MIC3205 High-Brightness LED Driver Controller with Fixed-Frequency Hysteretic Control General Description Features The MIC3205 is a hysteretic, step-down, high-brightness LED (HB LED) driver with a patent pending frequency regulation scheme that maintains a constant operating frequency over input voltage range. It provides an ideal solution for interior/exterior lighting, architectural and ambient lighting, LED bulbs, and other general illumination applications. The MIC3205 is well suited for lighting applications requiring a wide input voltage range. The hysteretic control provides good supply rejection and fast response during load transients and PWM dimming. The high-side current sensing and on-chip current-sense amplifier deliver LED current with 5% accuracy. An external high-side currentsense resistor is used to set the output current. The MIC3205 offers a dedicated PWM input (DIM) which enables a wide range of pulsed dimming. A high-frequency switching operation up to 1.5MHz allows the use of smaller external components minimizing space and cost. The MIC3205 operates over a junction temperature from –40°C to +125°C and is available in a 10-pin 3mm x 3mm ® MLF package. Data sheets and support documentation are available on Micrel’s web site at: www.micrel.com. 4.5V to 40V input voltage range Fixed operating frequency over input voltage range High efficiency (90%) 5% LED current accuracy High-side current sense Dedicated dimming control input Hysteretic control (no compensation!) Up to 1.5MHz switching frequency Adjustable constant LED current Over-temperature protection –40C to 125C junction temperature range Applications Architectural, industrial, and ambient lighting LED bulbs Indicators and emergency lighting Street lighting Channel letters 12V lighting systems (MR-16 bulbs, under-cabinet lighting, garden/pathway lighting) _________________________________________________________________________________________________________________________ Typical Application Normalized Switching Frequency vs. Input Voltage NORMALIZED FREQUENCY 2.0 ILED = 1A RCS = 0.2Ω 1.5 1 LED L = 22µH 1.0 4 LED L = 47µH 0.5 10 LED L = 33µH 6 LED L = 68µH 0.0 0 9 18 27 36 45 INPUT VOLTAGE (V) MIC3205 Buck LED Driver MLF and MicroLeadFrame are registered trademarks of Amkor Technology, Inc. Micrel Inc. • 2180 Fortune Drive • San Jose, CA 95131 • USA • tel +1 (408) 944-0800 • fax + 1 (408) 474-1000 • http://www.micrel.com October 2012 M9999-102312-A Micrel, Inc. MIC3205 Ordering Information Part Number Junction Temperature Range Package(1) MIC3205YML 40°C to 125°C 10-Pin 3mm x 3mm MLF Note: 1. MLF is a GREEN RoHS-compliant package. Lead finish is NiPdAu. Mold compound is Halogen Free. Pin Configuration 10-Pin 3mm x 3mm MLF (ML) Top View Pin Description Pin Number Pin Name 1 VCC 2 CS Current Sense Input. Negative input to the current sense comparator. Connect an external sense resistor to set the LED current. Connect the current sense resistor as close as possible to the chip. 3 VIN Input Power Supply. VIN is the input supply pin to the internal circuitry. Due to high frequency switching noise, a 10µF ceramic capacitor is recommended for bypassing and should be placed as close as possible to the VIN and PGND pins. See “PCB Layout Guidelines.” 4 VINS VIN Sense. Positive input to the current sense comparator. Connect as close as possible to the current sense resistor. 5 AGND Analog Ground. Ground for all internal low-power circuitry. 6 EN Enable Input. Logic high (greater than 2V) powers up the regulator. A logic low (less than 0.4V) powers down the regulator and reduces the supply current of the device to less than 2µA. A logic low pulls down the DRV pin turning off the external MOSFET. Do not drive the EN pin above VIN. Do not leave floating. 7 DIM PWM Dimming Input. A PWM input can be used to control the brightness of the LED. Logic high (greater than 2V) enables the output. Logic low (less than 0.4V) disables the output regardless of the EN state. Do not drive the DIM pin above VIN. Do not leave floating. 