Datasheet MAX253 (Maxim) - 6

MAX253 Transformer Driver for Isolated RS-485 Interface VIN 5V C1 0.1μF 6 R1 V 50Ω CC 4 1 SD D1 ON / OFF MAX253 R2 50Ω 3 8 FS D2 FREQUENCY SWITCH GND1 GND2 2 7 Figure 1. Test Circuit VIN 5V C1 5V @ 200mA V ISO OUTPUT CC D1 F / F MAX253 Q C3 C2 N FS T OSC D2 FREQUENCY ISO SWITCH Q N 400kHz/ GND 700kHz SD GND2 GND1 ON / OFF Figure 2. Block Diagram
Detailed Description
quency (Figure 2). These two signals drive the ground- referenced output switches. Internal delays ensure The MAX253 is an isolated power-supply transformer break-before-make action between the two switches. driver specifically designed to form the heart of a fully isolated RS-485 data interface. Completely isolated Ground SD for normal operation. When high, SD dis- communications are obtained by combining the ables all internal circuitry, including the oscillator and MAX253 with a linear regulator, a center-tapped trans- both power switches. former, optocouplers, and the appropriate Maxim inter- Pulling FS low reduces the oscillator frequency and low- face product (as described in the Isolated ers the supply current (see Supply Current vs. RS-485/RS-232 Data Interface section). Temperature in the Typical Operating Characteristics). The MAX253 consists of an RC oscillator followed by a FS includes a weak pull-up, so it will be set to the high- toggle flip-flop, which generates two 50% duty-cycle frequency state if not connected. square waves, out-of-phase at half the oscillator fre- www.maximintegrated.com Maxim Integrated | 6