ELM615DA 2A 18V 500kHz synchronous step down DC/DC converter http://www.elm-tech.com ■General description ELM615DA is a high-frequency, synchronous, rectified, step-down, switch-mode converter with internal power MOSFETs. It offers a very compact solution to achieve a 2A continuous output current over a wide input supply range, with excellent load and line regulation. ELM615DA has synchronous-mode operation for higher efficiency over the output current-load range. Current-mode operation provides fast transient response and eases loop stabilization. Protection features include over-current protection and thermal shutdown. ELM615DA requires a minimal number of readily available, standard external components and is available in space-saving SOP-8 package. ■Features ■Application • • • • • • • • • • • • • • • • Internal soft start Over current protection Over temperature protection Input voltage Output adjustable voltage Output current Integrated power MOSFET switches Shutdown current High efficiency Constant frequency Package : 4.75V to 18V : 0.923V to 15V : 2A : 129mΩ/87mΩ : Typ 3µA : Max 95% : Typ 500kHz : SOP-8 Distributed power systems Networking systems FPGA, DSP, ASIC power supplies Notebook computers Green electronics or appliance ■Maximum absolute ratings Parameter Supply voltage Switch node Boost voltage All other pins Power dissipation Junction temperature Operating temperature range Storage temperature range Symbol Vin Vsw Vboot Vall Pd Tj Top Tstg Limit -0.3 to +19.0 -0.3 to Vin+0.3 Vsw-0.3 to Vsw+6.0 -0.3 to +6.0 630 +150 -40 to +85 -65 to +150 Unit V V V V mW °C °C °C ■Selection guide ELM615DA-N Symbol a b c Package Product version Taping direction D: SOP-8 A N: Refer to PKG file ELM615 D A - N ↑ ↑ ↑ a b c * Taping direction is one way. Rev.1.1 8 - 1 ELM615DA 2A 18V 500kHz synchronous step down DC/DC converter http://www.elm-tech.com ■Pin configuration SOP-8(TOP VIEW) 1 8 2 7 3 6 4 5 Pin No. 1 2 3 4 5 6 7 8 Pin name BOOT VIN SW GND FB NC EN NC Pin description High-side gate drive boost input Power input Power switching output Ground Feedback input Not connected Enable input Not connected ■Standard circuit ( Adjustable Output Voltage ) 1 2 7 VIN BOOT EN SW FB GND 4 3 5 Note: C7 is optional. ■Block diagram RAMP FB 5 0.3V Oscillator Fosc1 or Fosc2 + - Soft start Current sense Amplifier + - Σ CLK S 0.923V + + + - Error amplifier Current comparator 200k 50pF R Q 1.2V 1 BOOT 3 SW 4 GND M2 0.9pF EN IN<3.75V 1.2V + VIN M1 Q thermal shutdown EN 7 2 5V VIN Internal Regulators Shutdown comparator 5V ELM615DA Rev.1.1 8 - 2 ELM615DA 2A 18V 500kHz synchronous step down DC/DC converter http://www.elm-tech.com ■Electrical characteristics Parameter Supply voltage Output voltage Shutdown supply current Supply current Feedback voltage Error amplifier voltage gain * Error amplifier transconductance High-side switch-on resistance * Low-side switch-on resistance * High-side switch leakage current Upper switch current limit Lower switch current limit Oscillation frequency Short circuit oscillation frequency Maximum duty cycle Minimum on time * EN falling threshold voltage EN rising threshold voltage Input under voltage lockout threshold Input under voltage lockout threshold hysteresis Soft-start period Thermal shutdown threshold * Guaranteed by design, not tested. Vin=+12V, Top=25°C, unless otherwise specified Symbol Test condition Min. Typ. Max. Unit Vin 4.75 18.00 V 0.923 15.000 V Vout Is Ven=0V 3 6 µA Ven=2.0V, Vfb=1.0V 2.0 mA Iin 0.900 0.923 0.946 V 4.75V ≤ Vin ≤ 18.00V Vfb Aea 1000 V/V Gea ∆Ic=±10µA 40 µA/V Rds(on)H 129 mΩ Rds(on)L 87 mΩ Ven=0V, Vsw=0V Ileak 10 µA Top=+125°C Iuswl Minimum duty cycle 3.0 3.6 A Ilswl From drain to source 0 A Fosc1 400 500 600 kHz Fosc2 Vfb=0V 100 125 150 kHz Dmax Vfb=0.5V 90 % to_min 100 ns VenL Ven falling 0.56 1.12 V VenH Ven rising 1.22 1.83 V Vuvlo Vin rising 3.5 V Vuvlo_hys 200 mV tss Tsd 2 150 ms °C ■Marking SOP-8 Mark a~r Content Assembly lot No. : 0 ~ 9 and A ~ Z Rev.1.1 8 - 3 ELM615DA 2A 18V 500kHz synchronous step down DC/DC converter http://www.elm-tech.com ■Application notes ELM615DA is a synchronous rectified, current-mode, step-down regulator. It regulates input voltages from 4.75V to 18V down to an output voltage as low as 0.923V, and supplies up to 2A of load current. ELM615DA uses current-mode control to regulate the output voltage. The output voltage is measured at FB through a resistive voltage divider and amplified through the internal trans-conductance error amplifier. The converter uses internal N-channel MOSFET switches to step-down the input voltage to the regulated output voltage. Since the high side MOSFET requires a gate voltage greater than the input voltage, a boost capacitor connected between SW and BOOT is needed to drive the high side gate. The boost capacitor is charged from the internal 5V rail when SW is low. 1) Pins description BOOT: High-side gate drive boost input. BOOT supplies the drive for the high-side N-channel MOSFET switch. Connect a 0.1μF or greater capacitor from SW to BOOT to power the high side switch. VIN: Power input. VIN supplies the power to the IC, as well as the step-down converter switches. Drive VIN with a 4.75V to 18V power source. Bypass VIN to GND with a suitably large capacitor to eliminate noise on the input to the IC. SW: Power switching output. SW is the switching node that supplies power to the output. Connect the output LC filter from SW to the output load. Note that a capacitor is required from SW to BOOT to power the high-side switch. GND: Ground. FB: Feedback input. FB senses the output voltage to regulate that voltage. Drive FB with a resistive voltage divider from the output voltage. The feedback threshold is 0.923V. EN: Enable input. EN is a digital input that turns the regulator on or off. Drive EN high to turn on the regulator, drive it low to turn it off. Pull up with 100kΩ resistor for automatic startup. *)EN terminal voltage is clamped to 5.7V by internal zenar diode when pulled up by 100kΩ resistor. 2) Setting the output voltage The output voltage is set using a resistive voltage divider from the output voltage to FB pin. The voltage divider divides the output voltage down to the feedback voltage by the ratio: Vfb = Vout × R2 / (R1 + R2) Where Vfb is the feedback voltage and Vout is the output voltage. Thus the output voltage is: Vout = 0.923 × (R1 + R2) / R2 R2 can be as high as 100kΩ, but a typical value is 10kΩ. Using the typical value for R2, R1 is determined by: R1 = 10.83 × (Vout − 0.923V) (KΩ) 3) Inductor The inductor is required to supply constant current to the output load while being driven by the switched input voltage. A larger value inductor will result in less ripple current that will result in lower output ripple voltage. However, the larger value inductor will have a larger physical size, higher series resistance, and/or lower saturation current. A good rule for determining the inductance to use is to allow the peak-to-peak ripple current in the inductor Rev.1.1 8 - 4 ELM615DA 2A 18V 500kHz synchronous step down DC/DC converter http://www.elm-tech.com to be approximately 30% of the maximum switch current limit. Also, make sure that the peak inductor current is below the maximum switch current limit. The inductance value can be calculated by: L = [ Vout / (fs × ΔIL) ] × (1 − Vout / Vin) Where Vout is the output voltage, Vin is the input voltage, fs is the switching frequency, and ΔIL is the peak-topeak inductor ripple current. Choose an inductor that will not saturate under the maximum inductor peak current. The peak inductor current can be calculated by: Ilp = Iload + [ Vout / (2 × fs × L) ] × (1 − Vout / Vin) Where Iload is the load current. The choice of which style inductor to use mainly depends on the price vs. size requirements and any EMI requirements. 4) Optional schottky diode During the transition between high-side switch and low-side switch, the body diode of the low-side power MOSFET conducts the inductor current. The forward voltage of this body diode is high. An optional Schottky diode may be paralleled between the SW pin and GND pin to improve overall efficiency. Table 1 lists example Schottky diodes and their Manufacturers. Voltage and Current Rating B130 30V, 1A SK13 30V, 1A MBRS130 30V, 1A Table 1 : Diode selection guide. Part number Vendor Diodes Inc. Diodes Inc. International Rectifier 5) Input capacitor The input current to the step-down converter is discontinuous, therefore a capacitor is required to supply the AC current to the step-down converter while maintaining the DC input voltage. Use low ESR capacitors for the best performance. Ceramic capacitors are preferred, but tantalum or low-ESR electrolytic capacitors may also suffice. Choose X5R or X7R dielectrics when using ceramic capacitors. Since the input capacitor (C1) absorbs the input switching current it requires an adequate ripple current rating. The RMS current in the