8 CTIMER 9 PGND 10 DRV Gate Drive Output. Connect to the gate of an external N-channel MOSFET. The drain of the external MOSFET connects directly to the inductor and provides the switching current necessary to operate in hysteretic mode. EP ePAD Exposed Pad. Must be connected to a GND plane for best thermal performance. October 2012 Pin Function Voltage Regulator Output. The VCC pin is the output of a linear regulator powered from VIN, which supplies power to the internal circuitry. A 4.7µF ceramic capacitor is recommended for bypassing. Place it as close as possible to the VCC and AGND pins. Do not connect to an external load. Timer Capacitor. A capacitor is required from CTIMER to ground sets the target switching frequency using the equation CTIMER=2.22*10-4 / FSW Power Ground. Ground for the power MOSFET gate driver. The current loop for the power ground should be as small as possible and separate from the analog ground loop. See “PCB Layout Recommendations.” 2 M9999-102312-A Micrel, Inc. MIC3205 Absolute Maximum Ratings (1) Operating Ratings (2) VIN to PGND .................................................. 0.3V to 42V VINS to PGND......................................... 0.3V to (VIN+0.3V) VCC to PGND ................................................ 0.3V to 6.0V CS to PGND ........................................ 0.3V to (VIN 0.3V) EN to AGND ........................................ 0.3V to (VIN 0.3V) DIM to AGND ...................................... 0.3V to (VIN 0.3V) CTIMER to AGND .............................. 0.3V to (VCC 0.3V) DRV to PGND .................................... 0.3V to (VCC 0.3V) PGND to AGND .......................................... 0.3V to 0.3V Junction Temperature ................................................ 150C Storage Temperature Range .................... 60°C to 150C Lead Temperature (Soldering, 10sec) ....................... 260C ESD Ratings (3) HBM ...................................................................... 1.5kV MM .........................................................................200V Supply Voltage (VIN).......................................... 4.5V to 40V Enable Voltage (VEN) .............................................. 0V to VIN Dimming Voltage (VDIM)................................................................. 0V to VIN Junction Temperature (TJ) ........................ 40C to 125C Junction Thermal Resistance 10-pin 3x3 MLF (JA).......................................60.7C/W 10-pin 3x3 MLF (JC).......................................28.7C/W Electrical Characteristics (4) VIN = VEN = VDIM = 12V; CVCC = 4.7µF; TJ = 25C; bold values indicate 40C TJ 125C, unless noted. Symbol Parameter Condition Min. Typ. Max. Units 40 V 1.3 3 mA 2 µA 4 4.5 V Input Supply VIN Input Voltage Range (VIN) IS Supply Current 4.5 DRV = Open ISD Shutdown Current VEN = 0V UVLO VIN UVLO Threshold VIN Rising UVLOHYS VIN UVLO Hysteresis 3.2 600 mV VCC Supply VCC VCC Output Voltage VIN = 12V, ICC = 5mA Average Current Sense Threshold ∆VCS =VINS VCS 4.5 5 5.5 V 190 200 210 mV 188 200 212 mV Current Sense ∆VCS VCS Rising 50 ns VCS Falling 70 ns ∆tCS Current Sense Response Time ICS CS Input Current VIN = VCS 0.5 Sense Voltage Hysteresis (5) VIN =12V, VLED =3V, L=47µH, FSW =250kHz, VD = 0.7V, ILED = 1A 46 mV 66 µA 1.189 V ∆VHYS 10 µA Frequency ITIMER CTIMER Pull-up Current VCTREF CTIMER Threshold (4*ITIMER)/ VCTREF Frequency Coefficient (6) October 2012 1.776 × 10-4 3 2.22 × 10-4 2.664 × 10-4 A/V M9999-102312-A Micrel, Inc. MIC3205 Electrical Characteristics (4) (Continued) VIN = VEN = VDIM = 12V; CVCC = 4.7µF; TJ = 25C; bold values indicate 40C TJ 125C, unless noted. Symbol Parameter Condition Min. Typ. Max. Units 0.4 V 60 µA 1 µA Enable Input ENHI EN Logic Level High ENLO EN Logic Level Low IEN EN Bias Current tSTART Start-Up Time 2.0 VEN = 12V V 20 VEN = 0V From EN pin going high to DRV going high 65 µs Dimming Input DIMHI DIM Logic Level High DIMLO DIM Logic Level Low IDIM DIM Bias Current 2.0 VDIM = 12V V 20 VDIM = 0V tDIM DIM Delay Time From DIM pin going high to DRV going high fDIM Maximum Dimming Frequency % of switching frequency 0.4 V 50 µA 1 µA 450 ns 2 % Pull-Up, ISOURCE = 10mA 4 Ω Pull-Down, ISINK = -10mA 1.5 Ω Rise Time, CLOAD = 1000pF 13 ns Fall Time, CLOAD = 1000pF 7 ns 160 C 20 C External FET Driver RON DRV On-Resistance tDRV DRV Transition Time Thermal Protection TLIM Overtemperature Shutdown TLIMHYS Overtemperature Shutdown Hysteresis TJ Rising Notes: 1. Exceeding the absolute maximum rating can damage the device. 2. The device is not guaranteed to function outside its operating rating. 