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A4401

Automotive Quasi-Resonant Flyback Control IC

Allegro MicroSystems 

A4401

Automotive Quasi-Resonant Flyback Control IC

In the event of an overload, the current demand signal

produced by the Gm amplifier restricts the output cur-

rent by introducing pulse-by-pulse current limiting.

Regulation Voltage

The feedback resistors, R5 and R6, determine the

voltage of the output rail to which they are connected,

according to the following formula:

VOUT =

VREF ×(R5 + R6)

R6

,

(1)

where R6 should be approximately 5 kΩ.

The internal 1.2 V reference has a ±2% worst case

tolerance, plus there is an input bias current, IBIAS, on

the feedback node, VA, that has a small influence. This

current flows into the ground referenced resistor, R6,

creating a small voltage offset.

In applications where the main control output (anode

or grid) can run at relatively light loads (relative to the

filament load), it may be necessary to “mix” the feed-

back signal. This involves adding an additional feed-

back from the filament output to the VA input. Note

that this only applies to DC filament outputs.

R3 Current Sense Resistor Selection

To determine the resistance value, the maximum peak

current needs to be determined. First determine the

average input current, IAV , as follows:

IAV =

POUT(max)

H% × VIN(max)

,

(2)

where POUT is the output power. Then, the peak cur-

rent through the sense resistor:

IPK

=

2 × IAV

D(max)

,

(3)

where D is limited to 0.7, or can be precisely found as

described in the Magnetics Design section. Note that

a D of 0.7 is chosen in order to achieve 0 V switching

with a VBAT of 13.5 V.

To determine the sense resistor value, assume that the

minimum sense voltage before current limiting occurs

is 600 mV. A reasonable maximum voltage to select

during normal operation would be 500 mV. Then, the

resistor value can be found as follows:

500 mV

RSENSE = IPK

.

(4)

The power losses in the resistor can be found by first

determining the rms current through it:

IRMS =

IPK ×

⎛

⎜⎜

⎝

D(max)

3

⎞½

⎟⎟

⎠

.

(5)

Then, the losses in sense resistor are:

PRDS = I ²RMS × RSENSE .

(6)

The power rating of the resistor can be selected based

on the power dissipation. When selecting a resistor

it is worth noting that the maximum power rating

is valid up to 70°C and derates linearly to 0 W at a

temperature of typically between 120°C and 140°C.

Check the resistor manufacturer guidelines.

Note that is imperative that this resistor be a low

inductance type; avoid wire wound. Standard surface

mount devices are usually acceptable.

Soft Start

When power is initially applied, assuming the input

voltage turn-on threshold is reached, and the EN input

is enabled, the controller is initiated and the MOSFET,

Q1, is turned on for the first switching cycle. Initially,

while the output volts are rising towards the target reg-

ulation point set by the soft start circuit, the MOSFET

will run at current limit.

During a soft start cycle, the reference voltage is

ramped from 0 to 1.2 V in 32 steps over a period of 10

ms under the control of a DAC. This forces the output

of the amplifier to vary between 0.8 and 1.5 V, which

in turn reduces the effects of inrush current and volt-

age overshoot on the outputs.

Allegro MicroSystems, Inc.

6

115 Northeast Cutoff, Box 15036

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

www.allegromicro.com

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