AIC1880 Просмотр технического описания (PDF) - Analog Intergrations

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производитель

AIC1880

Low Noise, Step-Up PWM DC/DC Converter

Analog Intergrations 

AIC1880

t SS

= CSS

× 1.0V

4µA

The soft-start capacitor is discharged to GND when the

SHDN pin is connected to GND.

Shutdown

By connecting the SHDN pin to GND, the AIC1880

can be shut down to reduce the supply current to

0.1µA (typ.). At this operation mode, the output voltage

of step-up converter is approximately equal to (VIN–VD).

VD is the forward drop voltage of catch diode.

Components Selection

Inductor

The inductor selection depends on the switching fre-

quency and current ripple of inductor.

L

≥

VIN

fOSC ⋅ ∆IL

⎜⎜⎝⎛1−

VIN

VOUT

⎟⎟⎠⎞

The AIC1880’s switching frequency can be selected to

be either 640kHz or 1.2MHz. A higher switching fre-

quency allows the use of a smaller inductance. How-

ever, higher switching frequency can cause more

switching losses, resulting in lower efficiency. By set-

ting the switching frequency and an acceptable current

ripple of inductor, a suitable inductance can be ob-

tained from above equation.

In addition, it is important to ensure the inductor satu-

ration current exceeds the peak value of inductor cur-

rent in application to prevent core saturation. The peak

value of inductor current can be calculated according

to the following equation.

IPEAK

= IIN(max ) +

2

×

VIN

fOSC

⋅L

⎜⎜⎝⎛1−

VIN

VOUT

⎟⎟⎠⎞

=

IOUT(MAX) ⋅ VOUT

η ⋅ VIN

+

VIN

2 × fOSC

⋅ L ⎜⎜⎝⎛1−

VIN

VOUT

⎟⎟⎠⎞

Diode

A Schottky diode with low forward drop voltage and

fast reverse recovery is the ideal choice for better effi-

ciency. The forward drop voltage of a Schottky diode

will result in the conduction losses in the diode, and the

diode capacitance (CT or CD) will cause the switching

losses. Therefore, it is necessary to consider both for-

ward voltage drop and diode capacitance for diode se-

lection. In addition, the rating of selected Schottky di-

ode should be able to handle the output voltage and

the maximum peak diode current.

Input Capacitor

To prevent the large input voltage ripple resulted from

high frequency switching, the use of low ESR ceramic

capacitor is recommended. A 10µF ceramic capacitor

is sufficient for most AIC1880 applications.

Output Capacitor

The selection of output capacitor depends on the re-

quired output voltage ripple. The output voltage ripple

can be expressed as:

∆VOUT

=

IOUT

fOSC ⋅ COUT

⎜⎜⎝⎛1 −

VIN

VOUT

⎟⎟⎠⎞

+ ESR ⋅ ∆IL

For lower output voltage ripple, the use of low ESR

ceramic capacitor is recommended. The tantalum ca-

pacitor can also be used well, but its ERS is larger than

that of ceramic capacitor.

Soft-Start Capacitor

The soft-start of AIC1880 begins from VSS=0V and

ends while VSS reaches 1.0V. During the soft-start pe-

riod, an internal current source of 4µA (typ.) charges

the soft-start capacitor. Hence, the soft-start capacitor

should be large enough to ensure that the output volt-

age has reached the regulation value before the

soft-start function has finished.

Output Voltage Programming

By connecting a resistive divider R1 and R2, the output

voltage of AIC1880 step-up converter can be set. VOUT

can be calculated as:

8

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