L4990AD datasheet(9/24 Pages) STMICROELECTRONICS

9 / 24 page

APPLICATION INFORMATION

Detailed Pin Functions Description

Pin 1. SYNC (In/Out Synchronization). This func-

tion allows the IC’s oscillator either to synchronize

other controllers (master) or to be synchronized to

an external frequency (slave).

As a master, the pin delivers positive pulses dur-

ing the ramp-down of the oscillator (see pin 2). In

slave operation the circuit is edge triggered. Refer

to fig. 21 to see how it works. When several IC

work in parallel no master-slave designation is

needed because the fastest one becomes auto-

matically the master.

During the ramp-up of the oscillator the pin is

pulled low by a 600

µA generator. During the

ramp-down, that is when the pulse is released,

the 600

µA pull-down is disconnected. The pin be-

comes a generator whose source capability is

typically 7mA (with a voltage still higher than

3.5V).

In fig. 20, some practical examples of synchroniz-

ing the L4990 are given.

L4990

VREF

SYNC

RCT

L4981A

(MASTER)

L4990

(SLAVE)

VREF

SYNC

RCT

ROSC

COSC

L4990

(MASTER)

L4981A

(SLAVE)

SYNC

ROSC

COSC

SYNC

(a)

(b)

(c)

D97IN494A

VREF

RCT

Figure 20. Synchronizing the L4990.

Pin 2. RCT (Oscillator). A resistor (RT) and a ca-

pacitor (CT), connected as shown in fig. 21 set the

operating frequency fosc of the oscillator.

CT is charged through RT until its voltage reaches

3V, then is quickly internally discharged. As the

voltage has dropped to 1V it starts being charged

again

CLAMP

Ω

600

µA

D97IN500B

REF

RCT

SYNC

CLK

Figure 21. Oscillator and synchronization internal schematic.

L4990 - L4990A

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