A8603KESTR-R datasheet(22/42 Pages) ALLEGRO

22 / 42 page

Multiple-Output Regulator for

Automotive LCD Displays

A8603

Allegro MicroSystems, LLC

115 Northeast Cutoff

Worcester, Massachusetts 01615-0036 U.S.A.

1.508.853.5000; www.allegromicro.com

Boost Switching Frequency

The boost stage switching frequency, fSW , of the A8603 can be

programmed by using an external resistor between the FSET pin

to GND, or it can be synchronized to an external clock frequency

between 350 kHz and 2.25 MHz.

During startup, the A8603 senses the FSET pin for any external

SYNC signal. If periodic logic transitions are detected (Low <

0.4 V or High > 1.5 V), this is evaluated as an external clock

signal, and the boost switching frequency is synchronized to it.

If no periodic signal is detected, the bias current flowing through

FSET_SYNC pin is used to determine the switching frequency.

The bias current is set by an external resistor, RFSET , on the

FSET_SYNC pin. The relation between RFSET and switching

frequency is given as:

RFSET = 10.21 / (fSW – 0.0025)

(3)

where RFSET is in kΩ and fSW is in MHz.

This relationship is charted in Figure 9. For example, to get a

switching frequency of 2 MHz requires an RFSET of 5.11 kΩ.

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

2.2

R(k)

FSET

Figure 9: Boost Switching Frequency as a Function of

FSET Resistance

Suppose the A8603 is started up with a valid external SYNC

signal, but the SYNC signal is lost during normal operation. In

that case, one of the following happens

• If the external SYNC signal is high impedance (open), the

A8603 continues normal operation, at the switching frequency

set by RFSET . No FAULT flag is generated

• If the external SYNC signal is stuck at low (shorted to

ground), the A8603 begins a shutdown sequence, at the

switching frequency set by the internal 1 MHz oscillator. The

FAULT pin is pulled low and the internal error counter is

increased by 1.

Note:

To prevent generating a fault when the external SYNC signal

is stuck at low, the circuit shown in Figure 10 can be used.

When the external SYNC signal goes low, the A8603 will con-

tinue to operate normally at the switching frequency set by

RFSET. No FAULT flag is generated.

A8603

Schottky

Barrier

Diode

FSET

10.2 k

220 pF

External

Synchronization

Signal

Figure 10: Countermeasure to Prevent External Sync

Signal Stuck-at-Low Fault

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