REV. 0

ADM1051/ADM1051A

–6–

GENERAL DESCRIPTION

The ADM1051/ADM1051A are dual, precision, voltage regulator

controllers intended for power rail generation and active bus ter-

mination in AGP and ICH applications on personal computer

motherboards. They contain a precision 1.2 V bandgap ref-

erence and two almost identical channels consisting of control

amplifiers driving external power devices. The main difference

between the two channels is the regulated output voltage, defined

by the resistor ratios on the voltage sense inputs of each channel.

Channel 1 has an output of nominally 1.515 V, but can be

switched to a 3.3 V output, while Channel 2 has a nominal

output of 1.818 V. Channel 1 is also optimized for driving

MOSFETs with lower on-resistance and higher gate capaci-

tance, as explained later.

Each channel has a shutdown input to turn off amplifier output

and protection circuitry for the external power device. The

shutdown input of Channel 1 has additional functionality as

described later.

The ADM1051A has some minor differences from the ADM1051

to support power-supply sequencing and voltage requirements

of some I/O control hub chipsets, which dictate that the 1.818 V

rail must never be more than 2 V below the 3.3 V rail.

Reset threshold (6 V–9 V). POR does not apply to Channel 2 of

the ADM1051A. This output will begin to rise as soon as there

is sufficient gate drive to turn on the external MOSFET.

The outputs from the ADM1051/ADM1051A are used to drive

external N-channel MOSFETs, operating as source-followers.

This has the advantage that N-channel devices are cheaper than

P-channel devices of similar performance, and the circuit is easier

to stabilize than one using P-channel devices in a common-

source configuration.

The external power devices are protected by a “Hiccup Mode”

circuit that operates if the circuit goes out of regulation due to

an output short-circuit. In this case the power device is pulsed

on/off with a 1:40 duty-cycle to limit the power dissipation until

the fault condition is removed. Again, to prevent Channel 2

falling more than 2 V below Channel 1, Hiccup Mode does not

operate on Channel 2 of the ADM1051A.

CIRCUIT DESCRIPTION

CONTROL AMPLIFIERS

The reference voltage is amplified and buffered by the control

amplifiers and external MOSFETs, the output voltage of each

channel being determined by the feedback resistor network

between the sense input and the inverting input of the control

amplifier.

The two control amplifiers in the ADM1051/ADM1051A are

almost identical, apart, from the ratios of the feedback resistor

networks on the sense inputs. A power-on reset circuit disables

old (not Channel 2 of ADM1051A).

Each amplifier output drives the gate of an N-channel power

MOSFET, whose drain is connected to the unregulated supply

input and whose source is the regulated output voltage, which is

also fed back to the appropriate sense input of the ADM1051/

ADM1051A. The control amplifiers have high current-drive

capability so they can quickly charge and discharge the gate

capacitance of the external MOSFET, thus giving good transient

response to changes in load or input voltage. In particular,

Channel 1 is optimized to drive MOSFETs with very low on

resistance and correspondingly higher gate capacitance such as

the PHD55N03LT from Philips. This is to minimize voltage

drop across the MOSFET when Channel 1 is used in 3.3 V

mode, as explained later.

SHUTDOWN INPUTS AND TYPEDET COMPATIBILITY

Each channel has a separate shutdown input, which may be

controlled by a logic signal, and allows the output of the regula-

tor to be turned on or off. If the shutdown input is held high or

not connected, the regulator operates normally. If the shutdown

input is held low, the enable input of the control amplifier is turned

off and the amplifier output goes low, turning off the regulator.

The

SHDN1 input on Channel 1 has additional functionality that

can be controlled by the TYPEDET signal on PC motherboards.

The AGP bus on a PC motherboard can have two different modes

of operation, requiring different regulated voltages of 3.3 V or

1.5 V. These two modes are signaled by the TYPEDET signal on

the PC motherboard, as follows:

TYPEDET = 0 V – Regulated Voltage 1.5 V (4

× AGP Graphics)

TYPEDET Floating – Regulated Voltage 3.3 V (2

× AGP Graphics)

5VSB – V

1.855

1.555

3.7

3.9

4.1

4.7

1.805

1.755

1.655

1.455

1.605

5.3

1.505

1.705

1.405

1.355

1.305

4.3

4.5

4.9

5.1

25 C

VCH2 V's 5VSB ILOAD 500m

70 C

0 C

TPC 13. ADM1051A Channel 2 Output Voltage vs. 5 VSB

5VSB – V

1.855

1.555

3.7

3.9

4.1

4.7

1.805

1.755

1.655

1.455

1.605

5.3

1.505

1.705

1.405

1.355

1.305

4.3

4.5

4.9

5.1

25 C

0 C

70 C

VCH2 V's 5VSB ILOAD 1

TPC 14. ADM1051A Channel 2 Output Voltage vs. 5 VSB