AP1623 datasheet(5/6 Pages) ANACHIP

5 / 6 page

AP1623

PWM Control Step-Down Switching Regulator-Controllers

Anachip Corp.

www.anachip.com.tw

Rev. 0.1 Feb 10, 2004

5/6

Function Description

PWM Control (AP1623 Series)

The AP1623 consists of DC/DC converters that

employ a pulse-width modulation (PWM) system.

In converters of the AP1623, the pulse width varies

in a range from 0 to 100, according to the load

current, and yet ripple voltage produced by the

switching can easily be removed through a filter

because the switching frequency remains constant.

Therefore, these converters provide a low-ripple

power over broad ranges of input voltage and load

current.

External adjustment of output voltage

The AP1623 allows the user to select 3 types of the

output voltage, when external resistances RA, RB,

and capacitor CC are added, as illustrated in Typical

Application

Circuit.

Moreover,

temperature

gradient

can

obtained

inserting

thermal-resistor or other element in series with RA

and RB.

Therefore, the output voltage (OUT) is determined

by the output voltage value VOUT of the AP1623,

and the ratio of the parallel resistance value of

external resistance RB and internal resistance R1 +

R2 of the IC, to external resistance RA. The output

voltage is expressed by the following equation:

OUT = VOUT + VOUT × RA ÷ (RB // (R1+ R2))

(Note:// denotes a combined resistance in parallel.)

The voltage accuracy of the output OUT set by

resistances RA and RB is not only affected by the

IC’s output voltage accuracy (VOUT ± 2.4%), but also

by the absolute precision of external resistances RA

and RB in use and the absolute value deviations of

internal resistances R1 and R2 in the IC.

Let us designate the maximum deviations of the

absolute value of external resistances RA and RB by

RAmax and RBmax, respectively, the minimum

deviations by RAmin and RBmin, respectively, and

the maximum and minimum deviations of the

absolute value of internal resistances R1 and R2 in

the IC by (R1+ R2)max and (R1+ R2)min,

respectively. Then, the minimum deviation value

OUTmin and the maximum deviation value

OUTmax of the output voltage OUT are expressed

by the following equations:

OUTmin = VOUT × 0.976 + VOUT × 0.976 × RAmin ÷

(RBmax// (R1+ R2)max)

OUTmax = VOUT × 1.024 + VOUT × 1.024 × RAmax ÷

(RBmin// (R1+ R2)min)

The voltage accuracy of the output OUT cannot be

made higher than the output voltage accuracy (VOUT

± 2.4%) of the IC itself, without adjusting the

external resistances RA and RB involved. The closer

the voltage value of the output OUT and the output

voltage value (VOUT) of the IC are brought to one

other, the more the output voltage remains immune

to deviations in the absolute accuracy of externally

connected resistances RA and RB and the absolute

value of internal resistances R1 and R2 in the IC.

In particular, to suppress the influence of deviations

in internal resistances R1 and R2 in the IC, a major

contributor to deviations in the output OUT, the

external resistances RA and RB must be limited to a

much smaller value than that of internal resistances

R1 and R2 in the IC.

On the other hand, a reactive current flows through

external resistances RA and RB. This reactive

current must be reduced to a negligible value with

respect to the load current in the actual use of the

IC so that the efficiency characteristics will not be

degraded. This requires that the value of external

resistance RA and RB be made sufficiently large.

However, too large a value (more than 1 MΩ) for

the external resistances RA and RB would make the

IC vulnerable to external noise. Check the influence

of this value on actual equipment.

There is a tradeoff between the voltage accuracy of

the output OUT and the reactive current. This

should be taken into consideration based on the

requirements of the intended application.

Deviations in the absolute value of internal

resistances R1 and R2 in the IC vary with the

output voltage of the AP1623, and are broadly

classified as follows:

. Output voltage 2.5V → 4.44MΩ to 27.0MΩ

. Output voltage 3.3V → 3.60MΩ to 23.3MΩ

. Output voltage 5.0V → 2.45MΩ to 2.45MΩ

When a value of R1+R2 given by the equation

indicated below is taken in calculating the voltage

value of the output OUT, a median voltage deviation

will be obtained for the output OUT.

R1 + R2 = 2 ÷ ( 1 ÷ maximum deviation in absolute

value of internal resistances R1 and R2 in IC + 1 ÷

minimum deviation in absolute value of internal

resistances R1 and R2 of IC)

Moreover, add a capacitor CC in parallel to the

external resistance RA in order to avoid output

oscillations and other types of instability.

Make sure that CC is larger than the value given by

the following equation:

CC(F)

(

≥

)

Ω

(

kHz

If a large CC-value is selected, a longer soft-start

time than the one set up in the IC will be set.

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