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ADP3088ARM-REEL

1 MHz, 750 mA Buck Regulator

Analog Devices 

ADP3088

APPLICATION INFORMATION

junction to case or ambient, as desired, to determine the internal

Output Voltage Setting

temperature rise.

In its standard usage, the output voltage of the ADP3088 is

programmed to a desired fixed value by a resistor divider from

the output voltage into the feedback node, the FB pin, at which

node the control loop ensures regulation at the reference level,

VREF. The divider should be designed to satisfy the formula

If the input voltage were so much higher than the output voltage

that it required an average duty ratio less than an internally

preset threshold, then power savings mode (PSM)—which is

characterized by periodic shutdown and wake-up of the device

that reduces average quiescent current—would be active for all

VOUT

= VREF

¥ ÊËÁ1 +

RA

RB

ˆ

¯˜

load conditions rather than only at lighter loads, for which it is

(1) intended. PSM operation is characterized by low frequency

ripple on the output that appears similar to the behavior of a

where RA is the upper divider resistor (between the output and FB) hysteretic regulator. This is usually not a factor for consider-

and RB is the lower one (between FB and ground). RA and RB

ation and may be ignored if PSM operation is acceptable for all

are recommended to have values in the range of 2 kW~200 kW and

are likely to require a 1% tolerance or better to attain acceptable

output voltage tolerance.

In less conventional applications described separately, the resistor

feedback configuration can be modified or tapped with other

resistors to affect current flow into the FB node that, in turn,

E influences the output voltage. Even a switched voltage can be

summed into the FB node as long as it is sufficiently integrated

and does not intolerably compromise the transient response. This

T latter application is considered further below, for an application for

powering a DSP.

Input Voltage, Power Dissipation Considerations, and

Power Savings Mode

E The input voltage range is not typically considered a critical

parameter for electrical functionality, but there are several

considerations, upon which there is further elaboration below:

L 1. VIN must never exceed the maximum rated voltage.

2. VIN must be within the specified operating range when normal

operation is expected.

3. VIN must be greater than VOUT by at least the specified head-

O room when dc regulation is expected.

4. VIN, if not sufficiently greater than VOUT, may limit the large

signal transient response of a buck converter.

S 5. VIN, if much greater than VOUT, may give rise to such a low

duty ratio that it activates power savings mode even at static

higher load conditions or upon dynamic load changes when

it is not desired.

B 6. VIN affects the device power dissipation (a lower value causes

higher dissipation), which in turn affects die temperature

that must be kept below a maximum rating.

The lowest input voltage together with the maximum output

O voltage and maximum current create the conditions for the

load conditions. But in case it is relevant, the following recom-

mendation is offered:

VIN

<

VO +VF

DPSM(MAX)

(3)

It is not possible to prevent the duty ratio from tending toward

zero in nonsynchronous buck converters below a certain minimum

load current level called “borderline current” or “critical current”

for the power converter. That corresponds to the inductor

ripple current reaching zero at its bottom peak, sometimes called

the “valley current.” If PSM activation strains the lower regulation

limit due to the hysteretic ripple, the output voltage can be offset

slightly upward by readjusting the nominal voltage setpoint with

the resistor divider.

Even though a buck converter may have a low dropout voltage

that allows the static regulation to be maintained as the input

voltage drops near the output voltage, in buck converters, the

slew rate limitation of the inductor current can compromise the

dynamic regulation in response to the load current step increases.

That is because the maximum rate the current can be increased

to in the inductor is proportional to the voltage available to

impress across it, which is compromised as the input voltage

reduces toward the output voltage. This is not a limitation of

the device but of buck converters in general. The limitation is

considered part of the output filter design, although it could

also be considered in terms of a minimum acceptable input

voltage for a given output filter that will ensure that the dynamic

response is acceptably maintained.

Output Filter Components

In most applications, it is desirable to use the smallest inductor

value that does not introduce practical problems, since this tends

to yield the lowest cost inductor. One reason for using an even

larger inductor than the minimum tolerable might be to reduce

the output ripple voltage further. But cost being equal, this is

maximum power dissipation in the device, which determine the generally better accomplished with a better quality or propor-

maximum temperature rise that should be checked against the

tionally larger output capacitor instead, since a larger inductor

maximum junction temperature rating. The formula for maximum degrades the large signal transient performance capability.

power dissipation in the device is given by

A conservative nominal design target value for the inductor of a

PDMAX

= VO

+VF @ IO, MAX

VIN

¥ IO, MAX

¥VSW @ IO, MAX

typical application circuit is that which creates a peak-to-peak

(2) ripple current, DIL, for the nominal input voltage that is approxi-

mately a third of the nominal 500 mA rating of the ADP3088.

where VF is the diode forward voltage drop and VSW is the drop

across the internal switch and current sensing resistor that

appears between the VIN and SW pins of the ADP3088 during the

on state of the switch. Both of these variables can be approximated

from a combination of worst-case specifications and typical graphs.

Multiply the power dissipation by the thermal resistance from

The reason for not basing the ripple current on the maximum

load current is concern about the protection. Scaling the ripple

currents with lower load currents would yield higher inductor

values that might give satisfactory operation. However, in order for

overload operation up to the current limit level of the ADP3088

to be satisfactory, it would be necessary to choose an inductor

REV. C

–9–

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