ADP3088ARM-REEL 查看數據表（PDF） - Analog Devices

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ADP3088ARM-REEL

1 MHz, 750 mA Buck Regulator

Analog Devices 

ADP3088

rated up to that higher current, which would likely yield an

For a given inductor selection, the earlier formula is rearranged

unsatisfactory inductor size and cost. In any case, having chosen for convenience and skewed to the worst-case input voltage to

a target level for DIL, the recommended inductor value is

given by

L

=

(1 -

D) ¥ (VO +VF

fSW ¥ DIL

)

(4)

determine the maximum inductor ripple current, DIL.

DIL(MAX)

=

VIN(MAX)

VIN(MAX)

-VO

+VF

– VSW

– VSW

¥

VO +VF

fSW ¥ L

(7)

Performance degradation of the inductor, consisting of some

where D is the duty ratio, the suffix indicating continuous

loss of inductance or excessive power loss, may be encountered

inductor current, and is given by

at higher ripple currents, so the ripple current figure, together

D

=

VO +VF

VIN +VF -VSW

with the knowledge of the expected dc current, should be

(5) checked against the specifications of the inductor.

If the ESR of the output capacitor is substantial, as it is likely to

VSW and VF are assessed at full load, and fSW is the fixed

switching frequency of the ADP3088. The formula suggests the

calculation of L using a nominal input voltage; for applications

requiring a large range of VIN, the limitations of transient

response at VIN(MIN) versus the higher ripple at VIN(MAX) may

warrant deeper consideration of how to optimize the design. In

E applications where load transients are not severe, this conservative

design for L is recommended. A more aggressive minimization of

L is outlined below, but a few restrictions are noted.

T As inductance becomes smaller, the ripple current becomes

larger. If the ripple becomes particularly large or, as an addi-

tional factor, if the load is particularly dynamic, then there is

an increasing possibility that the peak inductor current will

E reach the current limit shutdown threshold, ICL, which is not

desirable. This should be avoided by restricting the minimum

inductor value to keep the ripple current moderated. An

alternative way to prevent excessive dynamic overshoot of the

L inductor current during a load transient is to reduce the dc

gain of the error amplifier by adding resistive feedback; this

idea is discussed below.

Another important restriction of the minimum inductor value may

O apply. The design should ensure against possible subharmonic

oscillation that can occur in all fixed frequency, current-controlled

switching power supplies when switching at high duty ratios.

The subharmonic oscillation phenomenon will not be explained

S here (there are many papers written on the subject) except to

say that it is characterized by alternating high and low duty

ratios, i.e., every other cycle, which produces additional ripple

on the output. To prevent subharmonic oscillation, the follow-

B ing restriction for the minimum inductor value is recommended:

( ) O L

>

2mH

V

¥

VO

+ VF

¥

VO

VIN

+ VF

( MIN )

–

0.35

(6)

be if an MLC capacitor is not used, then the ripple voltage on

the output, dominated by the ESR, may be substantial and of

concern for regulation specifications. The resistive component

of the output voltage ripple is simply the ripple current multi-

plied by the ESR, and if it is more than a few millivolts, it will

dominate the output capacitance in contributing to the output

ripple voltage.

The boundary condition of the inductor reaching the borderline

current, IO(BL), can be determined by the formula

IO(BL)

=

VO +VF

2 fSW L

¥

VIN

VIN

-VO

+VF

-VSW

-VSW

(8)

Below this output current level, the inductor current will be

discontinuous, and the duty ratio will be modulated to lower

values by factors substantially more than the losses that cause

only a small amount of the modulation in the continuous inductor

current operation. PSM is initiated automatically by a propri-

etary technique consisting of a duty ratio amplifier with an

internal time constant. As the load current drops well into the

low current region and the duty ratio passes below the threshold

of DPSM for a sufficient time, PSM is activated. The correspond-

ing level of output current is given by

IO(PSM)

=

1

2

¥

DP2SM

VIN +VF -VSW

VO +VF

¥ VIN

-VO -VSW

fSW ¥ L

(9)

It can be seen in the formula that this current threshold is inversely

proportional to inductance, so although it is usually not a relevant

concern, it is noted that an aggressively low output inductance

should be avoided to keep the PSM threshold current at a desir-

ably low level.

For the user’s reference, when current is below the borderline

level, the duty ratio is modulated according to the formula

The value used for VIN(MIN) should be only the minimum input

voltage for which normal high performance operation must be

ensured. Note that the value returned for L may be negative, in

DD =

2

¥

IO

¥

VIN

VO +VF

+VF -VSW

¥ VIN

fSW ¥ L

-VO -VSW

(10)

which case the restriction does not apply. If the preceding formula where the suffix indicates that the inductor current is discontinuous.

yields a lower inductor value than the conservative recommen-

dation given previously, as is likely for most applications, then

one should consider further limitations to see how low the value

can be minimized.

For controlling the capacitive component of the output ripple

voltage, the following constraint on the minimum output

capacitance should be applied:

CO

>

DIL

8 fSW DVR

(11)

–10–

REV. C

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