3. Devices are ESD sensitive. Handling precautions are recommended. Human body model, 1.5kΩ in series with 100pF. 4. Specification for packaged product only. 5. See “Sense Voltage Hysteresis Range” in the “Application Information” section. 6. See “Frequency of Operation” in the “Application Information” section. October 2012 4 M9999-102312-A Micrel, Inc. MIC3205 Typical Characteristics Efficiency (ILED = 1A) vs. Input Voltage VIN Supply Current vs. Input Voltage 3.0 VIN SUPPLY CURRENT (mA) 90 4 LED L = 47µH 85 6 LED L = 68µH 80 1 LED L = 22µH 75 10 LED L = 33µH 70 65 60 2.5 2.0 1.5 1.0 0.5 0.0 0 9 18 27 36 9 18 VCC Output Voltage vs. Input Voltage 36 5.5 5.0 4.5 4.0 0 36 27 36 1.5 1 LED L = 22µH 1.0 4 LED L = 47µH 0.5 10 LED L = 33µH 6 LED L = 68µH TA = 25°C RCS = 0.2Ω 1.05 1.00 1 LED L = 22µH 6 LED L = 68µH 0.95 4 LED L = 47µH 0.90 0 9 18 27 36 45 0 9 18 27 36 INPUT VOLTAGE (V) INPUT VOLTAGE (V) CTIMER Current vs. Input Voltage Enable Threshold vs. Input Voltage Enable Bias Current vs. Input Voltage 1.5 ENABLE THRESHOLD (V) 64 62 60 1.2 RISING 0.9 FALLING 0.6 0.3 HYST 0.0 9 18 27 INPUT VOLTAGE (V) October 2012 36 45 45 100 ILED = 1A TA = 25°C 66 45 1.10 INPUT VOLTAGE (V) VEN = VIN TA = 25°C 0 18 ILED Output Current vs. Input Voltage ILED = 1A RCS = 0.2Ω 45 70 68 9 INPUT VOLTAGE (V) 0.0 27 0.2 45 ILED OUTPUT CURRENT (A) NORMALIZED FREQUENCY VCC OUTPUT VOLTAGE (V) 27 2.0 TA = 25°C ILED = 1A 18 0.4 Normalized Switching Frequency vs. Input Voltage 6.0 9 0.6 INPUT VOLTAGE (V) INPUT VOLTAGE (V) 0 VEN = 0V ILED = 0A TA = 25°C 0.8 0.0 0 45 ENABLE BIAS CURRENT (µA) EFFICIENCY (%) 95 1.0 ILED = 0A TA = 25°C VIN SHUTDOWN CURRENT (µA) 100 CTIMER CURRENT (µA) VIN Shutdown Current vs. Input Voltage VEN = VIN TA = 25°C ILED = 0A 80 60 40 20 0 0 9 18 27 INPUT VOLTAGE (V) 5 36 45 0 9 18 27 36 INPUT VOLTAGE (V) M9999-102312-A 45 Micrel, Inc. MIC3205 Typical Characteristics (Continued) Enable Bias Current vs. Enable Voltage Thermal Shutdown vs. Input Voltage 200 80 VIN = 42V 60 40 20 0 2.0 RISING 160 FALLING 120 ILED = 1A 80 40 HYST 0 0 9 18 27 36 0 9 VIN Shutdown Current vs. Temperature 18 27 36 1.2 0.8 0.4 0.0 -25 0 25 50 75 100 0 25 50 75 ILED Output Current vs. Temperature Switching Frequency vs. Temperature 1.01 1.00 490 470 450 0.99 430 -25 0 25 50 75 100 -50 125 -25 0 25 50 75 TEMPERATURE (°C) TEMPERATURE (°C) VCC vs. Temperature Enable Threshold vs. Temperature Enable Bias Current vs. Temperature 1.6 5.0 4.5 4.0 1.2 RISING 0.8 FALLING 0.4 HYST 0.0 -25 0 25 50 75 TEMPERATURE (°C) October 2012 100 125 100 125 100 125 30 VIN = 12V ILED = 1A EN BIAS CURRENT (µA) ENABLE THRESHOLD (V) 5.5 125 VIN = 12V VLED = 3.5V L = 22µH CT = 470pF RCS = 0.2Ω 510 TEMPERATURE (°C) VIN = 12V ILED = 1A 100 530 -50 6.0 -50 -25 TEMPERATURE (°C) VIN = 12V VLED = 3.5V RCS = 0.2Ω 1.02 125 1.2 -50 0.98 -50 1.4 INPUT VOLTAGE (V) FREQUENCY (kHz) 1.6 1.6 45 1.03 VIN = 12V ILED = 0A VEN = 0V ILED OUTPUT CURRENT (A) SUPPLY CURRENT (µA) 2.0 VIN = 12V ILED = 0A 1.8 1.0 45 ENABLE VOLTAGE (V) VCC (V) SUPPLY CURRENT (mA) VEN ≠ VIN TA = 25°C ILED = 0A THERMAL SHUTDOWN (°C) ENABLE BIAS CURRENT (µA) 100 VIN Supply Current vs. Temperature VIN = 12V ILED = 0A VEN = 12V 25 20 15 10 -50 -25 0 25 50 75 TEMPERATURE (°C) 6 100 125 -50 -25 0 25 50 75 TEMPERATURE (°C) M9999-102312-A Micrel, Inc. MIC3205 Typical Characteristics (Continued) VIN UVLO Threshold vs. Temperature VIN UVLO THRESHOLD (V) 5 RISING 4 FALLING 3 2 1 HYST 0 -50 -25 0 25 50 75 100 125 TEMPERATURE (°C) October 2012 7 M9999-102312-A Micrel, Inc. MIC3205 Functional Characteristics October 2012 8 M9999-102312-A Micrel, Inc. MIC3205 Functional Characteristics (Continued) October 2012 9 M9999-102312-A Micrel, Inc. MIC3205 Functional Diagram Figure 1. MIC3205 Block Diagram October 2012 10 M9999-102312-A Micrel, Inc. Functional Description The MIC3205 is a hysteretic step-down driver that regulates the LED current with a patent pending frequency regulation scheme. This scheme maintains a fixed operating frequency over a wide input voltage range. Theory of Operation The device operates from a 4.5V to 40V input MOSFET voltage. At turn-on, after the VIN input voltage crosses 4.5V, the DRV pin is pulled high to turn on an external MOSFET. The inductor and series LED current builds up linearly. This rising current results in a rising differential voltage across the current sense resistor (RCS). When this differential voltage reaches an upper threshold, the DRV pin is pulled low, the MOSFET turns off, and the Schottky diode takes over and returns the series LEDs and inductor current to VIN. Then, the current through the inductor and series LEDs starts to decrease. This decreasing current results in a decreasing differential voltage across RCS. When this differential voltage reaches a lower threshold, the DRV pin is pulled high, the MOSFET is turned on, and the cycle repeats. The average of the CS pin voltage is 200mV below VIN voltage. This is the average current sense threshold (∆VCS). Thus, the CS pin voltage switches about VIN – 200mV with a peak-to-peak hysteresis that is the product of the peak-to-peak inductor current times the current sense resistor (RCS). The average LED current is set by RCS, as explained in the “Application Information” section. MIC3205 dynamically adjusts hysteresis to accommodate fixed-frequency operation. Average frequency is programmed using an external capacitor connected to the CTIMER pin, as explained in the “Frequency of Operation” subsection in the “Application Information” section. The internal frequency regulator dynamically adjusts the inductor current hysteresis every eight switching cycles to make the average switching frequency a constant. If the instantaneous frequency is higher than the programmed average value, the hysteresis is increased to lower the frequency and vice versa. In other hysteretic control systems, current sense hysteresis is constant and frequency can change with input voltage, inductor value, series LEDs voltage drop, or LED current. However, with this patent pending frequency regulation scheme, the MIC3205 changes inductor current hysteresis and keeps the frequency fixed even upon changing input voltage, inductor value, series LEDs voltage drop, or LED current. The MIC3205 has an on-board 5V regulator, which is for internal use only. Connect a 4.7µF capacitor on VCC pin to analog ground. October 2012 MIC3205 The MIC3205 has an EN pin that gives the flexibility to enable and disable the output with logic high and low signals. The maximum EN voltage is VIN. Figure 2. Theory of Operation LED Dimming The MIC3205 LED driver can control the brightness of the LED string through the use of pulse width modulated (PWM) dimming. A DIM pin is provided, which can turn on and off the LEDs if EN is in an active-high state. This DIM pin controls the brightness of the LED by varying the duty cycle of DIM pin from 1% to 99%. An input signal from DC up to 20kHz can be applied to the DIM pin (see “Typical Application”) to pulse the LED string on and off. A logic signal can be applied on the DIM pin for dimming, independent of input voltage (VIN). Using PWM dimming signals above 120Hz is recommended to avoid any recognizable flicker by the human eye. Maximum allowable dimming frequency is 2% of operating frequency that is set by the external capacitor on the CTIMER pin (see “Frequency of Operation”). See “Functional Characteristics” on page 9 for PWM dimming waveforms. Maximum DIM voltage is VIN. PWM dimming is the preferred way to dim an LED to prevent color/wavelength shifting. Color/wavelength shifting occurs with analog dimming. By using PWM dimming, the output current level remains constant during each DIM pulse. The hysteretic buck converter switches only when the DIM pin is high. When the DIM pin is low, no LED current flows and the DRV pin is low turning the MOSFET off. 11 M9999-102312-A Micrel, Inc. MIC3205 CTIMER pin, gives the average frequency of operation, as seen in the following equation: Application Information The internal block diagram of the MIC3205 is shown in Figure 1. The MIC3205 is composed of a current-sense comparator, voltage reference, frequency regulator, 5V regulator, and MOSFET driver. Hysteretic mode control, also called bang-bang control, is a topology that does not use an error amplifier, instead using an error comparator. The frequency regulator dynamically adjusts hysteresis for the current sense comparator to regulate frequency. The inductor current is sensed by an external sense resistor (RCS) and controlled within a hysteretic window. It is a simple control scheme with no oscillator and no loop compensation. The control scheme does not need loop compensation. This makes design easy, and avoids instability problems. Transient response to load and line variation is very fast and depends only on propagation delay. This makes the control scheme very popular for certain applications. LED Current and RCS The main feature in MIC3205 is that it controls the LED current accurately within 5% of set current. Choosing a high-side RCS resistor is helpful for setting constant LED current regardless of wide input voltage range. The following equation and Table 1 give the RCS value for required LED current: 200mV ILED RCS Eq. 1 RCS (Ω) ILED (A) I2R (W) Size (SMD) 1.33 0.15 0.03 0603 0.56 0.35 0.07 0805 0.4 0.5 0.1 0805 0.28 0.7 0.137 0805 0.2 1.0 0.2 1206 0.13 1.5 0.3 1206 0.1 2.0 0.4 2010 0.08 2.5 0.5 2010 0.068 3.0 0.6 2010 FSW 2.22 10 -4 CT Eq. 2 The actual average frequency can vary depending on the variation of the frequency co-efficient and the parasitic board capacitances in parallel to the external capacitor CT. As shown in the Electrical Characteristics table, part to part variation for the frequency co-efficient is ±20% over temperature, from the target frequency co-efficient of 2.22 × 10-4. Switching frequency selection is based on the trade-off between efficiency and system size. Higher frequencies result in smaller, but less efficient, systems and vice versa. The operating frequency is independent of input voltage, inductor value, series LEDs voltage drop, or LED current, as long as 40mv ≤ ∆VHYS ≤ 100mV is maintained as explained in the next sections. Sense Voltage Hysteresis Range The frequency regulation scheme requires that the hysteresis remain in a controlled window. Components and operating conditions must be such that the hysteresis on the CS pin is between 40mV and 100mV. Hysteresis less than 40mV or more than 100mV can result in loss of frequency regulation. After average LED current (ILED) has been set by RCS and operating frequency has been set by external capacitor CT, the hysteresis ∆VHYS is calculated as follows: As seen in Figure 2, for the inductor, IL VHYS RCS Eq. 3 where: ∆IL = inductor ripple current ∆VHYS = hysteresis on CS pin For rising inductor current (MOSFET is on): tr Table 1. RCS for LED Current L IL VL_RISE Eq. 4 where: Frequency of Operation The patent pending frequency regulation scheme allows for operating frequency to be programmed by an external capacitor from the CTIMER pin to AGND. The frequency co-efficient (typically 2.22 × 10-4 A/F) divided by the value of this external capacitor connected to the October 2012 VL_RISE = VIN ILED × RCS VLED VLED is the total voltage drop of the LED string VIN is the input voltage RCS is the current sense resistor ILED is the average LED current 12 M9999-102312-A Micrel, Inc. MIC3205 tr is the MOSFET ON-time L is the inductor For falling inductor current (MOSFET is off): tf L I L VL_FALL Eq. 5 where: RCS (Ω) ILED (A) VIN (V) L (µH) ∆VHYS 0.56 0.35 5 22 64.1 0.56 0.35 12 68 57.7 0.28 0.7 5 10 70.5 0.28 0.7 12 33 59.4 0.2 1.0 5 6.8 72.6 0.2 1.0 12 22 62.4 0.1 2.0 5 3.6 68.5 0.1 2.0 12 10 68.6 (mV) VL_FALL = VD + ILED × RCS VLED VD is the freewheeling diode forward drop tf is the MOSFET OFF-time Table 2. Inductor for FSW = 400 kHz, VD = 0.4V, 1 LED Operating frequency and time period are given by: RCS (Ω) ILED (A) VIN (V) L (µH) ∆VHYS 0.56 0.35 24 150 55.8 0.56 0.35 36 220 56.8 0.28 0.7 24 68 61.6 0.28 0.7 36 100 62.5 0.2 1.0 24 47 62.4 0.2 1.0 36 68 64.3 0.1 2.0 24 22 66.6 0.1 2.0 36 33 66.2 FSW 1 T T tr tf Eq. 6 Eq. 7 Using Equations 3, 4, 5, 6, and 7: VHYS (VIN - ILED RCS - VLED) (VD ILED RCS VLED) RCS ( VIN VD) L FSW Eq. 8 The value of ∆VHYS calculated in this way must be between 40mV and 100mV to ensure frequency regulation. Table 3. Inductor for FSW = 400 kHz, VD = 0.4V, 4 LED Inductor According to the above equations, the inductor value can be calculated once average LED current, operating frequency and an appropriate hysteresis ∆VHYS value have been chosen. Thus, inductor L is given by: L (VIN - ILED RCS - VLED) (VD ILED RCS VLED) RCS Eq. 9 ( VIN VD) VHYS FSW Table 2, Table 3, and Table 4 give reference inductor values for an operating frequency of 400 kHz, for a given LED current, freewheeling diode forward drop, and number of LEDs. By selecting ∆VHYS in the 55mV to 75mV range, we get the following inductor values: (mV) RCS (Ω) ILED (A) VIN (V) L (µH) ∆VHYS 0.56 0.35 