LOW DROPOUT OPERATION

Using separate VIN and CONTROL power supplies allows for lower

dropout and improved efficiency. Figure 2 shows the MSK5985RH

output transistor collector is connected to the VIN pin. The regulator

control circuitry is powered by the CONTROL input. The dropout of

the regulator is determined by the saturation voltage of the output

transistor, typical 300mV at 1.9A ILOAD. The CONTROL supply must

supply the base drive current for the output transistor. The CONTROL

current minus the 50µA SET current is supplied to the load. See the

Typical Performance Characteristics curves for expected VIN dropout

voltage, CONTROL pin dropout voltage and current requirements

under various conditions. With separate supplies for VIN and

CONTROL, power dissipation is reduced and efficiency improves.

FIGURE 2

ADDITIONAL STABILITY

A capacitor placed in parallel with the SET pin resistor to ground,

will improve the output transient response and filter noise in the

system. To reduce output noise, typically 500-1000pF is required.

Capacitors up to 1µF can be used, however consideration must be

given to the effect the time constant created will have on the startup

time.

APPLICATION NOTES

OUTPUT VOLTAGE

A single resistor (RSET) from the SET pin to ground creates the

reference voltage for the internal Error Amplifier. The MSK5985RH

SET pin supplies a constant current of 50uA that develops the

reference voltage. The output voltage is simply RSET x 50uA. Since

the output is internally driven by a unity-gain amplifier, an alternative

to using RSET is to connect a high quality reference source to the

SET pin. With a minimum load requirement of 1mA on the Output, the

Output Voltage can be adjusted to near 0V. To bring the output voltage

to 0V, the load must be connected to a slightly negative voltage supply

to sink the 1mA minimum load current from a 0V output.

FIGURE 1

INPUT CAPACITANCE

Pins 3-8 are the connection to the collector of the power device

of the MSK5985RH. Output load current is supplied through these

pins. Minimum input capacitance for these devices is 10uF. Low ESR,

ceramic input capacitors are acceptable for applications without long

input leads. For applications with long input leads, the self inductance

of the wires can cause instability. Care must be taken to minimize the

inductance of the input wires. This can be accomplished through the

use of series resistance or higher ESR input capacitors. A minimum

of 10uF of low ESR tantalum bulk capacitance in parallel with low

value ceramic decoupling capacitance is recommended.

CONTROL PIN

The control pin is the bias supply for the control circuitry of the

MSK5985RH. Minimum input capacitance on the control pin is 2.2uF.

Approximately 1.7% of the output current flows into this pin. For

proper regulation, the control pin voltage must be 1.6V greater than

the output voltage. (See Control Dropout Voltage Specification).

OUTPUT CAPACITANCE

For stability purposes, the MSK5985RH requires a minimum output

capacitor of 10µF with an ESR of 0.5Ω or less. Tantalum or ceramic

capacitors are recommended. A larger capacitance value will improve

transient response for increased load current changes. Consideration

must also be given to temperature characteristics of the capacitors

used.

3

8548-143 Rev. E 12/17