
VREF
Leading Edge Blanking Time
τLEB
CS Bias Current
ICS
Gate Drive Source Current
Gate Drive Sink Current
Drive Low Level Output Voltage
Drive High Level Output
Voltage to Rail
Condition
DC voltage at Pin VIN
Min
8
Typ
Max
18
Units
V
VPWM = 0V
0.6
mA
VPWM = 5 V
0.78
mA
VIN=10V (VIN12V)
VIN=18V (VIN>12V)
VIN rising with a DC input
VIN falling with a DC input
VPWM rising
VPWM falling
VPWM=0V
7.5
7
8.5
9.8
9
7.4
1
9.5
7.8
1.5
0.75
10.6
V
V
V
V
V
V
V
uA
Bus input voltage rising
1.15
1.2
1.25
V
Bus input voltage falling
50
86
120
mV
1.3
2
us
230
350
ns
270
285
302
302
330
315
mV
mV
200
320
450
ns
0
PWM rising edge to Drive rising
edge
PWM falling edge to Drive falling
edge
-40°C ≤TA ≤+85°C
TA=+25°C
IDRV Source
IDRV Sink
VDRVL
VDRV=0V
VDRV=VCC
IDRV=10mA
550
-1.2
54
A
mA
A
mV
VDRVH
IDRV=-10mA
122
mV
VCS=0.3V
-0.5
Gate Minimal Turn-On Time
τON_Min
320
Max Switching Frequency
Over-Temperature Protection
Threshold
Over-Temperature Protection
Threshold Hysteresis
fSW
160
kHz
TOTP
150
°C
TOTP_Hys
30
°C
Output Short Shut Down Time
τshut-down
2.8
ms
MP4700 Rev. 1.0
10/29/2012
Max
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450
ns
3
MP4700 – HIGH-BRIGHTNESS BCM, LOW SIDE BUCK WLED DRIVE WLED DRIVER
PIN FUNCTIONS
Pin #
1
Name
INUV
2
PWM
3
GND
4
VIN
5
VCC
6
7
TST
DRV
8
CS
MP4700 Rev. 1.0
10/29/2012
Description
Input Bus Voltage UVLO. Sense with a voltage divider.
PWM Dimming Input. Connect directly to the PWM dimming signal. Apply a high voltage or
leave open for no applied dimming control.
Ground.
Input Supply. Apply a voltage in the range of 8V to 18V.
Internal Regulated Supply Voltage Output. Must be locally bypassed. Provides power for IC
logic and to drive the external MOSFET.
Test. Connect to GND.
External MOSFET Drive Signal. Also detects the zero current crossing.
LED Current Sense Input. Connect to the current sense resistor that programs the LED
current.
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4
MP4700 – HIGH-BRIGHTNESS BCM, LOW SIDE BUCK WLED DRIVE WLED DRIVER
TYPICAL CHARACTERISTICS
PWM Pull Up Current vs.
Temperature
CS Pin Current vs.
Temperature
VCS=0.3V
12.000
0.000
1.300
INUV_UVLO_LH (V)
-0.100
11.000
-0.200
10.000
-0.300
9.000
-0.400
8.000
-50 -30 -10 10 30 50 70 90 110 130
0.110
Input Bus Voltage
UVLO Threshold vs.
Temperature
1.200
1.150
1.100
-50 -30 -10 10 30 50 70 90 110 130
-0.500
-50 -30 -10 10 30 50 70 90 110 130
Input Bus Voltage
UVLO Hysteresis vs.
Temperature
1.250
VCC UVLO vs.
Temperature
VCC Voltage vs.
Temperature
VIN=10V
8.550
7.450
0.090
0.080
0.070
0.060
7.350
7.300
7.250
7.200
-50 -30 -10 10 30 50 70 90110 130
0.050
-50 -30 -10 10 30 50 70 90 110 130
PWM Input High Threshold
vs. Temperature
PWM Low High Threshold
vs. Temperature
0.800
8.450
8.400
8.350
8.300
8.250
8.200
8.150
8.100
-50 -30 -10 10 30 50 70 90110 130
CS Reference Volatage vs.
Temperature
303.500
303.000
1.450
DIM_PWM_INPUT_L (V)
DIM_PWM_INPUT_H (V)
1.500
1.400
1.350
1.300
1.250
0.780
0.760
0.740
0.720
-50 -30 -10 10 30 50 70 90110 130
302.500
302.000
301.500
301.000
300.500
300.000
299.500
0.700
1.200
MP4700 Rev. 1.0
10/29/2012
VCC_DC_REG_10V (V)
7.400
VCC_UVLO_LH (V)
INUV_UVLO_HYS (V)
8.500
0.100
-50 -30 -10 10 30 50 70 90110 130
299.000
-50 -30 -10 10 30 50 70 90110 130
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5
MP4700 – HIGH-BRIGHTNESS BCM, LOW SIDE BUCK WLED DRIVE WLED DRIVER
TYPICAL CHARACTERISTICS (continued)
PWM Dimming On
Propagation Delay Time vs.
Temperature
PWM Dimming Off
Propagation Delay Time vs.
Temperature
Leading Edge Blanking
Time vs.Temperature
1500.000
300.000
345.000
1450.000
250.000
340.000
1400.000
200.000
335.000
1350.000
150.000
330.000
1300.000
100.000
325.000
1250.000
50.000
320.000
1200.000
-50 -30 -10 10 30 50 70 90 110130
315.000
0.000
-50 -30 -10 10 30 50 70 90110 130
-50 -30 -10 10 30 50 70 90 110 130
Maximum Off Time vs.
Temperature
2.900
2.800
2.700
2.600
2.500
2.400
-50 -30 -10 10 30 50 70 90110 130
MP4700 Rev. 1.0
10/29/2012
www.MonolithicPower.com
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© 2012 MPS. All Rights Reserved.
6
MP4700 – HIGH-BRIGHTNESS BCM, LOW SIDE BUCK WLED DRIVE WLED DRIVER
TYPICAL PERFORMANCE CHARACTERISTICS
VIN =12.5VDC, VPWM= 5V, VBUS=220V, VLED=180V, ILED=0.22A, TA = +25°C, unless otherwise noted.
98.8
230
0.25
98.6
225
LED CURRENT (A)
98.4
98.2
220
98.0
215
97.8
97.6
210
97.4
205
97.2
97
200
200
180 200 220 240 260 280 300
INPUT BUS VOLTAGE (V)
0.2
0.15
0.1
0.05
0
220
240
260
280290
INPUT BUS VOLTAGE (V)
0
IC VIN Turn On
20
40
60
80 100
120
IC VIN Turn Off
2.5
2
ILED
100mA/div.
1.5
1
VCC
2V/div.
VIN
2V/div.
VDRV
5V/div.
ILED
100mA/div.
VCC
2V/div.
VIN
2V/div.
VDRV
5V/div.
0.5
0
0.00 0.20 0.40 0.60 0.80 1.00 1.20
PWM Turn On
PWM Turn Off
VIN = 12.5V
VIN = 10.7V
VCC
2V/div.
VIN = 12.5V
VCC
2V/div.
VCS
200mV/div.
VCS
200mV/div.
ILED
100mA/div.
ILED
100mA/div.
VCC
2V/div.
VDRV
5V/div.
VDRV
5V/div.
MP4700 Rev. 1.0
10/29/2012
VBUS Turn On
ILED
100mA/div.
VBUS
50V/div.
VDRV
5V/div.
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7
MP4700 – HIGH-BRIGHTNESS BCM, LOW SIDE BUCK WLED DRIVE WLED DRIVER
TYPICAL PERFORMANCE CHARACTERISTICS (continued)
VIN =12.5VDC, VPWM= 5V, VBUS=220V, VLED=180V, ILED=0.22A, TA = +25°C, unless otherwise noted.
VPWM
2V/div.
VPWM
2V/div.
ILED
100mA/div.
ILED
100mA/div.
VCS
200mV/div.
VCS
200mV/div.
VDRV
5V/div.
VDRV
10V/div.
VDRV
10V/div.
VPWM
2V/div.
VPWM
2V/div.
ILED
100mA/div.
VCC
2V/div.
VBUS
50V/div.
