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ADM1052JR

Precision Dual Voltage Regulator Controller

Analog Devices 

ADM1052

12V

3.3V

1␮F

VCC

FORCE 1

100␮F

LEAVE OPEN OR

CONNECT TO

LOGIC SIGNALS

IF SHUTDOWN

REQUIRED

SENSE 1

SHDN1

ADM1052

SHDN2

FORCE 2

VOUT1

2 ؋ 100␮F

3.3V

100␮F

GND

SENSE 2

VOUT2

2 ؋ 100␮F

Figure 4. Typical Application Circuit

SUPPLY DECOUPLING

The supply to the drain of an external MOSFET should be decoupled

as close as possible to the drain pin of the device, with a 100 µF

capacitor to ground. The output from the source of the MOSFET

should be decoupled as close as possible to the source pin of the

device. Decoupling capacitors should be chosen to have a low

Equivalent Series Resistance (ESR). With the MOSFETs specified

and two 100 µF capacitors in parallel, the circuit will be stable for

load currents up to 2 A. The VCC pin of the ADM1052 should be

decoupled with a 1 µF capacitor to ground, connected as close as

possible to the VCC and GND pins.

In practice, the amount of decoupling required will depend on

the application. PC motherboards are notoriously noisy environ-

ments, and it may be necessary to employ distributed decoupling

to achieve acceptable noise levels on the supply rails.

CHOICE OF MOSFET

As previously discussed, the load current at which an output

goes into hiccup mode depends on the on-resistance of the

external MOSFET. If the on-resistance is too low this current

may be very high. While the Test Circuit (Figure 1) shows the

use of the lower resistance PHD55N03LT from Philips on

Channel 1 and the use of the higher resistance MTD3055VL

from Motorola on Channel 2, the MTD3055VL is, in fact,

suitable for both channels. Similarly, the PHB11N06LT from

Philips is also suitable for both channels.

THERMAL CONSIDERATIONS

Heat generated in the external MOSFET must be dissipated

and the junction temperature of the device kept within accept-

able limits. The power dissipated in the device is, of course, the

drain-source voltage multiplied by the load current. The required

thermal resistance to ambient is given by

␪JA = TJ(MAX) – TAMB(MAX)/(VDS(MAX) × IOUT(MAX))

Surface-mount MOSFETs such as those specified must rely on

heat conduction through the device leads and the PCB. One

square inch of copper (645 sq. mm) gives a thermal resistance of

around 60°C/W for a SOT-223 surface-mount package and

80°C/W for a SO-8 surface-mount package.

For higher power dissipation than can be accommodated by a

surface-mount package D2PAK or TO-220 devices are recom-

mended. These should be mounted on a heatsink with a thermal

resistance low enough to maintain the required maximum junc-

tion temperature.

OUTLINE DIMENSIONS

Dimensions shown in inches and (mm).

8-Lead Small Outline Package

(Narrow Body, SO-8)

0.1968 (5.00)

0.1890 (4.80)

8

0.1574 (4.00)

0.1497 (3.80) 1

5

0.2440 (6.20)

4 0.2284 (5.80)

PIN 1

0.0500 (1.27)

BSC

0.0196 (0.50)

0.0099 (0.25) ؋ 45؇

0.0098 (0.25)

0.0040 (0.10)

SEATING

PLANE

0.0688 (1.75)

0.0532 (1.35)

8؇

0.0192 (0.49)

0.0098 (0.25) 0؇

0.0138 (0.35)

0.0075 (0.19)

0.0500 (1.27)

0.0160 (0.41)

–8–

REV. A

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