adm1051a Analog Devices, Inc., adm1051a Datasheet - Page 9

• Manufacturer Part Number: adm1051a
• Description: Precision Dual Voltage Regulator Controllers
• Manufacturer: Analog Devices, Inc.

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 V
with at least 1 µF to ground, connected as close as possible to
the V
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.
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.
LEAVE OPEN OR
LOGIC SIGNALS
IF SHUTDOWN
CONNECT TO
REQUIRED
CC
CC
and GND pins.
pin of the ADM1051/ADM1051A should be decoupled
SHDN1
SHDN2
V
ADM1051
CC
12V
1 F
FORCE 1
SENSE 1
FORCE 2
SENSE 2
PHD55N03LT
MTD3055VL
3.3V
3.3V
V
V
IN
IN
100 F
100 F
2 100 F
2 100 F
V
V
OUT2
OUT1
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, V
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 V
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.
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
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, D
ommended. These should be mounted on a heat sink with a
thermal resistance low enough to maintain the required maxi-
mum junction temperature.
10k
3.3V
5VSB
12V
JA
CC
SHDN1
SHDN2
= T
BAT45C
BAT45C
is supplied from the 5 V standby rail (5 VSB) and
J(MAX)
CC
ADM1051A
V
CC
is below the POR threshold. When the main
– T
AMB(MAX)
ADM1051/ADM1051A
2
0.1
F
PAK or TO-220 devices are rec-
FORCE 1
SENSE 1
FORCE 2
SENSE 2
/(V
PHD55N03LT
DS(MAX)
MTD3055VL
× I
3.3V
3.3V
V
V
IN
IN
OUT(MAX)
100 F
100 F
2 100 F
2 100 F
)
V
V
OUT2
OUT1