MIC2875/6 Evaluation Board 4.8A I , Synchronous Boost Regulator SW with Bi-Directional Load Disconnect General Description Getting Started The MIC2875/6 evaluation board is a compact and highly- 1. Connect an external supply to the VIN terminals. efficient 2MHz synchronous boost regulator with a 4.8A Apply the desired input voltage to the VIN (J1) and switch. It features a bi-directional load disconnect function GND (J2) terminals of the evaluation board, paying which prevents any leakage current between the input and careful attention to polarity and supply range (2.5V to output when the device is disabled. 5.5V). An ammeter can be placed between the power supply and the input terminal of the evaluation board. The MIC2875/6 operates in bypass mode automatically Ensure that the supply voltage is monitored at the VIN when the input voltage is greater than the target output (J1) terminal, as the ammeter and/or power-lead voltage. At light loads, the boost converter goes to PFM resistance can reduce the voltage supplied to the mode to improve the efficiency. The part also features an input. integrated anti-ringing switch to minimize EMI. 2. Connect a load to VOUT and GND terminals. The MIC2875 is designed to operate with a minimum switching frequency of 45kHz from no load to full load to The load connected between VOUT (J3) and GND minimize switching artifacts in the audio band. (J4) terminals of the evaluation board can be either passive (resistive) or active (electronic load). In case Datasheets and support documentation are available on of electronic load, keep the load disabled until the Micrels web site at: www.micrel.com. MIC2875/6 has been powered up. Requirements An ammeter may be placed between the load and the output terminal. Ensure the output voltage is monitored The MIC2875/6 evaluation board requires a bench input across the VOUT (J3) and GND (J4) terminals. power source that is adjustable from 2.5V to 5.5V and is able to deliver greater than 4.5A at 2.5V. 3. Enable/Disable the MIC2875/6. Precautions The MIC2875/6 evaluation board has an enable (EN) pin (J8).The MIC2875/6 is in standby mode when the The evaluation board does not have reverse polarity EN pin is asserted high. Setting the jumper at JP1 protection. Applying a negative voltage across the VIN connects the EN pin to VIN through a 10k pull-up terminal to the ground (GND) terminal can damage the resistor. If this pin is driven low, the IC is shut down. device. It should not exceed 5.5V on the input. This pin has an internal pull-down resistor and can be Input power leads should be kept as short as possible to left open. minimize the inductance. VIN measurements should be 4. Power Good (/PG). made at the VIN and GND terminals, to ensure that the voltage drop on the input power leads are not included. A power good (/PG) test pin (J5) is provided to monitor the power good function. It is an open drain active low output power good output. Setting the jumper at JP2 connects the /PG pin to VIN through a 1M pull-up resistor. This pin is asserted low when the output is above the power good threshold. Ordering Information Part Number Description MIC2875-AYMT EV MIC2875 Adjustable Output MIC2875-5.0YMT EV MIC2875-5.0V Fixed Output MIC2876-AYMT EV MIC2876 Adjustable Output MIC2876-5.0YMT EV MIC2876-5.0V Fixed Output Micrel Inc. 2180 Fortune Drive San Jose, CA 95131 USA tel +1 (408) 944-0800 fax + 1 (408) 474-1000 Micrel, Inc. MIC2875/6 Evaluation Board Integrated Anti-Ringing Switch Evaluation Board Description The MIC2875/6 includes an anti-ringing switch that eliminates the ringing on the SW node of a conventional boost converter operating in the discontinuous conduction mode (DCM). At the end of a switching cycle during DCM operation, both the NMOS and PMOS are turned off. The anti-ringing switch in the MIC2875/6 clamps the SW pin voltage to IN to dissipate the remaining energy stored in the inductor and the parasitic elements of the power switches. Automatic Bypass Mode (when V > V ) IN OUT The MIC2875/6 automatically operates in bypass mode when the input voltage is higher than the target output voltage. In bypass mode, the NMOS is turned off while the PMOS is fully turned on to provide a very low impedance path from IN to OUT. Soft-Start The MIC2875/6 integrates an internal soft-start circuit to Figure 1. MIC2875-AYMT Evaluation Board limit the inrush current during start-up. When the device is enabled, the PMOS is turned on slowly to charge the Bi-Directional Output Disconnect output capacitor to a voltage close to the input voltage. The power stage of the MIC2875/6 consists of a NMOS Then, the device begins boost switching cycles to transistor as the main switch and a PMOS transistor as the gradually charge up the output voltage to the target VOUT. synchronous rectifier. A control circuit turns off the back Output Voltage Programming gate diode of the PMOS to isolate the output from the input The MIC2875/6 has an adjustable version that allows the supply when the chip is disabled (V = 0V). An always EN output voltage to be set by an external resistor divider R2 on maximum supply selector switches the cathode of the and R3. The typical feedback voltage is 900mV and the backgate diode to either the IN or the OUT, whichever pin recommended maximum and minimum output voltage is has a higher voltage. As a result, the output of the 5.5V and 3.2V respectively. The current through the MIC2875/6 is bi-directionally isolated form the input as resistor divider should be significantly larger than the long as the device is disabled. The maximum supply current into the FB pin (0.01A typical). It is recommended selector and hence the output disconnect function requires that the total resistance of R2 + R3 should be only 0.3V at the IN pin to operate. approximately 1M. The appropriate R2 and R3 values for Minimum Switching Frequency (MIC2875 Only) the desired output voltage are calculated by: When the MIC2875 enters the pulse-skipping mode for more than 20s, an internal control circuitry forces the PMOS to turn on briefly to discharge VOUT to VIN through V OUT R2 = R3 1 the inductor. When the inductor current reaches a Eq. 1 0.9V predetermined threshold, the PMOS is turned off and the NMOS is turned on so that the inductor current can decrease gradually. Once the inductor current reaches zero, the NMOS is eventually turned off. The above cycle repeats if there is no switching activity for another 20s, effectively maintaining a minimum switching frequency of 45kHz. The frequency control circuit is disabled when VOUT is less than or within 200mV of VIN. This minimum switching frequency feature is advantageous for applications that are sensitive to low frequency EMI such as audio systems. October 21, 2014 2 Revision 1.0