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參數資料
| 型號： | L4990 |
| 廠商： | 意法半導體 |
| 英文描述： | PRIMARY CONTROLLER |
| 中文描述： | 主控制器 |
| 文件頁數： | 9/24頁 |
| 文件大?。?/td> | 322K |
| 代理商： | L4990 |
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 synchronizedto
an externalfrequency(slave).
As a master, the pin delivers positive pulses dur-
ing the ramp-down of the oscillator (see pin 2). In
slave operationthe circuit is edgetriggered. 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-
maticallythe master.
During the ramp-up of the oscillator the pin is
pulled low by a 600
μ
A generator. During the
ramp-down, that is when the pulse is released,
the 600
μ
A 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 examplesof synchroniz-
ing the L4990are given.
L4990
L4990
R
T
VREF
SYNC
SYNC
RCT
RCT
L4981A
(MASTER)
L4990
(SLAVE)
R
T
VREF
SYNC
RCT
R
OSC
C
OSC
C
T
L4990
(MASTER)
L4981A
(SLAVE)
SYNC
R
OSC
C
T
C
OSC
SYNC
(a)
(b)
(c)
R
T
D97IN494A
C
T
VREF
4
1
2
1
2
16
18
17
4
2
1
RCT
1
2
4
16
17
18
Figure 20. Synchronizingthe L4990.
Pin 2.
RCT (Oscillator). A resistor (R
T
) and a ca-
pacitor (C
T
), connectedas shown in fig. 21 setthe
operating frequencyf
osc
of theoscillator.
C
T
is charged through R
T
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
R
T
C
T
D
R
Q
600
μ
A
D97IN500B
V
REF
RCT
SYNC
CLK
2
4
1
Figure 21. Oscillatorand synchronizationinternalschematic.
L4990 - L4990A
9/24
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