MAX1856 is a low-cost SLIC (ringer and off-hook) power supply solution that uses standard off-the-shelf transformers from multiple vendors. It generates various output voltages: -24V and -72V (dual output) for both ringer and off-hook supplies for voice-enabled broadband consumer premises equipment (CPE), -48V for IP phones and routers, -5V and -15V (single or dual output) for DSL CO line drivers, or negative voltages as high as -185V for MEMS bias supplies. The output voltages are adjusted with an external voltage divider. Due to its wide operating voltage range, the MAX1856 operates from a low-cost, unregulated DC power supply for cost-sensitive applications like xDSL, cable modems, set-top boxes, LMDS, MMDS, WLL, and FTTH CPE. The MAX1856 provides low audio-band noise for talk battery and a sturdy output capable of handling the ring trip conditions for ring battery. The operating frequency can be set between 100kHz and 500kHz with an external resistor in free-running mode. For noise-sensitive applications, the MAX1856's operating frequency can be synchronized to an external clock over its operating frequency range. The flyback topology allows operation close to 50% duty cycle, offering high transformer utilization, low ripple current, and less stress on input and output capacitors. Internal soft-start minimizes startup stress on the input capacitor, without any external components. The MAX1856's current-mode control circuit provides excellent output voltage accuracy, resulting in lower system noise.
The MAX1856 is a current‑mode PWM controller designed for SLIC (ringer and off‑hook) power supplies. It generates multiple negative output voltages (e.g., -24 V, -48 V, -72 V, up to -185 V) from a 3 V‑28 V input using off‑the‑shelf transformers. The device offers low audio‑band noise, internal soft‑start, and a wide operating frequency range of 100 kHz to 500 kHz, which can be synchronized to an external clock.
Key features include a built‑in 5 V low‑dropout regulator, programmable current‑limit, shutdown control via the SYNC/SHDN pin, and a 10‑pin µMAX package. The datasheet provides absolute maximum ratings, electrical characteristics, pin descriptions, and typical operating curves for design reference.
| Input Voltage Range | 3 V to 28 V |
|---|---|
| Switching Frequency Range | 100 kHz to 500 kHz |
| Reference Output Voltage (VREF) | 1.25 V |
The MAX1856 operates from a low‑cost, unregulated DC supply ranging from 3 V to 28 V.
The switching frequency is set by an external resistor on the FREQ pin, allowing selection between 100 kHz and 500 kHz, or it can be synchronized to an external clock via the SYNC/SHDN pin.
Shutdown is controlled by the SYNC/SHDN pin; pulling it low puts the device into shutdown mode.