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參數(shù)資料
| 型號： | L6997 |
| 廠商： | 意法半導(dǎo)體 |
| 英文描述： | STEP DOWN CONTROLLER FOR LOW VOLTAGE OPERATIONS |
| 中文描述： | 降壓控制器低電壓運(yùn)作 |
| 文件頁數(shù)： | 18/23頁 |
| 文件大小： | 291K |
| 代理商： | L6997 |
L6997
18/23
Eq 24
α
is the temperature coefficient of RDS
ON
(typically,
α
= 510
-3
°C
-1
for these low-voltage classes) and T the
admitted temperature rise. It is worth noticing, however, that generally the lower RDS
ON
, the higher is the gate
charge Q
G
, which leads to a higher gate drive consumption. In fact, each switching cycle, a charge Q
G
moves
from the input source to ground, resulting in an equivalent drive current:
Eq 25
The SCHOTTKY diode to be placed in parallel to the synchronous rectifier must have a reverse voltage V
RRM
greater than VIN
MAX
; for low current application the SCHOTTKY is not necessary to increase the efficiency. In
order to use less space than possible, a double MOSFET in a single package is chosen: STS5DNF20V
3.5 Output voltage setting
The first step is choosing the output divider to set the output voltage. To select this value there isn't a criteria,
but a low divider network value (around 100
) decries the efficiency at low current; instead a high value divider
network (100K
) increase the noise effects. A network divider values from 1K
to 10K
is right. We chose:
R3 = 1K
R2 = 1.1K
The device output voltage is adjustable by connecting a voltage divider from output to VSENSE pin. Minimum
output voltage is V
OUT
=VREF=0.6V. Once output divider and frequency divider have been designed as to obtain
the required output voltage and switching frequency, the following equation gives the smallest input voltage,
which allows L6997 to regulate (which corresponds to T
OFF
=T
OFFMIN
):
Eq 26
3.6 Voltage Feedforward
From the equations 1,2 3 choosing the switching frequency around 270kHz it can be selected the input divider.
For example:
R3 = 470K
R4 = 8.5K
3.7 Current limit resistor
From the equation 8 it can be set the valley current limit considering the STS5DNF20V Ultra logic Level Mosfet
with a current around 5A:
R8 = 120K
3.8 Integrator capacitor
Let it be F
U
= 15kHz, V
OUT
= 1.25V.
Since V
REF
= 0.6V, from equation 2, of the device description, it follows
α
O
UT
= 0.348 and, from equation 5 it
follows C = 250pF. The output ripple is around 22mV, so the system doesn't need the second integrator capac-
itor.
RDS
ON
P
(
Iout
2
1
α
T
+
)
------------------------------------------------
=
Iq
Qg F
SW
=
δ
1
α
α
OUT
--------------
K
T
OFF,MIN
-------------------------
MAX
---------------------------------------------
–
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|---|---|
| L6997-28 | 制造商:Johanson Manufacturing 功能描述:TUNING ELEMENT |
| L6997-5 | 制造商:Johanson Manufacturing 功能描述:TUNING ELEMENT; Accessory Type:Tuning Element; For Use With:High Quality Variable Reactance Tuning Microwave Cavities, Filters & other Resonant Structures |
| L6997-9 | 制造商:Johanson Manufacturing 功能描述:TUNING ELEMENT |
| L6997D | 功能描述:DC/DC 開關(guān)控制器 Adjustable Step-Down RoHS:否 制造商:Texas Instruments 輸入電壓:6 V to 100 V 開關(guān)頻率: 輸出電壓:1.215 V to 80 V 輸出電流:3.5 A 輸出端數(shù)量:1 最大工作溫度:+ 125 C 安裝風(fēng)格: 封裝 / 箱體:CPAK |
| L6997DTR | 功能描述:DC/DC 開關(guān)控制器 Adjustable Step-Down RoHS:否 制造商:Texas Instruments 輸入電壓:6 V to 100 V 開關(guān)頻率: 輸出電壓:1.215 V to 80 V 輸出電流:3.5 A 輸出端數(shù)量:1 最大工作溫度:+ 125 C 安裝風(fēng)格: 封裝 / 箱體:CPAK |
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