36 150 58.4 0.56 0.35 40 220 54.3 0.28 0.7 36 68 64.4 0.28 0.7 40 100 59.6 0.2 1.0 36 47 65.2 0.2 1.0 40 68 61.4 0.1 2.0 36 22 69.6 0.1 2.0 40 33 63.3 (mV) Table 4. Inductor for FSW = 400 kHz, VD = 0.4V, 8 LED Given an inductor value, the size of the inductor can be determined by its RMS and peak current rating. Because LEDs are in series with the inductor, IL ILED Eq. 10 From Equations 1, 3, and 10: IL VHYS IL 200m October 2012 13 Eq. 11 M9999-102312-A Micrel, Inc. MIC3205 With 40mv ≤ ∆VHYS ≤ 100mV: IL(RMS ) IL2 IL(PK) IL(1 1 2 IL IL 12 VHYS ) 400m Eq. 12 Eq. 13 where: IL is the average inductor current IL(PK) is the peak inductor current where: RGATE is total MOSFET gate resistance; Qgs2 and Qgd can be found in a MOSFET manufacturer data sheet. A gate resistor can be connected between the MOSFET gate and the DRV pin to slow down MOSFET switching edges. A 2Ω resistor is usually sufficient. The total power loss is: PLOSS( TOT ) = PLOSS( CON) + PLOSS( TRAN) The MOSFET junction temperature is given by: TJ = PLOSS( TOT ) × R θJA + TA Select an inductor with a saturation current rating at least 30% higher than the peak current. For space-sensitive applications, smaller inductors with higher switching frequency could be used but regulator efficiency will be reduced. MOSFET N-channel MOSFET selection depends on the maximum input voltage, output LED current, and switching frequency. The selected N-channel MOSFET should have 30% margin on maximum voltage rating for high reliability requirements. The MOSFET channel resistance (RDSON) is selected such that it helps to get the required efficiency at the required LED currents and meets the cost requirement. Logic level MOSFETs are preferred as the drive voltage is limited to 5V. The MOSFET power loss has to be calculated for proper operation. The power loss consists of conduction loss and switching loss. The conduction loss can be found by: PLOSS(CON) 2 IRMS(FET) R DSON IRMS(FET) ILED D D VLED VIN The switching loss occurs during the MOSFET turn-on and turn-off transition and can be found by: PLOSS( TRAN) = IDRV = VIN × ILED × FSW × (Qgs2 + Qgd ) IDRV TJ must not exceed maximum junction temperature under any conditions. Freewheeling Diode The freewheeling diode should have a reverse voltage rating that is at least 20% higher than the maximum input supply voltage. The forward voltage drop should be small to get the lowest conduction dissipation for high efficiency. The forward current rating should be at least equal to the LED current. Schottky diodes with low forward voltage drop and fast reverse recovery are ideal choices and give the highest efficiency. The freewheeling diode average current (ID) is given by: ID (1 D) ILED Diode power dissipation (PD) is given by: PD VD ID Typically, higher current rating diodes have a lower VD and have better thermal performance, improving efficiency. Input Capacitor The ceramic input capacitor is selected by voltage rating and ripple current rating. A 10µF ceramic capacitor is usually sufficient. Select a voltage rating that is at least 30% larger than the maximum input voltage. LED Ripple Current The LED current is the same as inductor current ∆IL. A ceramic capacitor should be placed across the series LEDs to pass the ripple current. A 4.7µF capacitor is usually sufficient for most applications. Voltage rating should be the same as the input capacitor. VDRV RGATE October 2012 14 M9999-102312-A Micrel, Inc. PCB Layout Guidelines NOTE: To minimize EMI and output noise, follow these layout recommendations. PCB layout is critical to achieve reliable, stable, and efficient performance. A ground plane is required to control EMI and minimize the inductance in power, signal, and return paths. Follow these guidelines to ensure proper operation of the MIC3205. IC Use thick traces to route the input and output power lines. Keep signal and power grounds separate and connect them at only one location. Input Capacitor MIC3205 LED Ripple Current Carrying Capacitor Place this ceramic capacitor as close to the LEDs as possible. Use either X7R or X5R dielectric capacitors. Do not use Y5V or Z5U type capacitors. MOSFET To avoid trace inductance, place the N-channel MOSFET as close as possible to the MIC3205. Provide sufficient copper area on MOSFET ground to dissipate the heat. Freewheeling Diode Place the