Voltage dependent output driver adjustment
Original Publication Date: 2003-Oct-25
Included in the Prior Art Database: 2003-Oct-25
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For DDR DRAMs (Double Data Rate Dynamic Random Access Memory), the static output driver pull-up and pull-down I-V curves (I-current, V-Voltage) are specified for a voltage range and temperature range. Sensing the applied voltage, the strength of the output driver can be changed. This can be accomplished by changing the overdrive of the activated transistor, by changing the number of active output drive transistors or by changing a series resistance. Up to now, the problem was that the output drive current was depended of the voltage. The here presented idea solves this problem by sensing the external voltage, by comparing it to a internal reference voltage and by changing the driver strength. Figure 1 shows a possible circuit for voltage sensing that gives a signal to activate a strong or weak driver. In this way, more than two output settings can be realized, e.g. very weak, weak, strong, very strong. This requires more than a voltage comparator. Like already described, the changing of output range can be realized in different ways: By changing the overdrive of output transistor, by changing the numbers of the activated output drivers or by changing the output series resistor. The activation should have a hysteresis thus that a small voltage noise and a fluctuation don't cause output ringing or switching the output drivers on and off at high frequency.