REV. A

ADP3157

–7–

Power Good

The ADP3157 has an internal monitor that senses the output

voltage and drives the PWRGD pin of the device. This pin is an

open drain output whose high level (when connected to a pull-

up resistor) indicates that the output voltage has been within a

±5% regulation band of the targeted value for more than 500 µs.

The PWRGD pin will go low if the output is outside the regula-

tion band for more than 500

µs.

Output Crowbar

An added feature of using an N-channel MOSFET as the syn-

chronous switch is the ability to crowbar the output with the

same MOSFET. If the output voltage is 15% greater than the

targeted value, the ADP3157 will turn on the lower MOSFET,

which will current-limit the source power supply or blow its

fuse, pull down the output voltage, and thus save the micropro-

cessor from destruction. The crowbar function releases at ap-

proximately 50% of the nominal output voltage. 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 if

the fault condition has been removed.

Shutdown

The ADP3157 has a shutdown (SD) pin that is pulled down by

an internal resistor. In this condition the device functions nor-

mally. This pin should be pulled high to disable the output drives.

APPLICATION INFORMATION

Specifications for a Design Example

The design parameters for a typical 550 MHz Pentium III 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 8 A/

µs

The above requirements correspond to Intel’s published power

supply requirements based on Intel Pentium III specifications.

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

reaches 2.3 V, a new on-time cycle is initiated. The value of the

off-time is calculated using the continuous-mode operating

200 kHz at an output voltage of 2.0 V, the corresponding off

time is:

t

V

Vf

s

OFF

O

IN

NOM

=









=

1

1

30

–.

µ

The timing capacitor can be calculated from the equation:

C

tA

V

pF

T

OFF

=

×

=

65

1

200

µ

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), where:

is the input dc current

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: 5 m

Ω)

is the resistance of the inductor

(estimated value: 6 m

Ω)

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 III processor is 140 mV.

Taking into account the

±1% setpoint accuracy of the ADP3157,

and assuming a 0.5% (or 10 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

90 mV. 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:

R

ESR

mV

A

m

E MAX

MAX

()

.

==

=

1

90

16

56

Ω

The output filter capacitor must have an ESR of less than 5.6 m

Ω.

One can use, for example, two SP-Type OS-CON capacitors

from Sanyo, with 2200

µF capacitance, 7 V voltage rating, and

f

t

VI R

I

R

R

R

V

VI R

I

R

R

R

R

kHz

MIN

OFF

IN

IN

IN

OMAX

DS ON HSF

SENSE

L

O

IN

IN

IN

OMAX

DS ON HSF

SENSE

L

DS ON LSF

=×

++

++

=

1

180

––

(

) –

––

(

–

)

()

()

()

(1)