ILED
20mA/div.
ILED
200mA/div.
VCS
200mV/div.
VCS
200mV/div.
VDRV
10V/div.
VDRV
10V/div.
MP4700 Rev. 1.0
10/29/2012
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8
MP4700 – HIGH-BRIGHTNESS BCM, LOW SIDE BUCK WLED DRIVE WLED DRIVER
FUNCTIONAL BLOCK DIAGRAM
VBUS
Cin
VCC
UVLO
C1
RINUV_H
VIN
INUV
2.8ms Max off
time Control
Thermal
Protection
LDO
Output Short
Protection
1.2V
Zero-I
Detection
Circuit
Control
and
Protection
Circuit
160kHz Max
Frequency
Control
RINUV_L
DRIVE
driver
PWM
50k
Vref
Filter and
LEB=320nS
CS
Rs
GND
Figure 1: Block Diagram
MP4700 Rev. 1.0
10/29/2012
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9
MP4700 – HIGH-BRIGHTNESS BCM, LOW SIDE BUCK WLED DRIVE WLED DRIVER
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 through a resistor (Rsense) to regulate the
current to:
IL _ PEAK
302mV
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:
ILED
302mV
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.
MP4700 Rev. 1.0
10/29/2012
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 .
Add a capacitor (Cout) in parallel to the LED
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
switching frequency (fs) is within the 30kHz to
160kHz range.
L
(V Vout ) Vout
1
Bus
,
fs 2 ILED
VBus
Where VBus is the input voltage of the Buck
converter, and Vout is the LED voltage.
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10
MP4700 – HIGH-BRIGHTNESS BCM, LOW SIDE BUCK WLED DRIVE WLED DRIVER
Hiccup Output-Short Protection
If the entire LED string is shorted, VOUT is zero.
Due to the minimal on time limit, the inductor
current will be out of regulation. The MP4700
can detect this failure and shut down for about
2.8ms, and then re-tries the operation. This
hiccup protection can eliminate thermal issues
due to a short-circuit current, and also maintain
normal operation if the protection is mistriggered.
Input-Bus-Voltage Under-Voltage Lockout
(UVLO) Protection
The MP4700 implements UVLO protection for
the input bus voltage. The INUV pin senses the
input bus voltage through a voltage divider. The
IC is locked out until the input bus voltage rises
so that the INUV voltage pin exceeds its UVLO
threshold. This UVLO function protects against
a low input bus voltage. For best results, set the
input bus voltage UVLO point over 1.1 times the
output LED voltage.
Set the input bus UVLO as follows:
RINUV _ H
RINUV _ L
VBus _ UVLO
1
VINUV _ th
Where VINUV_th=1.2V is the INUV pin threshold
voltage.
VCC Under-Voltage Lockout (UVLO)
Protection
Under-voltage lockout (UVLO) protects the chip
from operating at an insufficient supply voltage.
The UVLO rising threshold is about 7.4V while
its falling threshold is a consistent 6.4V.
Thermal Shutdown Protection
An accurate temperature protection prevents
the chip from operating at exceedingly high
temperatures. When the silicon die temperature
exceeds its upper threshold, it shuts down the
whole chip. When the temperature drops below
its lower threshold, the chip is enabled again.
MP4700 Rev. 1.0
10/29/2012
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11
MP4700 – HIGH-BRIGHTNESS BCM, LOW SIDE BUCK WLED DRIVE WLED DRIVER
DESIGN EXAMPLE
TV LED Backlighting
The design example introduces an MP4700based high-performance TV LED backlighting
solution. Figure 2 shows the structure of this
total system solution—Flyback+MP4700 lowside buck for single-string TV LED backlight.
The flyback converts the AC input line voltage
to the system output supply voltage 13V/2A,
and also outputs a DC bus voltage (around
280V) to the LED driver. The flyback is based
on the MPS quasi-resonant flyback controller,
HFC0100. The MP4700 acts as the LED driver
stage as a boundary conduction mode (BCM)
low-side buck.
Figure 2: Power System Structure
Turn ratio:
Specifications:
Description
Parameter
Units
Np:Nbus1:Nbus2:N13V:NAu=45:51:51:5:10
Input voltage
110 to 265
VAC
Primary inductance: Lp=360μH.
