CM1581
5A LOW DROPOUT VOLTAGE REGULATOR
2002/02/05
Preliminary Rev. 1
Champion Microelectronic Corporation
Page 6
APPLICATION INFORMATION
General
The CM1581 series is designed to power the new generation of microprocessors. It is designed to make use of multiple power
supplies, present in most systems, to reduce the dropout voltage. One of the advantages of the two-supply approach is
maximizing the efficiency.
The second supply is at least 1V greater than output voltage and is providing the power for the control circuitry and supplies the
drive current to the NPN output transistor. This allows the NPN output transistor to be driven into saturation. For the control
voltage the current requirement is small, about 1% of the output current or approximately 50mA for a 5A load. This drive current
becomes part of the output current. The maximum voltage on the Control pin is 13V. The maximum voltage at the Power pin is 7V.
By trying the control and power inputs together the CM1581 can be operated as a single supply device. In single supply operation,
the dropout voltage will be determined by the minimum the Control voltage.
The new generation of microprocessors cycle load current from several hundred milliampers to several amperes in tens of
nanoseconds. Output voltage tolerances are tighter and include transient response as part of the specification. Designed to meet
the fast current load step requirements of these microprocessors, the CM1581 also saves total cost by needing less output
capacitance to maintain regulation.
The sense pin permits very accurate regulation of output voltage. As a result, over an output current range of 100mA to 5A, the
typical load regulation is less than 1mV.
In addition to the enhancements mentioned, the reference accuracy has been improved a factor of two with a guaranteed initial
tolerance of ±1% at 25℃ and 1.6% accuracy over the full temperature and load current load.
Typical applications for the CM1581 include 3.3V to 2.5V conversion with a 5V control supply, 5V to 4.2V conversion with a 12V
control supply or 5V to 3.6V conversion with a 12V control supply. It is easy to obtain dropout voltages of less than 0.5V at 2.5A
along with excellent static and dynamic specifications. The device is fully protected against over current and over temperature
conditions.
Grounding and Output Sensing
The CM1581 adjustable voltage version allows true Kelvin sensing for both the high and low side of the load. As a result the
voltage regulation at the load can be easily optimized. Voltage drops due to parasitic resistances between the regulator and the
load can be placed inside the regulation loop. The advantages of remote sensing are illustrated in figures 1 through 3.
Figure 1 shows the device connected as a conventional 3 terminal regulator with the Sense lead connected directly to the output
of the device. Rp is the parasitic resistance of the connections between the device and the load. Trace A of figure 3 illustrates the
effect of Rp.
Figure 2 shows the device connected to take advantage of the remote sense feature. The Sense pin and the top of the resistor
divider are connected to the top of the load; the bottom of the resistor divider is connected to the bottom of the load. The effect on
the output regulation can be seen in trace B of figure 3.
It is important to note that the voltage drops due to Rp are not eliminated; they will add to the dropout voltage of the regulator
regardless. The CM1581 adjustable can control the voltage at the load as long as the input-output voltage is greater than the total
of the dropout voltage of the device plus the voltage drop across Rp.
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