Datasheet
SLUS165C – MARCH 1999 - REVISED JANUARY 2002
5
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pin assignments
(–) SENSE: (Pin 4) This is a high-impedance pin allowing remote sensing of the system ground, bypassing any
voltage drops which might appear in the power return line. This point should be considered as the true ground. Unless
otherwise stated, all voltages are with respect to this point.
Artificial Ground: (Pin 6) This is a low-impedance-circuit ground which is exactly 250 mV above the (–) SENSE
terminal. This offset allows the ground buffer amplifier negative headroom to return all the control bias and operating
currents while maintaining a high impedance at the (–) SENSE input.
Power RTN: (Pin 5) This should be the most negative voltage available and can range from zero to 5 V below the
(–) SENSE terminal. It should be connected as close to the power source as possible so that voltage drops across
the return line and current-sensing impedances lie between this terminal and the (–) SENSE point.
VREF: (Pin 7) The internal voltage reference is a band-gap circuit set at 2.0 V with respect to the (–) SENSE input
(1.75 V above the artificial ground), and an accuracy of ±1.5%. This circuit, as well as all the other chip functions, will
work over a supply voltage range of 4.5 V to 35 V allowing operation from unregulated dc, an auxiliary voltage, or
the same output voltage that it is controlling. Under-voltage lockout has been included to insure proper startup by
disabling internal bias currents until the reference rises into regulation.
Voltage Amplifier: (Pins 11, 12) This circuit is the feedback-control-gain stage for the power module’s output-voltage
regulation, and overall-loop compensation will normally be applied around this amplifier. Its output will swing from
slightly above the ground return to an internal clamp of 2.0 V. The reference trimming is performed closed loop, and
measured at pin 11, (+) SENSE. The value is trimmed to 2 V ±1.25%.
Drive Amplifier: (Pins 8, 9, 12) This amplifier is used as an inverting buffer between the voltage amplifier’s output
and the medium used to couple the feedback signal to the power controller. It has a fixed-voltage gain of 2.5 and is
usually configured with a current-setting resistor to ground. This establishes a current-sinking output optimized to
drive optical couplers biased at any voltage from 4.5 V to 35 V, with current levels up to 20 mA. The polarity of this
stage is such that an increasing voltage at the voltage amplifier’s sense input (as, for example, at turnon) will increase
the opto’s current. In a nonisolated application, a voltage signal ranging from 0.25 V to 4.1 V may be taken from the
current-setting output but it should be noted that this voltage will also increase with increasing sense voltage and an
external inverter may be required to obtain the correct feedback polarity.
Current Amplifier: (Pins 1, 2, 3) This amplifier has differential-sensing capability for use with an external shunt in
the power-return line. The common mode range of its input will accommodate the full range between the power return
point and VCC-2 V which will allow undefined-line impedances on either side of the current shunt. The gain is
internally set at 20, giving the user the ability to establish the maximum-voltage drop across the current-sense resistor
at any value between 50 mV and 500 mV. While the bandwidth of this amplifier may be reduced with the addition of
an external-output capacitor to ground, in most cases this is not required as the compensation of the adjust amplifier
will typically form the dominant pole in the adjust loop.
Buffer Amplifier: (Pins 1, 15) This amplifier is a unidirectional buffer which drives the current-share bus. The line
which will interconnect all power modules paralleled for current sharing. Since the buffer amplifier will only source
current, it insures that the module with the highest-output current will be the master and drive the bus with a
low-impedance drive capability. All other buffer amplifiers will be inactive with each exhibiting a 10-kΩ load impedance
to ground. The share bus terminal is protected against both shorts to ground and accidental voltages in excess of
50 V.