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B1585J-1.5

5.0A Low Dropout Voltage Regulator

Bay Linear 

APPLICATION HINTS

The Bay Linear B1585 incorporates protection against

over-current faults, reversed load insertion, over

temperature operation, and positive and negative

transient voltage. However, the use of an output

capacitor is required in order to insure the stability and

the performances.

Stability

The output capacitor is part of the regulator’s

frequency compensation system. Either a 220µF

aluminum electrolytic capacitor or a 47µF solid

tantalum capacitor between the output terminal and

ground guarantees stable operation for all operating

conditions.

However, in order to minimize overshoot and

undershoot, and therefore optimize the design, please

refer to the section ‘Ripple Rejection’.

Ripple Rejection

Ripple rejection can be improved by adding a

capacitor between the ADJ pin and ground. When

ADJ pin bypassing is used, the value of the output

capacitor required increases to its maximum (220µF

for an aluminum electrolytic capacitor, or 47µF for a

solid tantalum capacitor). If the ADJ pin is not

bypass, the value of the output capacitor can be

lowered to 100µF for an electrolytic aluminum

capacitor or 15µF for a solid tantalum capacitor.

However the value of the ADJ-bypass capacitor

should be chosen with respect to the following

equation:

C = 1 / ( 6.28 * FR * R1 )

Where C = value of the capacitor in Farads (select

an equal or larger standard value),

FR = ripple frequency in Hz,

R1 = value of resistor R1 in Ohms.

If an ADJ-bypass capacitor is use, the amplitude of the

output ripple will be independent of the output

voltage. If an ADJ-bypass capacitor is not used, the

output ripple will be proportional to the ratio of the

output voltage to the reference voltage:

M = VOUT / VREF

Where M = multiplier for the ripple seen when the

ADJ pin is optimally bypassed.

VREF = Reference Voltage

B1585

Reducing parasitic resistance and inductance

One solution to minimize parasitic resistance and

inductance is to connect in parallel capacitors. This

arrangement will improve the transient response of

the power supply if your system requires rapidly

changing current load condition.

Thermal Consideration

Although the B1585 offers some limiting circuitry

for overload conditions, it is necessary not to exceed

the maximum junction temperature, and therefore to

be careful about thermal resistance. The heat flow

will follow the lowest resistance path, which is the

Junction-to-case thermal resistance. In order to

insure the best thermal flow of the component, a

proper mounting is required. Note that the case of

the device is electrically connected to the output. In

case the case has to be electrically isolated, a

thermally conductive spacer can be used. However

do not forget to consider its contribution to thermal

resistance.

Assuming:

VIN = 7V, VOUT = 5V, IOUT = 5A, TA = 90°C, θCASE=

1°C/W (no external heat sink, no wind)

Power dissipation under these conditions

PD = (VIN – VOUT) * IOUT = 15W

Junction Temperature

TJ = TA + PD * (θCASE+ θJC)

For the Control Section

TJ = 90°C + 15W*(1°C/W + 0.6°C/W) = 114°C

114°C < TJUNCTION MAX for the control section.

For the Power Section

TJ = 90°C + 15W*(1°C/W + 1.6°C/W) = 129°C

129°C < TJUNCTION MAX for the power transistor.

In both case reliable operation is insured by

adequate junction temperature.

Bay Linear, Inc 2478 Armstrong Street, Livermore, CA 94550 Tel: (925) 606-5950, Fax: (925) 940-9556

www.baylinear.com

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