A4401 Просмотр технического описания (PDF) - Allegro MicroSystems

Номер в каталоге

Компоненты Описание

производитель

A4401

Automotive Quasi-Resonant Flyback Control IC

Allegro MicroSystems 

A4401

Automotive Quasi-Resonant Flyback Control IC

for more information). The current that flows in and

out of the capacitor is similar to the current that flows

through the corresponding rectifier diode.

Worst case power dissipation due to the ESR will

occur at VBAT(min) and maximum load. The duty

cycle under these conditions is a maximum of 0.7.

The rms current in the capacitor can be found as fol-

lows:

IRMS =

⎛

4 × ILOAD × ⎜⎜

⎝

0.7

3

⎞½

⎟⎟

⎠

.

(16)

When selecting a suitable capacitor, the rms current

rating should have reasonable margin with respect to

the above value. In addition, the current rating should

be derated to take into account the frequency correc-

tion at values of less than 100 kHz.

The impedance of the output capacitor will affect the

amount of voltage ripple and noise that appears on

the output. The impedance is composed of two com-

ponents: ESR and reactance, XC. Even with a modest

amount of capacitance on the output, the ESR will

tend to dominate the overall impedance.

The ESR can be estimated by multiplying the capaci-

tance reactance by the dissipation factor, DF (tan δ):

ESR =

1

× DF .

2 × × fSW × COUT

(17)

Note that DF is normally quoted at 25°C, however,

it is usually fairly constant as the temperature is

increased.

The peak to peak voltage can then be found:

Vpk-pk = ESR × ILOAD × 4

.

(18)

The voltage rating should be chosen to provide at least

a 20% margin above the maximum output voltage.

Control Loop

As the converter operates in discontinuous mode,

the inductor does not feature in the power stage. The

power stage effectively contains one pole formed by

the output capacitors and loads.

In terms of “closing the loop,” the optimal shaping

components are shown in the Functional Block dia-

gram, connected to the COMP pin. The control loop is

optimized for an equivalent output capacitor of 22 μF.

Larger capacitor values can be used, however, those

will tend to reduce the bandwidth of the control loop.

Smaller values should not be used, as they may cause

instability issues.

Magnetics Design

The following are the known variables:

• Maximum output power, POUT .

• Minimum battery voltage, VBAT(min).

• Minimum switching frequency, fSW(min). This

occurs at VBAT(min) and maximum load. Note, it

is recommended that the lowest possible switching

frequency be used, in order to minimize switching

losses and to shift the differential harmonics down

the frequency spectrum. This is a compromise with

the sizing of the magnetics and filtering components.

• Maximum duty cycle. This should typically not

exceed 0.7, in order to avoid excessive losses in the

secondary output circuits.

• Efficiency of converter, η% , at VBAT(min). This will

typically be between 80% and 85%.

• Resonant capacitor, C11, connected between the

LX pin and ground. The resonant half period of 1 μs

may be achieved with the parasitic capacitance that

exists in the circuit. Fine tuning can be performed by

adding additional capacitance.

• Known magnetic core set. Material selection should

be based on performance at elevated temperature,

and include consideration of flux density, losses,

Curie temperature, and so forth.

Allegro MicroSystems, Inc.

9

115 Northeast Cutoff, Box 15036

Worcester, Massachusetts 01615-0036 (508) 853-5000

www.allegromicro.com

Share Link: