ADM1051A View Datasheet(PDF) - Analog Devices

Part Name

Description

Manufacturer

ADM1051A

Precision Dual Voltage Regulator Controllers

Analog Devices 

ADM1051/ADM1051A

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

at least 100 µF 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), typically 50 mΩ or

lower. 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 ADM1051/ADM1051A should be decoupled

with at least 1 µF 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 envi-

ronments, and it may be necessary to employ distributed decoupling

to achieve acceptable noise levels on the supply rails.

12V

VCC

VIN

1␮F

3.3V

PHD55N03LT

FORCE 1

100␮F

LEAVE OPEN OR

CONNECT TO

LOGIC SIGNALS

IF SHUTDOWN

REQUIRED

SHDN1

SHDN2

SENSE 1

VOUT1

2؋100␮F

ADM1051

VIN

3.3V

MTD3055VL

FORCE 2

100␮F

SENSE 2

VOUT2

2؋100␮F

Figure 6. Typical ADM1051 Application Circuit

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; if the on resistance is high, the trip current may be

lower than the maximum required load current. For the primary

application of AGP and ICH power supplies and bus termina-

tion on personal computer motherboards, devices with very

low on resistance, such as the PHD55N03LT from Philips,

or the SUB60N06-18 from Siliconix, are suitable. For Channel 2,

suitable devices are the MTD3055VL from Motorola and the

PHB11N06LT from Philips.

POWERING SUPPLY SEQUENCING

Some I/O control hub chipsets have power-supply sequencing

requirements, which dictate that the 1.818 V supply must never

be more than 2 V below the 3.3 V supply. This requirement can

be met using the ADM1051A, as shown in Figure 7. In this

circuit, VCC is supplied from the 5 V standby rail (5 VSB) and

from the 12 V rail via Schottky diodes. 5 VSB is always present

when ac power is supplied to the system, so the ADM1051A is

powered up, but VCC is below the POR threshold. When the main

power supplies are turned on, the Channel 2 output will rise at

the same rate as the 3.3 V rail until it regulates at 1.818 V. The

12 V supply will take over from 5 VSB when it exceeds the 5 VSB

rail, and Channel 1 will then be subject to the POR delay. This

ensures that Channel 2 can never be more than 2 V below

Channel 1.

12V

5VSB

BAT45C

BAT45C VCC

0.1␮

F

VIN

3.3V

PHD55N03LT

FORCE 1

100␮F

3.3V

10k⍀

SHDN1

ADM1051A

SHDN2

SENSE 1

VIN

3.3V

MTD3055VL

FORCE 2

VOUT1

2؋100␮F

100␮F

SENSE 2

VOUT2

2؋100␮F

Figure 7. Typical ADM1051A Application Circuit

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 an SOT-223 surface-mount package and

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

For high power dissipation that can be accommodated by a

surface-mount package, D2PAK or TO-220 devices are rec-

ommended. These should be mounted on a heat sink with a

thermal resistance low enough to maintain the required maxi-

mum junction temperature.

REV. 0

–9–

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