input capacitor can be estimated by: Ic1 = Iload × [ (Vout / Vin) × (1 − Vout / Vin) ] 1/2 The worst-case condition occurs at Vin = 2Vout, where Ic1 = Iload/2. For simplification, choose the input capacitor whose RMS current rating greater than half of the maximum load current. The input capacitor can be electrolytic, tantalum or ceramic. When using electrolytic or tantalum capacitors, a small, high quality ceramic capacitor, i.e. 0.1μF, should be placed as close to the IC as possible. When using ceramic capacitors, make sure that they have enough capacitance to provide sufficient charge to prevent excessive voltage ripple at input. The input voltage ripple for low ESR capacitors can be estimated by: ΔVin = [ Iload / (C1 × fs) ] × (Vout / Vin) × (1 − Vout / Vin) Where C1 is the input capacitance value. 6) Output capacitor The output capacitor is required to maintain the DC output voltage. Ceramic, tantalum, or low ESR electrolytic capacitors are recommended. Low ESR capacitors are preferred to keep the output voltage ripple low. The output Rev.1.1 8 - 5 ELM615DA 2A 18V 500kHz synchronous step down DC/DC converter http://www.elm-tech.com voltage ripple can be estimated by: ΔVout = [ Vout / (fs × L) ] × (1 − Vout / Vin) × [ Resr + 1 / (8 × fs × C2) ] Where C2 is the output capacitance value and Resr is the equivalent series resistance (ESR) value of the output capacitor. In the case of ceramic capacitors, the impedance at the switching frequency is dominated by the capacitance. The output voltage ripple is mainly caused by the capacitance. For simplification, the output voltage ripple can be estimated by: ΔVout = [ Vout / (8 × fs2 × L × C2) ] × (1 − Vout / Vin) In the case of tantalum or electrolytic capacitors, the ESR dominates the impedance at the switching frequency. For simplification, the output ripple can be approximated to: ΔVout = [ Vout / (fs × L) ] × (1 − Vout / Vin) × Resr The characteristics of the output capacitor also affect the stability of the regulation system. ELM615DA can be optimized for a wide range of capacitance and ESR values. 7) External bootstrap diode An external bootstrap diode may enhance the efficiency of the regulator, the applicable conditions of external BOOT diode are: • Vout = 5V or 3.3V; and • Duty cycle is high : D = Vout / Vin > 65% Figure 1: Add optional external bootstrap diode to enhance efficiency. In these cases, an external BOOT diode is recommended from the output of the voltage regulator to BOOT pin, as shown in Figure 1. The recommended external BOOT diode is IN4148, and the BOOT capacitor is 0.1 ~ 1μF. When Vin≤ 6V, for the purpose of promote the efficiency, it can add an external Schottky diode between VIN and BOOT pins, as shown in Figure 2. Figure 2: Add a Schottky diode to promote efficiency when Vin ≤ 6V. Rev.1.1 8 - 6 ELM615DA 2A 18V 500kHz synchronous step down DC/DC converter http://www.elm-tech.com 8) PCB layout guide PCB layout is very important to achieve stable operation. Please follow the guidelines below. 1) Keep the path of switching current short and minimize the loop area formed by Input capacitor, high-side MOSFET and low-side MOSFET. 2) Bypass ceramic capacitors are suggested to be put close to the VIN Pin. 3) Ensure all feedback connections are short and direct. Place the feedback resistors and compensation components as close to the chip as possible. 4) Rout SW away from sensitive analog areas such as FB. 5) Connect VIN, SW, and especially GND respectively to a large copper area to cool the chip to improve thermal performance and long-term reliability. 9) BOM of ELM614xA Please refer to the Standard circuit. Item 1 2 3 4 Reference C1 C5 C7 R4 Table 2: BOM selection table I. Vout = 5.0V Vout = 3.3V Vout = 2.5V Vout = 1.8V Vout = 1.2V Vout = 1.0V L 6.8μH 4.7μH 4.7μH 3.3μH 2.2μH 2.2μH Table 3: BOM selection table II. R1 44.2K 25.7K 17.1K 9.50K 3.00K 0.834K Part 10μF 100nF 0.1μF 100K R2 10K 10K 10K 10K 10K 10K C2 10μF×2 10μF×2 10μF×2 10μF×2 10μF×2 10μF×2 Rev.1.1 8 - 7 ELM615DA 2A 18V 500kHz synchronous step down DC/DC converter http://www.elm-tech.com ■Typical characteristics Vin=12V, Vout=3.3V, L=4.7µH, C1=10µF, C2=10µF×2, Top=+25°C, unless otherwise noted. Start Up & Inrush Current (12V→3.3V, Load=1A) Shut Down (Iout 1A→Shut down) Output Rippie (12V=>3.3V, Load=2A) Output Rippie (12V=>3.3V, Load=1A) Output Rippie (12V=>3.3V, Load=0A) Dynamic Load (Iload=0.2A_2A, Vout=3.3V) Short Circuit Protection Efficiency Rev.1.1 8 - 8