50
Hz
Output DC Bus voltage
13.4
V
Output DC Bus current
2
A
LED Voltage
210
V
LED Current
250
mA
Input frequency
Protection: short LED protection, open LED
protection and 13V over-voltage protection.
Schematics:
Figure 3 and Figure 4 show the schematics of
the HFC0100 flyback stage and the MP4700
LED driver stage. The parameters of the power
transformer T2 are as follow:
MP4700 Rev. 1.0
10/29/2012
In this application example, the bus voltage
uses 2 windings. Each winding outputs half of
the bus voltage (140V) after the rectifier,
dividing the voltage stress across each rectifier
diode. The reduced stress allows for the use of
800V diodes.
Both the 13V voltage and the 280V bus voltage
feed back to the flyback control circuit, which
decreases the influence of the cross regulation.
The 13V system voltage also supplies the
MP4700.
The following describes the design of the
MP4700 LED driver. The design procedure for
the HFC0100 flyback is not described here, but
rather in the HFC0100 design materials.
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12
MP4700 – HIGH-BRIGHTNESS BCM, LOW SIDE BUCK WLED DRIVE WLED DRIVER
R1 2k/1206
C1
22pF/1000V
R2 2k/1206
L1
D1 NC
CY2
CY1
1.5nF/400V 1.5nF/400V
F1
D2 HER208 800V/2A
5A/250V
CN1
R4
1M
1206
VA1
TVR10431
R5
1M
1206
T1
CX1
275V
F2
5A/250V
R7 R8
150k 150k
LX1
R12 R13
1M
1M
1206 1206
1
C7
4.7nF/1kV
4
C11
PG5406 PG5406
C14
RT1
CX2
275V
1nF/400V
12mH
CY5
CY6
1nF/400V
220pF/400V
D7
FR107
1000V/1A
Output
8
5
CY8
1nF/400V
1nF/400V
D11
2
R17 200/1206
D9 V40120C 120V/40A
3
L2
R19
10k
5
C17
PGND C21
R26
20k
6
7
R27
91k
3
NA
R23
10k
R25
U2
PC817A
8
Q4
23.7k/1%
R28
C22
33pF
ADIM
2k
7
C24
220pF
8
GND
2
Vcc
FB
Demag
1
C25
33pF
R35
C23
402k
8.2nF
R30
6.81k/1% R31
47k
U3
R32
10k
R36
510
Q5
Q6
TL431K
2N7002
R39
NC
R38
8.2k/1%
PGND
R37
68k
R40
10k
C26
D14 1N4148/SOD-123
R29 5.11k
R33
300m/1%
1206
CS
6
R22
510
4
HV
DRV
5
AGND
U1
10
4
C16
C19
GND
C20
R24
13V
Q2
SS8050
13V
R21
51k
R34
NS
CN2
1
AGND
AGND
R18
1k
D12
R20
NS
C8
R14
10k
R15
374k/1%
1206
C13
C18
D10
C5
100nF/400V
470pF/100V
D8 BAV21 200V/0.2A
PGND
Q3
V700C
R11
510
C9
NC
D4 NC
R16 200/1206
Feedback
PG5406 PG5406
CY7
C10
7
C15
5.1 Ohms CY4
220pF/400V
LX2
R10
1M/1%
1206
D3 HER208 800V/2A
Output
9
6
C4
C3
NC
Q1
10
C12
D6
D5
CY3
Output
11
Primary
12mH
C2
R6 2k/1206
C6
R9 2k/1206 22pF/1000V
12
280V
R3
30k/2W
9
PS_ON 10
D13
1N4148/SOD-123
PWM 11
ON/OFF 12
C27
AGND
Figure 3: HFC0100 Flyback Schematic
280V
LED_P
R41
C28
D15
WEGP30G/400V/3A
R41X
L3
ON/OFF
LED_N
R43
D16
R44
R45
2k
2k
C29
1
AGND
R46
20k
2
3
C31
1nF
4
INUV
CS
PWM
ORV
GND
TST
VIN
VCC
8
Q7
7
STD3NK60ZT4
6
2k
C32
C33
R48
R49
1206
1206
R51
AGND
R55
2k
FMMT558TA/SOT23
-400V
R47
AGND
ADIM
Q8
5
MP4700
13V
R42
300k
C30
10pF/630V
0805
U2
1nF
1N4148W
PWM
LED_N
AGND
Q10
R52 30k
R53
10k
C34
1nF
R54
10k
Option for Open Protection
AGND
Figure 4: MP4700 Low-Side Buck LED Driver Schematic
MP4700 Rev. 1.0
10/29/2012
www.MonolithicPower.com
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© 2012 MPS. All Rights Reserved.
