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參數(shù)資料
| 型號(hào)： | L5991AD |
| 廠商： | 意法半導(dǎo)體 |
| 英文描述： | PRIMARY CONTROLLER WITH STANDBY |
| 中文描述： | 主控制器具有待機(jī) |
| 文件頁(yè)數(shù)： | 8/19頁(yè) |
| 文件大?。?/td> | 192K |
| 代理商： | L5991AD |
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STANDBYFUNCTION
The standby function, optimized for flyback topol-
ogy, automatically detects a light load condition
for the converter and decreases the oscillator fre-
quency on that occurrence. The normal oscillation
frequency is automaticallyresumed when the out-
put load builds up and exceeds a defined thresh-
old.
This function allows to minimize power losses re-
lated to switching frequency, which represent the
majority of losses in a lightly loaded flyback, with-
out givingup the advantagesof a higherswitching
frequencyat heavy load.
This is accomplished by monitoring the output of
the Error Amplifier (V
COMP
) that depends linearly
on thepeak primary current, exceptfor an offset.
If the the peak primary currentdecreases(as a re-
sult of a decreaseof the power demandedby the
load) and V
COMP
falls below a fixed threshold
(V
T1
), the oscillator frequency will be set to a
lower value (f
SB
). When the peak primary current
increases and V
COMP
exceedsa secondthreshold
(V
T2
) the oscillator frequency is set to the normal
value (f
osc
). An appropriate hysteresis (V
T2
-V
T1
)
prevents
undesired frequency change when
power is such that V
COMP
moves close to the
threshold.This operationis shown in fig.20.
Both the normal and the standby frequency are
externally programmable. V
T1
and V
T2
are inter-
nally fixed but it is possible to adjust the thresh-
olds in terms of inputpower level.
APPLICATIONINFORMATION
Detailed Pin Function 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 falling edge of the oscillator (see pin 2). In
slave operationthe circuit is edge triggered. Refer
to fig. 22 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 internal sink current gener-
ator. During the falling edge, that is when the
pulse is released, the 600
μ
A pull-down is discon-
nected. The pin becomes a generator whose
source capability is typically 7mA (with a voltage
still higher than3.5V).
In fig. 21, some practical examples of synchroniz-
ing the L5991are given.
Since the device automatically diminishes its op-
erating frequency under light load conditions, it is
reasonable to suppose that synchronization will
referto normal operationand not to standby.
Pin 2.
RCT (Oscillator).Two resistors (R
A
and R
B
)
and one capacitor (C
T
), connected as shown in
fig. 22, allow to set separately the operating fre-
quency of the oscillator in normal operation (f
osc
)
and in standbymode (f
SB
).
C
T
is chargedfrom Vrefthrough R
A
and R
B
in nor-
mal operation (STANDBY = HIGH), through R
A
only in standby ( STANDBY = LOW). See pin 16
descriptionto see howthe STANDBYsignal isgen-
erated.
When the voltage on C
T
reaches 3V, the capaci-
tor is quickly internally discharged. As the voltage
has droppedto 1V it startsbeing charged again.
1
2
3
4
VCOMP
Pin
f
osc
f
SB
Stand-by
Normaloperation
V
T1
P
NO
P
SB
V
T2
Figure20. Standby dynamic operation.
L5991
L5991
R
A
VREF
SYNC
SYNC
RCT
RCT
L4981A
(MASTER)
L5991
(SLAVE)
R
A
VREF
SYNC
RCT
R
OSC
C
OSC
C
T
L5991
(MASTER)
16
L4981A
(SLAVE)
SYNC
R
OSC
C
T
C
OSC
SYNC
(a)
(b)
(c)
R
A
D97IN728A
C
T
VREF
4
1
2
1
2
16
18
17
4
2
1
RCT
1
2
4
16
17
18
ST-BY
16
R
B
ST-BY
16
R
B
R
B
ST-BY
Figure 21. Synchronizingthe L5991.
L5991 - L5991A
8/23
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| L5991AD13TR | 功能描述:電流型 PWM 控制器 Prog Current Mode RoHS:否 制造商:Texas Instruments 開(kāi)關(guān)頻率:27 KHz 上升時(shí)間: 下降時(shí)間: 工作電源電壓:6 V to 15 V 工作電源電流:1.5 mA 輸出端數(shù)量:1 最大工作溫度:+ 105 C 安裝風(fēng)格:SMD/SMT 封裝 / 箱體:TSSOP-14 |
| L5991D | 功能描述:電流型 PWM 控制器 Prog Current Mode RoHS:否 制造商:Texas Instruments 開(kāi)關(guān)頻率:27 KHz 上升時(shí)間: 下降時(shí)間: 工作電源電壓:6 V to 15 V 工作電源電流:1.5 mA 輸出端數(shù)量:1 最大工作溫度:+ 105 C 安裝風(fēng)格:SMD/SMT 封裝 / 箱體:TSSOP-14 |
| L5991D013TR | 功能描述:電流型 PWM 控制器 Prog Current Mode RoHS:否 制造商:Texas Instruments 開(kāi)關(guān)頻率:27 KHz 上升時(shí)間: 下降時(shí)間: 工作電源電壓:6 V to 15 V 工作電源電流:1.5 mA 輸出端數(shù)量:1 最大工作溫度:+ 105 C 安裝風(fēng)格:SMD/SMT 封裝 / 箱體:TSSOP-14 |
| L5993 | 功能描述:開(kāi)關(guān)變換器、穩(wěn)壓器與控制器 Prog Current Mode RoHS:否 制造商:Texas Instruments 輸出電壓:1.2 V to 10 V 輸出電流:300 mA 輸出功率: 輸入電壓:3 V to 17 V 開(kāi)關(guān)頻率:1 MHz 工作溫度范圍: 安裝風(fēng)格:SMD/SMT 封裝 / 箱體:WSON-8 封裝:Reel |
| L5993D | 功能描述:開(kāi)關(guān)變換器、穩(wěn)壓器與控制器 Prog Current Mode RoHS:否 制造商:Texas Instruments 輸出電壓:1.2 V to 10 V 輸出電流:300 mA 輸出功率: 輸入電壓:3 V to 17 V 開(kāi)關(guān)頻率:1 MHz 工作溫度范圍: 安裝風(fēng)格:SMD/SMT 封裝 / 箱體:WSON-8 封裝:Reel |
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