Click here to ask about the production status of specific part numbers. Evaluates: MAX25256 MAX25256 Evaluation Kit General Description Quick Start The MAX25256 evaluation kit (EV kit) is a fully assembled Required Equipment and tested PCB that contains the MAX25256 10W iso- MAX25256 EV kit lated H-bridge DC-DC converter. The EV kit operates +24V, 1A DC power supply from an 8V to 36V DC power source and the on-board 1:1 turns-ratio transformer from HALO sets the output voltage Electronic load capable of 150mA range from 6.8V to 34.8V with a 215mA current limit. Ammeter The EV kit provides greater than 90% overall effi- Voltmeter ciency at +24V between 2.2W and up to 8.3W output power using an H-bridge DC-DC converter topology. Procedure Input-ripple current and radiated noise are minimized The EV kit is fully assembled and tested. Follow the steps by the inherent balanced nature of the design with below to verify board operation. no interruption in the input current. Undervoltage Caution: Do not turn on the power until all connec- lockout (UVLO), adjustable current limit, and thermal tions are completed. shutdown provide for a robust 10W isolated supply. The 1) Verify that jumpers JU2, and JU3 are in their default surface-mount transformer provides galvanic isolation positions, as shown in Table 1 and Table 2. with the output powered from a full-wave rectifier circuit, reducing the output-voltage ripple. 2) Set the DC power supply to 24V. The EV kit circuit is configured as a full-wave rectifier, 3) Set the electronic load to 150mA and disable with an output voltage that follows the input voltage but the output. is configurable for other topologies including a voltage 4) Connect the voltmeter between the +VOUT and doubler, bipolar outputs, half-wave rectification, and a SGND PCB pads on the EV kit. push-pull rectifier. 5) Connect the ammeter between the +VOUT PCB The device is available in a 10-pin (3mm x 3mm) TDFN pad on the EV kit and the positive terminal on the package with an exposed pad. electronic load. The negative terminal on the electronic load is connected to the SGND PCB pad Features on the EV kit. 8V to 36V Input Supply Range 6) Connect the power supply between the VDD and Up to 90% Efficiency GND PCB pads on the EV kit. Full-Wave Rectified Output 7) Turn on the power supply. Configurable for a Voltage Doubler, Bipolar 8) Enable the electronic load. Half-Wave Rectifier, and Push-Pull Rectifier Outputs 9) Verify that the ammeter reads approximately 150mA. Internal or External Clock Operation Option 10) Verify that the voltmeter reads approximately 22.8V. Proven PCB Layout Fully Assembled and Tested Ordering Information appears at end of data sheet. 319-100612 Rev 4 6/21Evaluates: MAX25256 MAX25256 Evaluation Kit Clock Source Detailed Description The device has two modes of operation: internal The MAX25256 EV kit is a 10W, isolated H-bridge oscillator or external clock. To use the internal 425kHz DC-DC converter that provides an unregulated output oscillator, place a shunt in the 1-2 position on jumper that is two diode-voltage drops less than its input supply, JU2. When using an external clock, remove the shunt with respect to the isolated ground. In the default from JU2 and apply a clock signal on the CLK PCB configuration, the maximum load is limited by the device pad on the EV kit. An internal flip-flop divides the and the on-board transformer. external clock by two, generating a switching signal with a The device is an integrated primary-side controller guaranteed 50% duty cycle. As a result, the device and H-bridge driver for isolated power-supply circuits. outputs switch at 1/2 the external clock frequency. The device contains an on-board oscillator, protec tion circuitry, and internal MOSFETs to provide up to 215mA Overcurrent Limiting of current to the transformers primary winding. The Resistor R2 sets the current-limit threshold to 650mA. To device can be operated using the internal 425kHz oscil- change the current-limit threshold, replace resistor R2 lator, or driven by an external clock to synchronize with a 0603 surface-mount resistor using the following multiple devices and control EMI behavior. Regardless equation: of the clock source being used, an internal flip-flop stage guarantees a fixed 50% duty cycle, preventing DC R2 k = 650/I (mA) ( ) LIM current flow in the transformer as long as the clocks period is constant. where I is the desired current threshold in the range of LIM 215mA < I < 650mA. LIM The device operates from a single-supply voltage and includes UVLO and an active-low enable input for An overcurrent or overtemperature condition triggers a controlled startup. If the input voltage at VDD falls below fault on the device and the red LED (D5) turns on. 6.3V, or the EN input is pulled above 2V, the device shuts Evaluating Other down and ST1 and ST2 are high impedance. Transformer Configurations The device features an adjustable output current limit The EV kit PCB layout provides an easy method to at the transformer driver outputs (ST1 and ST2). When reconfigure transformer T1 secondary windings for other the current reaches the limit for longer than the 1.2ms configurations, including a half-wave rectifier, voltage blanking time, the drive outputs are disabled and the doubler, bipolar outputs, and other full-wave-rectifier FAULT output asserts. The drivers are reenabled configurations. Use Table 3 to reconfigure the device for after the 38.2ms autoretry time. If a continuous fault the appropriate output configuration. condition is present, the duty cycle of the fault current is approximately 3%. Output Snubbers The bottom PCB GND plane under device U1 is utilized For VDD greater than 27V, use a simple RC snubber as a thermal heatsink for power dissipation of the devices circuit on ST1 and ST2 to ensure that the peak voltage thermally enhanced TDFN package with exposed pad. is less than 40V during switching. Maxim recommends Test points GND and SGND are provided on the PCB for installing 91 0603 surface-mount resistors at R5 and R6, probing the respective ground planes, or to connect the and 330pF 0603 surface-mount capacitors at C2 and C3 GND and SGND planes for nonisolated evaluation of the when operating under these conditions. circuit. Maxim Integrated 2 www.maximintegrated.com