13
MP4700 – HIGH-BRIGHTNESS BCM, LOW SIDE BUCK WLED DRIVE WLED DRIVER
Select the Inductor
This allows for a current rating within 1A to 2A.
The MP4700 operates at BCM. Select a proper
inductor to ensure that the operating frequency
is within a desired range.
Select the Current Sense Resistor
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.
L
(VBus _ max VLED _ min ) VLED _ min
1
fs _ max 2 ILED
VBus _ max
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.
The MP4700 operates in BCM and the current
sense resistor is:
Rsense
302mV
600m
2 ILED
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
resistance of the LED string (RLED).
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:
IRMS _ MOS _ max 2ILED *
VLED _ max
3Vbus _ min
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:
MP4700 Rev. 1.0
10/29/2012
VLED _ min
Vbus _ max
RLED
VLED
I LED
ΔVLED
0.27A
Choose a MOSFET with a current rating around
2A to 3A.
IAvg _ Diode _ max 2ILED * (1
ΔILED
) 0.21A
Figure 5: LED Equivalent Dynamic Resistance
Calculate the value of the ceramic output
capacitor as:
COUT
IL _ PEAK
8fs _ min RLED ILED _ ripple _ pk _ pk
Where fs_min is the minimum operating
frequency, which occurs at the minimum input
voltage and maximum output voltage condition
for this application.
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© 2012 MPS. All Rights Reserved.
14
MP4700 – HIGH-BRIGHTNESS BCM, LOW SIDE BUCK WLED DRIVE WLED DRIVER
fs _ min
(Vbus _ min VLED _ max ) * VLED _ max
L * IL _ PEAK * Vbus _ min
Considering that IL_PEAK=2×ILED, the output
capacitor value is:
2
COUT
L * ILED * Vbus _ min
2(Vbus _ min VLED _ max ) * VLED _ max RLED ILED _ ripple _ pk _ pk
If using an electrolytic output capacitor, its ESR
dominates its impedance. The ESR value
should be:
RESR _ COUT
RLED ILED _ ripple _ pk _ pk
IL _ PEAK
Usually, a 1μF to 2.2μF ceramic capacitor or a
10μF to 22μF electrolytic capacitor should
suffice for this LED current level.
Select the Input Capacitor
Select the input capacitor to ensure that the
input voltage ripple is within a desired range
(1% to 5% of the input bus voltage). The input
capacitor is usually electrolytic and its ESR
dominates its impedance. The ESR of this input
capacitor should be:
RESR _ Cin
PCB Layout Guide
For best results, follow these layout guidelines
for the MP4700 low-side buck:
(1) Make the high-frequency switching loop
(the input capacitor, the diode, the power
MOSFET and the current sense resistor) as
small and tight as possible.
(2) Place the current sense resistor close to
the IC and make the current sense loop as
small as possible.
(3) Separate the GND of MP4700 from the
power ground of system, which conducts
current of power stage.
Figure 6 and Figure 7 show an example of the
PCB layout. Figure 7 shows the high-frequency
switching loop (composed of the input capacitor,
the diode, the power MOSFET and the current
sense resistor) marked in red. Make sure to
minimize this high-frequency switching loop.
Vbus _ ripple _ PK _ PK
IL _ PEAK
A 4.7μF to 22μF electrolytic capacitor will
usually suffice.
MP4700 Rev. 1.0
10/29/2012
www.MonolithicPower.com
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© 2012 MPS. All Rights Reserved.
