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 MPS Proprietary Information. Patent Protected. Unauthorized Photocopy and Duplication Prohibited. © 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. www.MonolithicPower.com MPS Proprietary Information. Patent Protected. Unauthorized Photocopy and Duplication Prohibited. © 2012 MPS. All Rights Reserved. 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 (VIN<12V) VIN=18V (VIN>12V) 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 www.MonolithicPower.com MPS Proprietary Information. Patent Protected. Unauthorized Photocopy and Duplication Prohibited. © 2012 MPS. All Rights Reserved. 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. www.MonolithicPower.com MPS Proprietary Information. Patent Protected. Unauthorized Photocopy and Duplication Prohibited. © 2012 MPS. All Rights Reserved. 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 www.MonolithicPower.com MPS Proprietary Information. Patent Protected. Unauthorized Photocopy and Duplication Prohibited. © 2012 MPS. All Rights Reserved. 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 MPS Proprietary Information. Patent Protected. Unauthorized Photocopy and Duplication Prohibited. © 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. www.MonolithicPower.com MPS Proprietary Information. Patent Protected. Unauthorized Photocopy and Duplication Prohibited. © 2012 MPS. All Rights Reserved. 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 www.MonolithicPower.com MPS Proprietary Information. Patent Protected. Unauthorized Photocopy and Duplication Prohibited. © 2012 MPS. All Rights Reserved. 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 www.MonolithicPower.com MPS Proprietary Information. Patent Protected. Unauthorized Photocopy and Duplication Prohibited. © 2012 MPS. All Rights Reserved. 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. www.MonolithicPower.com MPS Proprietary Information. Patent Protected. Unauthorized Photocopy and Duplication Prohibited. © 2012 MPS. All Rights Reserved. 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 www.MonolithicPower.com MPS Proprietary Information. Patent Protected. Unauthorized Photocopy and Duplication Prohibited. © 2012 MPS. All Rights Reserved. 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. www.MonolithicPower.com MPS Proprietary Information. Patent Protected. Unauthorized Photocopy and Duplication Prohibited. © 2012 MPS. All Rights Reserved. 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 MPS Proprietary Information. Patent Protected. Unauthorized Photocopy and Duplication Prohibited. © 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. www.MonolithicPower.com MPS Proprietary Information. Patent Protected. Unauthorized Photocopy and Duplication Prohibited. © 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 MPS Proprietary Information. Patent Protected. Unauthorized Photocopy and Duplication Prohibited. © 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 www.MonolithicPower.com MPS Proprietary Information. Patent Protected. Unauthorized Photocopy and Duplication Prohibited. © 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