L9637

Functional description

Doc ID 1765 Rev 8

9/15

3

Functional description

The L9637 is a monolithic bus driver designed to provide bidirectional serial communication

in automotive diagnostic applications according to the specification "Diagnostic Systems

ISO9141".

The device provides a bidirectional link, called K, to the VBat related diagnosis bus. It also

includes a separate comparator L which is also able to be linked to the VBat bus. The input

TX and output RX of K are related to VCC with her integrated pull up resistances. Also the L

comparator output LO has a pull up resistance connected to VCC.

The maximum external pull up resistance at K related to VS should not be higher than RKO 

5 k

to achieve clear output ON conditions.

All VBat bus defined inputs LI and K have supply voltage dependent thresholds together with

sufficient hysteresis to suppress line spikes. These pins are protected against overvoltages,

shorts to GND and VS and can also be driven beyond VS and GND.

These features are also given for TX, RX and LI only taking into account the behavior of the

internal pull up resistances. The thermal shut down function switches OFF the K output if

the chip temperature increases above the thermal shut down threshold. To reactivate K

again the temperature must decrease below the K switch ON temp. To achieve no fault for

VS undervoltage conditions the outputs will be switched OFF and stay at high impedance.

The device is also protected against reverse battery condition. During lack of VS or GND all

pins shows high impedance characteristic. To realize a lack of the VS related bus line LI and

K the outputs LO and RX shows defined ON status.

Suppressing all 4 classes of "Schaffner" signals all pins can be load with short energy

pulses of max. ±0.2 mJ. All these features together with a high possible baud rate

>50Kbaud, controlled output slopes for low EMI, a wide power supply voltage range and a

very small quiescent current during OFF (TX LI K=High) condition ISoff typ  120 µA, and a

real standby function with zero power consumption ISSB typ  1 µA during system de

powering VCC  0.5 V make this device high efficient for automotive bus system.

After wake up of the system from OFF or SB condition the first output signal will have an

additional delay time tdtyp  5 µs see also Figure 5.

The typical output voltage behavior for the K, LO, RX outputs as a function of the output

current is shown in Figure 6. Figure 7 shows a waveform of the output signal when the low

level changes from RON * IOUT to IOUT * 2 * RON + UBE state. This variation occurs due to too

low output current or after a negative transient forced to the output or to the supply voltage

line.