15
MP4700 – HIGH-BRIGHTNESS BCM, LOW SIDE BUCK WLED DRIVE WLED DRIVER
Figure 6: Top Layer
Figure 7: Bottom Layer
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.
16
MP4700 – HIGH-BRIGHTNESS BCM, LOW SIDE BUCK WLED DRIVE WLED DRIVER
System Performance
LED Driver Stage Efficiency
98.4
System Efficiency
89
300
88.5
250
98.2
98.0
97.8
97.6
200
88
97.4
150
97.2
87.5
100
97.0
96.8
87
50
96.6
96.4
250
260
270 280 290
BUS VOLTAGE (V)
300
86.5
100
140
180
220
260 280
INPUT AC LINE VOLTAGE (V)
0
Analog Dimming Curve
300
300
250
250
200
200
150
150
100
100
50
50
0
20
40
60
80
100
Steady State
VCS
500mV/div.
VLED+
100V/div.
IL
500mA/div.
0
0.5 1
1.5 2 2.5
VADIM (V)
3
3.5
Startup
Analog Dimming
VLED+
100V/div.
VPWM
2V/div.
VSW
100V/div.
VCS
200mV/div.
VLED+
100V/div.
VLED100V/div.
VCS
200mV/div.
IL
500mA/div.
ILED
200mA/div.
VADIM
2V/div.
ILED
200mA/div.
VSW
100V/div.
MP4700 Rev. 1.0
10/29/2012
20
40
60
80
100
PWM DIMMING DUTY(%)
VSW
100V/div.
0
0
0
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© 2012 MPS. All Rights Reserved.
17
MP4700 – HIGH-BRIGHTNESS BCM, LOW SIDE BUCK WLED DRIVE WLED DRIVER
13V
VLED+
100V/div.
VLED100V/div.
IL
500mA/div.
MP4700 Rev. 1.0
10/29/2012
VPWM_IC
2V/div.
VLED+
100V/div.
VPWM_IC
2V/div.
VLED+
100V/div.
VLED100V/div.
VLED100V/div.
ILED
200mA/div.
ILED
200mA/div.
www.MonolithicPower.com
MPS Proprietary Information. Patent Protected. Unauthorized Photocopy and Duplication Prohibited.
© 2012 MPS. All Rights Reserved.
18
MP4700 – HIGH-BRIGHTNESS BCM, LOW SIDE BUCK WLED DRIVE WLED DRIVER
PACKAGE INFORMATION
SOIC8
0.189(4.80)
0.197(5.00)
8
0.050(1.27)
0.024(0.61)
5
0.063(1.60)
0.150(3.80)
0.157(4.00)
PIN 1 ID
1
0.228(5.80)
0.244(6.20)
0.213(5.40)
4
TOP VIEW
RECOMMENDED LAND PATTERN
0.053(1.35)
0.069(1.75)
SEATING PLANE
0.004(0.10)
0.010(0.25)
0.013(0.33)
0.020(0.51)
0.0075(0.19)
0.0098(0.25)
SEE DETAIL "A"
0.050(1.27)
BSC
SIDE VIEW
FRONT VIEW
0.010(0.25)
x 45o
0.020(0.50)
GAUGE PLANE
0.010(0.25) BSC
0o-8o
0.016(0.41)
0.050(1.27)
DETAIL "A"
NOTE:
1) CONTROL DIMENSION IS IN INCHES. DIMENSION IN
BRACKET IS IN MILLIMETERS.
2) PACKAGE LENGTH DOES NOT INCLUDE MOLD FLASH,
PROTRUSIONS OR GATE BURRS.
3) PACKAGE WIDTH DOES NOT INCLUDE INTERLEAD FLASH
OR PROTRUSIONS.
4) LEAD COPLANARITY (BOTTOM OF LEADS AFTER FORMING)
SHALL BE 0.004" INCHES MAX.
5) DRAWING CONFORMS TO JEDEC MS-012, VARIATION AA.
6) DRAWING IS NOT TO SCALE.
NOTICE: The information in this document is subject to change without notice. Users should warrant and guarantee that third
party Intellectual Property rights are not infringed upon when integrating MPS products into any application. MPS will not
assume any legal responsibility for any said applications.
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.
19
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