ADP3152AR Analog Devices, ADP3152AR Datasheet - Page 7

• Manufacturer Part Number: ADP3152AR
• Description: 5-Bit Programmable Synchronous Switching Regulator Controller for Pentium II Processor
• Manufacturer: Analog Devices

expensive microprocessor from destruction. The crowbar func-
tion releases at approximately 50% of the nominal output volt-
age. For example, if the output is programmed to 2.0 V, but is
pulled up to 2.3 V or above, the crowbar will turn on the lower
MOSFET. If in this case the output is pulled down to less than
1.0 V, the crowbar will release, allowing the output voltage to
recover to 2.0 V.
Shutdown
The ADP3152 has a shutdown pin which is pulled logic low by
an internal resistor. In this condition the device functions nor-
mally. This pin should be pulled high externally to disable the
output drives.
Calculation of Component Values
The design parameters for a typical 300 MHz Pentium II appli-
cation (Figure 2) are as follows:
Maximum output current:
Minimum output current:
Static tolerance of the supply voltage for the processor core:
Transient tolerance (for less than 2 s) of the supply voltage for
the processor core when the load changes between the minimum
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 ADP3152 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
by 1 V. The value of the off time is calculated from the pre-
ferred continuous-mode operating frequency. Assuming a nomi-
nal operating frequency of f
of V
REV. 0
T
T
Input voltage: V
Auxiliary input: V
Output voltage: V
I
I
OMAX
OMIN
Selection for Operating Frequency
is discharged by a constant current of 65 A to 2.3 V, that is
V
V
V
V
O
OST+
OST–
OTR+
OTR–
= 2.8 V, the corresponding off time is:
f
MIN
= 0.8 Adc
T
= 14.2 Adc
= –60 mV
= 100 mV
= –130 mV
is reset to approximately 3.3 V. During the off time,
= 130 mV
t
OFF
1
t
OFF
IN
V
CC
O
= 5 V
IN
= 2.8 V
= 12 V
V
– I
1–
IN
IN
NOM
V
V
– I
R
IN
O
IN
IN
= 200 kHz at an output voltage
R
– I
f
IN
NOM
1
OMAX
– I
OMAX
2.2 s
(R
DS(ON )HSF
T
(R
. Each time the
DS(ON )HSF
R
SENSE
OFF
R
SENSE
–7–
R
L
– R
R
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 below). Where
C
The selection of the output capacitor is driven by the required
ESR and capacitance C
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, C
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 ADP3152,
and assuming a 0.5% (or 14 mV) peak-to-peak ripple, the al-
lowed static voltage deviation of the output voltage when the
load changes between the minimum and maximum values is
0.08 V. Assuming a step change of I = I
and allocating all of the total allowed static deviation to the
contribution of the ESR sets the following limit:
L
O
I
R
R
R
R
R
DS(ON )LSF
) –V
IN
Selection—Determining the ESR
L
IN
DS(ON)HSF
DS(ON)LSF
SENSE
O
)
R
E MAX
160 kHz
O
, must be large enough that the output is held
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 )
C
T
O
= 2.8 V is calculated to be 160 kHz (see
O
O
t
ESR
OFF
. The ESR must be small enough that
and at light load. At higher V
IN
1V
= 9 A)
MAX 1
65 A
13. 4
0.08
143 pF
OMAX
5.9 m
ADP3152
–I
OMIN
= 13.4 A,
O
, and
(1)