Schottky diode on the same side of the board as the IC and input capacitor. Keep the connection from the Schottky diode’s anode to the switching node as short as possible. Keep the diode’s cathode connection to the RCS as short as possible. Place the input capacitors on the same side of the board and as close to the IC as possible. Keep both the VIN and PGND traces as short as possible. RC Snubber If the application requires vias to the ground plane, place them close to the input capacitor ground terminal, but not between the input capacitors and IC pins. Use either X7R or X5R dielectric input capacitors. Do not use Y5V or Z5U type capacitors. Do not replace the ceramic input capacitor with any other type of capacitor. Any type of capacitor can be placed in parallel with the ceramic input capacitor. If a tantalum input capacitor is placed in parallel with the ceramic input capacitor, it must be recommended for switching regulator applications and the operating voltage must be derated by 50%. In “Hot-Plug” applications, place a tantalum or electrolytic bypass capacitor in parallel to the ceramic capacitor to limit the overvoltage spike seen on the input supply when power is suddenly applied. In this case, an additional tantalum or electrolytic bypass input capacitor of 22µF or higher is required at the input power connection. If an RC snubber is needed, place the RC snubber on the same side of the board and as close to the Schottky diode as possible. A 1.2Ω resistor in series with a 1nF capacitor is usually a good choice. RCS (Current-Sense Resistor) VINS pin and CS pin must be as close as possible to RCS. Make a Kelvin connection to the VINS and CS pin, respectively, for current sensing. For low values of ∆VHYS (around 40mV) the switching noise could cause faulty switching on the DRV pin. If this occurs, place two 30Ω resistors and a 1nF capacitor, as shown in Figure 3, to filter out switching noise for low values of ∆VHYS. Alternatively, as seen in Equation 8, a smaller inductor value can be used to increase ∆VHYS and make the system more noise tolerant. Inductor Keep the inductor connection to the switch node (MOSFET drain) short. Do not route any digital lines underneath or close to the inductor. To minimize noise, place a ground plane underneath the inductor. October 2012 15 M9999-102312-A Micrel, Inc. MIC3205 For FSW = 400 kHz CT = 550pF The actual frequency may vary as explained in “Frequency of Operation” in the “Application Information” section. 3. INDUCTOR SELECTION From Equation 9: L Figure 3. Input Filter for Low Values of ∆VHYS Trace Routing Recommendation Keep the power traces as short and wide as possible. There is one current flowing loop during the MOSFET ON-time; the traces connect the input capacitor (CIN), RCS, the LEDs, the inductor, the MOSFET, and back to CIN. There is another current flowing loop during the MOSFET OFF-time; the traces for this loop connect RCS, the LED, the inductor, the freewheeling diode, and back to RCS. These two loop areas should kept as small as possible to minimize noise interference Keep all analog signal traces away from the switching node and its connecting traces. Design Example SPECIFICATIONS: FSW = 400 kHz VSUPPLY = 24V rectified AC ILED = 1A Voltage drop per LED = 3.5V Number of LEDs = 4 Schottky diode drop at 1A = 0.4V 1. CURRENT SENSE RESISTOR From Equation 1: RCS 200mV ILED (VIN - ILED RCS - VLED) (VD ILED RCS VLED) RCS ( VIN VD) VHYS FSW Given VSUPPLY = 24V rectified AC The peak voltage = √2 x VSUPPLY Thus for MIC3205, VIN ≈ 34V VLED = 3.5 x 4 = 14, VD = 0.4V Select ∆VHYS = 60mV Thus, L = 70µH Chose L = 68µH as closest available value. As a side note, for this example, L = 68µH can be used even if VSUPPLY = 24V DC. This is because ∆VHYS calculates to around 44mV (with VIN = VSUPPLY = 24V) which is acceptable. From Equations 12 and 13: IL(PK) = 1.15A Thus, we choose L = 68µH with an RMS saturation current of 1.5A or higher. 4. MOSFET SELECTION For this example, VIN = 34V, a 50V rating or greater Nchannel MOSFET is required. A high current rating MOSFET is a good choice because it has lower RDSON. A 60V, 12A MOSFET with 10mΩ RDSON is a good choice. 