ADP3155 Analog Devices, ADP3155 Datasheet - Page 8

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

ADP3155
Transient tolerance (for less than 2 s) of the supply voltage for
the processor core when the load changes between the mini-
mum and maximum values with a di/dt of 30 A/ s:
Input current di/dt when the load changes between the mini-
mum and maximum values: less than 0.1 A/ s
The above requirements correspond to Intel’s published power
supply requirements based on VRM 8.2 guidelines.
C
The ADP3155 uses a constant-off-time architecture with t
determined by an external timing capacitor C
high side N-channel MOSFET switch turns on, the voltage
across C
C
reaches 2.3 V, a new on-time cycle is initiated. The value of the
off-time is calculated using the continuous-mode operating
frequency. Assuming a nominal operating frequency of f
200 kHz at an output voltage of 2.8 V, the corresponding off
time is:
The timing capacitor can be calculated from the equation:
The converter operates at the nominal operating frequency only
at the above specified V
heavy load, the operating frequency decreases due to the para-
sitic voltage drops across the power devices. The actual mini-
mum frequency at V
Equation 1), where:
I
R
R
R
R
R
IN
IN
DS(ON)HSF
DS(ON)LSF
SENSE
L
T
T
Selection for Operating Frequency
V
V
is discharged by a constant current of 65 A. Once C
OTR+
OTR–
T
= –130 mV
is reset to approximately 3.3 V. During the off time,
= 130 mV
is the input dc current
(assuming an efficiency of 90%, I
is the resistance of the input filter
(estimated value: 7 m )
is the resistance of the high side MOSFET
(estimated value: 10 m )
is the resistance of the low side MOSFET
(estimated value: 10 m )
is the resistance of the sense resistor
(estimated value: 7 m )
is the resistance of the inductor
(estimated value: 6 m )
f
MIN
t
C
OFF
T
OUT
t
OFF
1
t
OFF
OUT
= 2.8 V is calculated to be 160 kHz (see
1
–
V
V
1
V
and at light load. At higher V
IN
IN
O
V
65
V
–
IN
I R
f
IN
NOM
A
1
–
I R
IN
IN
143
–
2 2
IN
I
.
OMAX
pF
–
T
. Each time the
I
s
IN
OMAX
(
= 9 A)
R
DS ON HSF
(
(
R
NOM
DS ON HSF
OUT
)
(
OFF
T
=
or
)
R
–8–
SENSE
R
C
The required ESR and capacitance drive the selection of the
type and quantity of the output capacitors. The ESR must be
small enough that both the resistive voltage deviation due to a
step change in the load current and the output ripple voltage
stay below the values defined in the specification of the supplied
microprocessor. The capacitance must be large enough that the
output is held up while the inductor current ramps up or down
to the value corresponding to the new load current.
The total static tolerance of the Pentium II processor is 160 mV.
Taking into account the 1% setpoint accuracy of the ADP3155,
and assuming a 0.5% (or 14 mV) peak-to-peak ripple, the allowed
static voltage deviation of the output voltage when the load
changes between the minimum and maximum values is 80 mV.
Assuming a step change of I = I
allocating all of the total allowed static deviation to the contri-
bution of the ESR sets the following limit:
The output filter capacitor must have an ESR of less than 5.9 m .
One can use, for example, six FA-type capacitors from
Panasonic, with 2700 F capacitance, 10 V voltage rating, and
34 m ESR. The six capacitors have a total ESR of 5.7 m when
connected in parallel, which gives adequate margin.
Inductor Selection
The minimum inductor value can be calculated from ESR, off-
time, dc output voltage and allowed peak-to-peak ripple voltage.
The minimum inductance gives a peak-to-peak ripple current of
2.15 A, or 15% of the maximum dc output current I
The inductor peak current in normal operation is:
The inductor valley current is:
The inductor for this application should have an inductance
of 2.6 H at full load current and should not saturate at the
worst-case overload or short circuit current at the maximum
specified ambient temperature. A suitable inductor is the
CTX12-13855 from Coiltronics, which is 4.4 H at 1 A and
about 2.5 H at 14.2 A.
SENSE
L
OUT
MIN
R
Selection—Determining the ESR
L
1
–
R
R
V t
L
DS ON LSF
O OFF
)–
R
V
RIPPLE p p
(
E MAX
V
(
O
I
LPEAK
)
R
I
LVALLEY
E MAX
,
(
)
)
= I
ESR
160
)
OMAX
= I
kHz
MAX
2 8
LPEAK
.
+ I
V
1
OMAX
RPP
– I
13 4
80
2 2
14
RPP
/2 = 15.3 A
.
–I
.
mV
mV
OMIN
= 13 A
A
s
5 9
= 13.4 A, and
5 9
.
.
m
m
OMAX
2 6
REV. A
.
.
(1)
H