US 6,664,821 B2Granted

Line driver with current source output and low sensitivity to load variations

AssigneeKoninklijke Philips Electronics N.V.

InventorsClemens Gerhardus Johannes De Haas, Hendrik Boezen, Aloysius Johannes Maria Boomkamp, Gerrit Jan Bollen, Abraham Klaas Van Den Heuvel, Ruurd Anne Visser

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Abstract

Line driver for a LIN-bus. The line driver has a current source output transistor (T1) for pulling down the LIN-bus wire (LB) to ground (GND). The LIN-bus wire (LB) is connected to a positive supply voltage (VBAT) through a pull-up resistor (R1). The output transistor (T1) provides a ramp shaped output current under control of a ramp generator (RG). The ramp shaped output current in combination with the pull-up resistor (R1) asserts a ramp shaped output voltage on the LIN-bus wire (LB). According to the LIN-bus specifications the resistance of the pull-up resistor may vary between 500 Ohm and 1 kOhm. When the resistance is 1 kOhm, the output voltage is clamped to signal ground (GND) and a delay occurs between the edge in the data signal (TXD) and the corresponding rising ramp in the output voltage. This delay is reduced considerably by a comparator (CMP, T2) which monitors the output voltage and which increases the slew rate of the ramp generator (RG) when the output voltage is clamped and drops below a certain threshold.

Claims6

Dependent claims

A line driver comprising:

a driver output terminal (LB) for connection to a line to be driven;
a reference terminal (GND) for connection to a reference voltage;
a ramp generator (RG) for providing a substantially linearly inclining and declining ramp signal (RS) in response to a data signal (TXD), the ramp generator (RG) having a control input (CI) for connecting a control signal, the ramp signal (RS) having a slew rate substantially proportional to the control signal;
a current source output transistor (T1) having its first main electrode coupled to the reference terminal (GND) and its second main electrode coupled to the driver output terminal (LB) for providing between the driver output terminal (LB) and the reference terminal (GND) a current which is proportional to the ramp signal (RS);
a comparator (CMP) for comparing a voltage drop across the current source output transistor (T1) to a threshold voltage;
and
the comparator (CMP) having an output (T2) coupled to the control input (CI) of the ramp generator (RG) and being operable to increase the slew rate of the ramp signal when the voltage drop is smaller than the threshold voltage.

4 more elements

Dependencies are inferred from the claim text.