MIC28500 4A Evaluation Board 75V/4A Hyper Speed Control Synchronous DC/DC Buck Regulator meter and V at Vin and GND terminals with IN General Description voltmeter. Do not apply power until Step 5. The MIC28500 DC/DC regulator operates over an input 2. Connect Load and Monitor Output supply range of 30V to 75V and provides a regulated Connect a load to the Vout and GND terminals. The output at up to 4A of output current. The output voltage is load can be either a passive (resistive) or an active adjustable down to 0.8V with a typical accuracy of 1% . (as in an electronic load) type. A current meter may be The switching frequency is adjustable and once adjusted placed between the Vout terminal and load to monitor the switching frequency remains fairly constant with the output current. Ensure the output voltage is changes in input voltage and output load. monitored at the Vout terminal. 3. V 30V Limiter Circuit IN The basic parameters of the evaluation board are: To prevent overstressing of the internal top FET of the 1. Input 30V to 75V MIC28500, the minimum input voltage at full load (1) (2) 2. Output 0.8V to 5V at 4A , 12V at 2A should be limited to 30V. This is implemented by using external VIN limiter circuit installed on either EN 3. 250kHz Switching Frequency (Adjustable 100kHz pin (dash-line block 1) or VDD Pin (dash-line block 2) to 500kHz) as shown in the Evaluation board schematic. On the Notes: MIC28500 evaluation board the limiter circuit on EN 1,2. Refer to thermal de-rating curves shown in Typical Pin is installed, which shuts off the MIC28500 until the Characteristics section. input supply voltage reaches 30V. 4. Enable Input Datasheets and support documentation can be found The EN pin has an internal 100k pull-up resistor to on Micrels web site at: www.micrel.com. VDD, which allows the output to be turned on when Requirements VDD exceeds its UVLO threshold. An EN connector is The MIC28500 evaluation board requires only a single provided on the evaluation board for users to easily power supply with at least 10A current capability. A access the enable feature. Applying an external logic linear regulator, which includes a Zener and an NPN signal on the EN pin to pull it low or using a jumper to transistor, has been installed on the board to provide short the EN pin to GND will shut off the output of the housekeeping (V ) for the MIC28500. The output DD MIC28500 evaluation board. If in certain application load can either be a passive or an active load. the EN needs to be controlled externally the V limiter IN Precautions circuit should be implemented on VDD pin. 5. Turn on the Power The MIC28500 evaluation board does not have reverse polarity protection. Applying a negative urn on the VIN supply and verify that the output T voltage to the PVIN and GND terminals may damage voltage is regulated to 3.3V. the device. The maximum PVIN of the board is rated at 75V. Exceeding 75V on the PVIN could damage the device Ordering Information Getting Started Part Number Description 1. V Supply IN Connect a supply to the Vin and GND terminals, MIC28500 4A EV Evaluation Board up to 5V Output paying carefu l attention to the polarity and the supply MIC28500 2A EV Evaluation Board 12V Output < 75V). Monitor I with a current range (30V < V IN IN Hyper Speed Control is a trademark of Micrel, Inc Micrel Inc. 2180 Fortune Drive San Jose, CA 95131 USA tel +1 (408) 944-0800 fax + 1 (408) 474-1000 Micrel, Inc. MIC28500 4A Evaluation Board Features Feedback Resistors The output voltage on the MIC28500 evaluation board, which is preset to 3.3V, is determined by the feedback divider: R4 V = V 1 + OUT REF R BOTTOM where V = 0.8V, and R is one of R5, R6, R7, REF BOTTOM R8, R9, R10, R11, R12, which corresponds to 0.9V, 1.0V, 1.2V, 1.5V, 1.8V, 2.5V, 3.3V, or 5V. Leaving the R open gives a 0.8V output voltage. All other BOTTOM voltages not listed above can be set by modifying R value according to: BOTTOM R4 V REF R = BOTTOM V V OUT REF Note that the output voltage should not be set to exceed 5V due to the 6.3V voltage rating on the output capacitors. If higher than 5V output is desired, it is recommended to use the designs shown in Figure 2, where the output capacitors, L1, R3 and R17 are optimized for 12V/2A output. SW Node Test point J11 (VSW) is placed for monitoring the switching waveform, one of the most critical waveforms for the converter. Current Limit The MIC28500 has internal FETs, and the current limit is implemented by sensing the R of bottom FET. DS-ON The MIC28500 has a fixed current limit of 7A (TYP). Loop Gain Measurement The resistor, R13, is placed in series with the regulator feedback path. The control loop gain can be measured by connecting an impedance analyzer across the resistor and selecting the resistor value in between 20 to 50 . 2 M9999-071911 July 2011 hbwhelp micrel.com or (408) 955-1690