adm1051a Analog Devices, Inc., adm1051a Datasheet - Page 6

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

ADM1051/ADM1051A
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.
The ADM1051/ADM1051A operates from a 12 V V
The outputs are disabled until V
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.
1.855
1.805
1.755
1.705
1.655
1.605
1.555
1.505
1.455
1.405
1.355
1.305
3.7
3.9
4.1
VCH2 V's 5VSB ILOAD 500m
70 C
4.3
25 C
5VSB – V
CC
4.5
climbs above the Power-On
4.7
0 C
4.9
5.1
CC
5.3
supply.
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
the amplifier output until V
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)
1.855
1.805
1.755
1.705
1.655
1.605
1.555
1.505
1.455
1.405
1.355
1.305
3.7
3.9
4.1
VCH2 V's 5VSB ILOAD 1
CC
4.3
has risen above the reset thresh-
70 C
5VSB – V
4.5
25 C
4.7
0 C
4.9
5.1
5.3