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L4990AD 数据表(PDF) 9 Page - STMicroelectronics

部件名 L4990AD
功能描述  PRIMARY CONTROLLER
PDF  24 Pages
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制造商  STMICROELECTRONICS [STMicroelectronics]
网页  http://www.st.com
标志 STMICROELECTRONICS - STMicroelectronics

L4990AD 数据表(HTML) 9 Page - STMicroelectronics

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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
L4990
RT
VREF
SYNC
SYNC
RCT
RCT
L4981A
(MASTER)
L4990
(SLAVE)
RT
VREF
SYNC
RCT
ROSC
COSC
CT
L4990
(MASTER)
L4981A
(SLAVE)
SYNC
ROSC
CT
COSC
SYNC
(a)
(b)
(c)
RT
D97IN494A
CT
VREF
4
1
2
1
2
16
18
17
4
2
1
RCT
1
2
4
16
17
18
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
+
-
R2
R3
R1
CLAMP
D1
50
Ω
R
T
C
T
D
R
Q
600
µA
D97IN500B
V
REF
RCT
SYNC
CLK
2
4
1
Figure 21. Oscillator and synchronization internal schematic.
L4990 - L4990A
9/24



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