Diodes AUR9705 Manuel d'utilisateur télécharger pdf (Page 9)

Data Sheet

1.5MHz, 1A, STEP DOWN DC-DC CONVERTER AUR9705

Sep. 2013 Rev. 1. 2

BCD Semiconductor Manufacturing Limited

Application Information

The basic AUR9705 application circuit is shown in

Figure 16, external components selection is determined

by the load current and is critical with the selection of

inductor and capacitor values.

1. Inductor Selection

For most applications, the value of inductor is chosen

based on the required ripple current with the range of

2.2μH to 4.7μH.

GND

VOUT

AUR9705

The largest ripple current occurs at the highest input

voltage. Having a small ripple current reduces the ESR

loss in the output capacitor and improves the efficiency.

The highest efficiency is realized at low operating

frequency with small ripple current. However, larger

value inductors will be required. A reasonable starting

point for ripple current setting is

△

=40%I

MAX

　. For

a maximum ripple current stays below a specified

value, the inductor should be chosen according to the

following equation:

The DC current rating of the inductor should be at

least equal to the maximum output current plus half

the highest ripple current to prevent inductor core

saturation. For better efficiency, a lower

DC-resistance inductor should be selected.

2. Capacitor Selection

The input capacitance, C

, is needed to filter the

trapezoidal current at the source of the top MOSFET.

To prevent large ripple voltage, a low ESR input

capacitor sized for the maximum RMS current must

be used. The maximum RMS capacitor current is

given by:

It indicates a maximum value at V

=2V

OUT

, where

RMS

OUT

/2. This simple worse-case condition is

commonly used for design because even significant

deviations do not much relieve. The selection of C

OUT

is determined by the Effective Series Resistance

(ESR) that is required to minimize output voltage

ripple and load step transients, as well as the amount

of bulk capacitor that is necessary to ensure that the

control loop is stable. Loop stability can be also

checked by viewing the load step transient response

as described in the following section. The output

ripple,

△

OUT

, is determined by:

The output ripple is the highest at the maximum input

voltage since

△

increases with input voltage.

3. Load Transient

A switching regulator typically takes several cycles to

respond to the load current step. When a load step

occurs, V

OUT

immediately shifts by an amount equal

△

LOAD

×ESR, where ESR is the effective series

resistance of output capacitor. △I

LOAD

also begins to

charge or discharge C

OUT

generating a feedback error

signal used by the regulator to return V

OUT

to its

steady-state value. During the recovery time, V

OUT

can be monitored for overshoot or ringing that would

indicate a stability problem.

4. Output Voltage Setting

The output voltage of AUR9705 can be adjusted by a

resistive divider according to the following formula:

The resistive divider senses the fraction of the output

voltage as shown in Figure 15.

Figure 15. Setting the Output Voltage

OUTINOUT

OMAXRMS

VVV

)]([ −

×=

)1(

OUT

OUTL

I −

=Δ

]

)(

1][

)(

[

MAXV

MAXIf

OUT

−

Δ×

]

[

OUT

LOUT

ESRIV

××

+Δ≤Δ

)1(6.0)1(

OUT

+×=+×=V

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

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Diodes AUR9705 Manuel d'utilisateur Page 9

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