5. CAPACITOR SELECTION Use a 10µF/50V X7R type ceramic capacitor for the input capacitor. Use a 4.7µF/50V X5R type ceramic capacitor for the LED ripple current carrying capacitor connected across the series connection of 4 LEDs 6. FREEWHEELING DIODE SELECTION With VIN = 34V, choose a 2A, 60V Schottky diode with a forward drop voltage of 0.4V at 1A forward current. For ILED = 1A RCS = 0.2Ω 2. SWITCHING FREQUENCY From Equation 2: FSW October 2012 2.22 10 -4 CT 16 M9999-102312-A Micrel, Inc. MIC3205 Evaluation Board Schematic October 2012 17 M9999-102312-A Micrel, Inc. MIC3205 Bill of Materials Item Part Number 12105C475KAZ2A C1, C2,C3,C4,C11 C5 C10 GRM32ER71H475KA88L Murata CGA4J3X7R1H105K TDK 06035C471K4T2A AVX GRM188R60J475KE19J CGA3E1X5R0J475K 06035C102KAT2A GRM188R71H102KA01D C1608X7R1H102K SK36-TP D1 SK36 SK36-7-F L1 SLF10145T-220M1R9-PF M1 FDS5672 Qty. 4.7µF/50V, Ceramic Capacitor, X7R, Size 1210 5 1µF/50V, Ceramic Capacitor, X7R, Size 0805 1 470pF/50V, Ceramic Capacitor, X7R, Size 0603 1 4.7µF/6.3V, Ceramic Capacitor, X5R, Size 0603 1 1nF/50V, Ceramic Capacitor, X7R, Size 0603 2 60V, 3A, SMC, Schottky Diode 1 22µH, 2.1A, 0.0591Ω, SMT, Power Inductor 1 MOSFET, N-CH, 60V, 12A, SO-8 1 0.2Ω Resistor, 1/2W, 1%, Size 1206 1 (3) GRM21BR71H105KA12L 06036D475KAT2A C7,C9 Murata(2) TDK GRM188R71H471KA01D Description AVX(1) CGA6P3X7R1H475K C1608X7R1H471K C8 Manufacturer Murata TDK AVX Murata TDK AVX Murata TDK MCC(4) Fairchild(5) (6) Diodes, Inc. TDK Fairchild Stackpole Electronics, Inc.(7) RCS CSR1206FKR200 R5, R8 CRCW0603100KFKEA Vishay Dale(8) 100kΩ Resistor, 1%, Size 0603 2 R2, R3 CRCW060330R0FKEA Vishay Dale 30Ω Resistor, 1%, Size 0603 2 R1, R9 CRCW06032R00FKEA Vishay Dale 2Ω Resistor, 1%, Size 0603 2 R4 CRCW060310K0FKEA Vishay Dale 10kΩ Resistor, 1%, Size 0603 1 R6 CRCW060351R0FKEA Vishay Dale 51Ω Resistor, 1%, Size 0603 1 R7 CRCW06030000Z0EA Vishay Dale 0Ω Resistor, Size 0603 1 U1 MIC3205YML High-Brightness LED Driver Controller with Fixed Frequency Hysteretic Control 1 Micrel, Inc.(9) Notes: 1. AVX: www.avx.com. 2. Murata: www.murata.com. 3. TDK: www.tdk.com. 4. MCC: www.mccsemi.com. 5. Fairchild: www.fairchildsemi.com. 6. Diodes Inc.: www.diodes.com. 7. Stackpole Electronics: www.seielect.com. 8. Vishay Dale: www.vishay.com. 9. Micrel, Inc.: www.micrel.com. October 2012 18 M9999-102312-A Micrel, Inc. MIC3205 PCB Layout Recommendations Top Assembly Top Layer October 2012 19 M9999-102312-A Micrel, Inc. MIC3205 PCB Layout Recommendations (Continued) Bottom Layer October 2012 20 M9999-102312-A Micrel, Inc. MIC3205 Package Information 10-Pin 3mm x 3mm MLF (ML) October 2012 21 M9999-102312-A Micrel, Inc. MIC3205 Recommended Landing Pattern 10-Pin 3mm x 3mm MLF (ML) Land Pattern October 2012 22 M9999-102312-A Micrel, Inc. MIC3205 MICREL, INC. 2180 FORTUNE DRIVE SAN JOSE, CA 95131 USA TEL +1 (408) 944-0800 FAX +1 (408) 474-1000 WEB http://www.micrel.com Micrel makes no representations or warranties with respect to the accuracy or completeness of the information furnished in this data sheet. This information is not intended as a warranty and Micrel does not assume responsibility for its use. Micrel reserves the right to change circuitry, specifications and descriptions at any time without notice. No license, whether express, implied, arising by estoppel or otherwise, to any intellectual property rights is granted by this document. Except as provided in Micrel’s terms and conditions of sale for such products, Micrel assumes no liability whatsoever, and Micrel disclaims any express or implied warranty relating to the sale and/or use of Micrel products including liability or warranties relating to fitness for a particular purpose, merchantability, or infringement of any patent, copyright or other intellectual property right. Micrel Products are not designed or authorized for use as components in life support appliances, devices or systems where malfunction of a product can reasonably be expected to result in personal injury. Life support devices or systems are devices or systems that (a) are intended for surgical implant into the body or (b) support or sustain life, and whose failure to perform can be reasonably expected to result in a significant injury to the user. A Purchaser’s use or sale of Micrel Products for use in life support appliances, devices or systems is a Purchaser’s own risk and Purchaser agrees to fully indemnify Micrel for any damages resulting from such use or sale. © 2012 Micrel, Incorporated. October 2012 23 M9999-102312-A