ADP3152

–7–

REV. 0

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:

∆V

∆V

Transient tolerance (for less than 2

µs) 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/

µs:

∆V

∆V

Input current di/dt when the load changes between the mini-

mum and maximum values: less than 0.1 A/

µs

The above requirements correspond to Intel’s published power

supply requirements based on VRM 8.2 guidelines.

high side N-channel MOSFET switch turns on, the voltage

µA to 2.3 V, that is

by 1 V. The value of the off time is calculated from the pre-

ferred continuous-mode operating frequency. Assuming a nomi-

= 1–









1

= 2.2 µs

The timing capacitor can be calculated from the equation:

=

×65 µA

1V

=143 pF

The converter operates at the nominal operating frequency only

heavy load, the operating frequency decreases due to the para-

sitic voltage drops across the power devices. The actual mini-

Equation 1 below). Where

is the input dc current (assuming an efficiency

is the resistance of the input filter (estimated

value: 7 m

Ω)

is the resistance of the high side MOSFET

(estimated value: 10 m

Ω)

is the resistance of the low side MOSFET

(estimated value: 10 m

Ω)

is the resistance of the sense resistor

(estimated value: 7 m

Ω)

is the resistance of the inductor (estimated

value: 6 m

Ω)

The selection of the output capacitor is driven by the required

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

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:

0.08

13. 4

= 5.9 mΩ

=

1

×

+ R

SENSE

+ R

+ R

SENSE

+ R

=160 kHz

(1)