The MAX22088 can be used in the Home Bus System (HBS), where power and data are carried on a single pair of wires. The MAX22088 operates with data rates up to 200kbps for bus-powered applications. For applications where power is consumed, the MAX22088 features an integrated Active Inductor to eliminate the use of external AC-blocking inductor. In typical applications where external power is sourced, the MAX22288 is recommended. See the MAX22288 data sheet for more information.
The MAX22088 can drive external system loads at 5V up to 70mA using the integrated voltage regulator. The MAX22088 also features dynamic cable termination, configurable receiver hysteresis and thresholds, and transmit driver slew rate adjustment for better signal quality and flexible design.
Power Supply
For applications where power is sourced, the MAX22088 is powered by the system voltage supplied at VRAW. In this configuration, an external AC-blocking inductor is required to superimpose the data on the Home Bus cable. Bypass the integrated Active Inductor by connecting CAPN to HNA, HN to GND, CAPP to HPA, and HP to VRAW.
For applications where power is consumed, the MAX22088 is powered by the voltage carried on the Home Bus cable. In this configuration, the MAX22088 eliminates the need for an external AC-blocking inductor and uses the integrated Active Inductor to separate data from power. Power is passed from the Home Bus cable to VRAW to supply the MAX22088 and drive external system loads.
VRAW Voltage Output
VRAW is the output of the integrated Active Inductor. The integrated Active Inductor can source up to 200mA (max) to VRAW to power the external loads, minus internal load current. Bypass VRAW to GND with a 100μF (min) capacitor to drive loads less than 70mA (max). Bypass VRAW to GND with a 200μF (min) capacitor to drive loads more than 70mA (max). To drive larger loads, or for a regulated output, connect a DC-DC converter, or an LDO, to VRAW. See the Typical Application Circuits section for more information.
Internal Voltage Regulator
The MAX22088 features an internal 5V linear regulator, powered by VRAW, capable of driving external loads up to 70mA (max), minus internal load current. Connect EN to VRAW to enable the 5V output at VCC. Connect EN to GND to disable the linear regulator. Do not use VCC to power external loads if an external LDO or DC-DC converter is connected at VRAW. See the Typical Application Circuits section for more information.
Active Inductor
The MAX22088 features an integrated Active Inductor to eliminate the need for an external AC-blocking inductor in applications where power is consumed. The differential inputs, HP and HN, to the integrated Active Inductor maintain a balanced termination for the Home Bus cable. The equivalent value of the integrated Active Inductor (LACT) is set by the values of the two capacitors (CACT) connected between CAPP and HPA, and between CAPN and HNA. Use the following approximate formula to calculate the typical value of the integrated Active Inductor:
where LACT is in Henry, CACT is in Farad, ILOAD is in Ampere (Figure 4).
For 57.6kbps operation, the recommended value for CACT is 100nF. Ensure that the CACT is in the range from 100nF to 500nF.
In parallel to CACT, connect a 180kΩ resistor and a 1μF ceramic capacitor in series between CAPP and HPA to provide dampening for the LC network formed by the integrated Active Indutor and the bypass capacitor on VRAW. This LC network also limits the inrush current that charges the bypass capacitor on VRAW.
The transient load current, in series with the output inductance, can cause fluctuations on output voltage at VRAW. In a Home Bus system, the total inductance is the serial combination of the passive inductor on the power-sourcing side (host) and the integrated Active Inductor on the power-consuming side (device). Limit the load current and carefully select the CACT value to avoid excessive voltage fluctuations at VRAW, as shown in Table 1.
Table 1. Capacitor Value and Maximum Transient Load Current
CACT (nF)
MAXIMUM TRANSIENT ILOAD (mA)
100
200
200
160
300
120
400
100
500
90
Figure 4. Integrated Active Inductor Circuit
Operation of MAX22088 Transceiver
The MAX22088 uses three pins for the logic interface: RST, DIN, and DOUT. AIO, BIO, and TERM are connected to the Home Bus network. RST is the bus reset control input. Drive RST low to enable the transmitter on AIO and BIO. Drive RST high to disable the transmitter. The MAX22088 Home Bus receiver is always enabled.
DIN is the logic input of the MAX22088. DOUT is the logic output. When DIN goes from high to low, the polarities of AIO and BIO invert. When DIN goes from low to high, AIO and BIO are set to high-impedance (Figure 5).
DOUT asserts low when the leading edge of VAIO - VBIO crosses VTVLor -VTVL. DOUT is high-impedance when the trailing edge of VAIO - VBIO crosses VTVT or -VTVT. See the Receiver Threshold Adjustment section for more information.
To improve signal quality, the MAX22088 features an internal switch that connects TERM to AIO for 34μs (typ) after the driver transitions to high-impedance. See the Dynamic Cable Termination section for more information. For a typical data rate of 57.6kbps, the recommended value for coupling capacitors is 2.2μF (min).
Figure 5. Operation of MAX22088 Transceiver
Dynamic Cable Termination
The MAX22088 supports Home Bus signals at data rates up to 200kbps. When operating at high data rates, the mismatch between the Home Bus cable impedance and cable termination resistor can negatively affect the signal quality. The MAX22088 features dynamic cable termination to improve the signal quality with longer cables. When the driver transitions to high-impedance, an internal switch connects AIO to TERM. The external termination resistor at TERM is then connected between AIO and BIO in parallel with the static cable termination resistor. The internal switch opens after 34μs (typ), when DOUT asserts low, or when RST is driven high. The optimized value of the dynamic termination resistor depends on the application. For typical applications, the value of the dynamic termination resistor is between 50Ω and 240Ω.
Transmit Slew Rate Adjustment
Connect an external resistor, RSRA, from SRA to GND to control the slew rate of the transmit signals at AIO and BIO. The transmit rise/fall time (tRLD, tFLD, tRTR, tFTR) is proportional to RSRA and is calculated using the following equation:
For most applications, it is recommended to use RSRA = 62kΩ resulting in 1μs (typ) output rise/fall time. Ensure that RSRA is in the range from 33kΩ to 470kΩ.
Receiver Threshold Adjustment
The threshold levels for the receiving signals are set by the voltages at TVL and TVT. The voltage at TVL sets the threshold for the leading edge of the pulse on the Home Bus signal (VAIO - VBIO). The voltage at TVT sets the threshold for the trailing edge of the pulse. Ensure that VTVL > VTVT.
DOUT asserts low when VAIO - VBIO crosses VTVL or -VTVL. DOUT is high-impedance when VAIO - VBIO crosses VTVT or -VTVT (Figure 6). Connect a pullup resistor from DOUT to a logic voltage supply. Ensure the pullup resistor is in the range from 1kΩ to 100kΩ.
Figure 6. Receiver Thresholds
RST (Reset) Functionality
The MAX22088 features a bus reset control input. Drive RST low to enable the transmitter. Drive RST high to disable the transmitter. RST also controls the internal switch used for dynamic cable termination. Ensure that RST remains low for at least 34μs (typ) after the internal switch is closed when the driver transitions to high-impedance. The internal switch opens when RST is driven high.
High-Pass Filter
The MAX22088 features an internal high pass filter on the receiver to filter out the low frequency voltage fluctuations at AIO and BIO. Connect HPEN to GND to enable the internal high-pass filter on the receiver input. Connect HPEN to VCC to disable the internal high pass filter. Ensure that HPEN is always connected.
