ADP3155 Analog Devices, ADP3155 Datasheet - Page 11

• Manufacturer Part Number: ADP3155
• Description: 5-Bit Programmable Triple Power Supply Controller for Pentium III Processors
• Manufacturer: Analog Devices

dc biasing, it is simplest to use two resistors to terminate the g
amplifier output, with the lower resistor tied to ground and the
upper resistor to the 12 V supply of the IC. The values of these
resistors can be calculated using:
and:
where V
ommended 12 V), and V
amplifier to produce the desired offset at the output. V
calculated using Equation 2, where V
the nominal VID-programmed value to the center of the speci-
fied regulation window for the output voltage. (Note this may be
either positive or negative.) For clarification, that offset is given
by:
where V
allowed for regulation.
Finally, the compensating capacitance is determined from the
equality of the pole frequency of the error amplifier gain and the
zero frequency of the impedance of the output capacitor:
Trade-Offs Between DC Load Regulation and AC Load
Regulation
Casual observation of the circuit operation—e.g., with a voltmeter
—would make it appear that the dc load regulation appears
to be rather poor compared to a conventional regulator. This
would be especially noticeable under very light or very heavy
loads where the voltage is “positioned” near one of the extremes
of the regulation window rather than near the nominal center
value. It must be noted and understood that this low gain char-
acteristic (i.e., loose dc load regulation) is inherently required to
allow improved transient containment (i.e., to achieve tighter ac
load regulation). That is, the dc load regulation is intentionally
sacrificed (but kept within specification) in order to minimize
the number of capacitors required to contain the load transients
produced by the CPU.
Linear Regulators
The two ADP3155 linear regulators provide a low cost, conve-
nient and versatile solution for generating additional lower sup-
ply rails that can be programmed in the range 1.2 V–5 V. The
maximum output load current is determined by the size and
thermal impedance of the external N-channel power MOSFET
that is placed in series with the supply and controlled by the
REV. A
DIV
HI
and V
is the resistor divider supply voltage (e.g., the rec-
V
OS
LO
V
R
OUT OS
LOWER
are the respective upper and lower limits
Rt
(
R
C
TOTAL
R
UPPER
C
COMP
OS
)
is the offset voltage required on the
1
2
R
(
V
C
0 8
C
R
HI
.
Rt
O
C
V
TOTAL
V
V
DIV
V
ESR
V
OUT(OS)
LO
DIV
V
OS
V
OS
)–
–
OUT OS
V
VID
OS
(
is the offset from
)
1 36
Rt
.
TOTAL
k
OS
is
– .
1 7
m
V
–11–
Rt
275
ADP3155. The output voltage, V
at the FB pin of the ADP3155 and compared to an internal
1.2 V reference in a negative feedback loop which keeps the
output voltage in regulation. If the load is being reduced or
increased, the FET drive will also be reduced or increased by the
ADP3155 to provide a well regulated 1% accurate output
voltage. The output voltage is programmed by adjusting the
value of the external resistor R
Efficiency of the Linear Regulators
The efficiency and corresponding power dissipation of each of
the linear regulators are not determined by the ADP3155.
Rather, these are a function of input and output voltage and
load current. Efficiency is approximated by the formula:
The corresponding power dissipation in the MOSFET, together
with any resistance added in series from input to output is given
by:
Minimum power dissipation and maximum efficiency are ac-
complished by choosing the lowest available input voltage that
exceeds the desired output voltage. However, if the chosen
input source is itself generated by a linear regulator, its power
dissipation will be increased in proportion to the additional
current it must now provide. For most PC systems, the lowest
available input source for the linear regulators, which is not
itself generated by a linear regulator, is 3.3 V from the main
power supply.
Assuming that the 3.3 V supply is used to provide input power
for a 1.5 V linear regulator output, the efficiency will inherently
be 1.5 V
demand in all of the low voltage power rails (e.g., 1.5 V, 1.8 V
and 2.5 V) can produce unacceptable dissipation and junction
temperatures in the linear regulators. For such systems, Analog
Devices recommends the ADP3156—a switching regulator that
generates one of the lower voltage outputs (e.g., 1.8 V), which can
also be used as a power source to the lower voltage outputs
(e.g., 1.5 V). This results is a highly efficient and reliable power
conversion system that can readily handle the combined loading
specifications for the lower system voltages, with room to spare
for the higher current demands and lower voltages of next gen-
eration PC systems.
Features
• Tight DC Regulation Due to 1% Reference and High Gain
• Output Voltage Stays Within Specified Limits at Load
• Fast Response to Input Voltage or Load Current Transients
Overcurrent protection may be provided by the addition of an
external NPN transistor and an external resistor R
specification and procedure is given below.
TOTAL
Current Step with 30 A/ s Slope
k
6
P
3.3 V, which is less than 50%. The total current
R I
LDO
CS OMAX
= (V
= 100%
IN(LDO)
– V
PROG
(V
OUT(LDO)
OLDO1, 2
OUT
, shown in Figure 15.
)
V
in Figure 14, is sensed
IN
I
)
OUT(LDO)
ADP3155
S2
. The design
(2)