APPLICATION NOTES

PARALLELING DEVICES

When currents greater than 0.9A are needed, the MSK5979RH's may

be paralleled to multiply the current capacity. As shown in Figure 4, the Vin

and SET pins must be tied together. The VOUT pins are connected to the

load with consideration to the conductor resistance. The conductor resis-

tance of each MSK5979RH VOUT connection to the load, must be equal

to create equal load sharing. As little as 10mΩ ballast resistance typically

ensures better than 80% equal sharing of load current at full load. Additional

consideration must be given to the effect the additional VOUT conductor

resistance has on load regulation; see paragraph titled "Load Regulation".

OUTPUT CAPACITANCE

For stability purposes, the MSK5979RH requires a minimum output ca-

pacitor of 10µF with an ESR of 0.5Ω or less. Tantalum or ceramic capac-

itors 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.

FIGURE 1

OUTPUT VOLTAGE

3

FIGURE 3

FIGURE 4

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

voltage for the internal Error Amplifier. The MSK5979RH SET pin supplies

a constant current of 10uA that develops the reference voltage. The output

voltage is simply Rset x 10uA. 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.

ADDITIONAL STABILITY

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

prove the output transient response and filter noise in the system. To reduce

output noise, typically less than 100pF is all that will be 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.

LOAD REGULATION

The MSK5979RH specified load regulation is Kelvin Sensed, therefore

the parasitic resistance of the system must be considered to design an ac-

ceptable load regulation. The overall load regulation includes the specified

MSK5979RH load regulation plus the parasitic resistance multiplied by

the load current as shown in Figure 3. RSO is the series resistance of all

conductors between the MSK5979RH output and the load. It will directly

increase output load regulation error by a voltage drop of ∆Io x Rso. Rss

is the series resistance between the set pin and the load. RSS will have

little effect on load regulation if the set pin trace is connected as close to

the load as possible keeping the load return current on a separate trace as

shown. RSR is the series resistance of all of the conductors between the

load and the input power source return. RSR will not effect load regulation

if the set pin is connected with a Kelvin Sense type connection as shown

in Figure 3, but it will increase the effective dropout voltage by a factor of

IO x RSR. Keeping RSO and RSR as low as possible will ensure minimal

voltage drops and wasted power.

 

FIGURE 2

LOW DROPOUT OPERATION

Using separate VIN and Vcontrol power supplies allows for lower dropout

and improved efficiency. Figure 2 shows the MSK5979RH output transis-

tor collector is connected to the VIN pin. The regulator control circuitry is

powered by the Vcontrol input. The dropout of the regulator is determined

by the saturation voltage of the output transistor, typical 300mV at 0.9A

load. The Vcontrol supply must supply the base drive current for the output

transistor. The Vcontrol current minus the 10µA 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 Vcontrol,

power dissipation is reduced and efficiency improves.

8548-133 Rev. B 8/15