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參數(shù)資料
| 型號： | NCP1351 |
| 廠商： | ON SEMICONDUCTOR |
| 英文描述： | Variable Off Time PWM Controller |
| 中文描述： | 變關閉時間PWM控制器 |
| 文件頁數(shù)： | 8/19頁 |
| 文件大?。?/td> | 222K |
| 代理商： | NCP1351 |
NCP1351
http://onsemi.com
8
APPLICATION INFORMATION
The Negative Sensing Technique
Standard currentmode controllers use the positive
sensing technique as portrayed by Figure 4. In this
technique, the controller detects a positive voltage drop
across the sense resistor, representative of the flowing
current. Unfortunately, this solution suffers from the
following drawbacks:
1. Difficulties to precisely adjust the peak current. If
1 V is the maximum sense level, you must
combine low valued resistors to reach the exact
limit you need.
2. The voltage developed across the sense resistor
subtracts from the gate voltage. If your VCC
(min)
is 7 V, then the actual gate voltage at the end of the
on time, assuming a full load condition, is 7 V –
1 V = 6 V.
3. The current in the sense resistor also includes the
C
iss
current at turnon. This narrow spike often
disturbs the controller and requires adequate
treatment through a LEB circuitry for instance.
Figure 5 represents the negative current sense technique.
In this simplified example, the source directly connects to
the controller ground. Hence, if V
CC
is 8 V, the effective
gatesource voltage is very close to 8 V: no sense resistor
drop. How does the controller detect a negative excursion
In lack of primary current, the voltage on the CS pin reaches
R
offset
x I
CS
. Let us assume that these elements lead to have
1 V on this pin. Now, when the power MOSFET activates,
the current flows via the sense resistor and develop a
negative voltage by respect to the controller ground. The
voltage seen on the CS is nothing else than a positive voltage
(R
offset
x I
CS
) plus the voltage across the sense resistor which
is negative. Thus, the CS pin voltage goes low as the primary
current increases. When the result reaches the threshold
voltage (around 20 mV), the comparator toggles and resets
the main latch. Figure 3 details how the voltage moves on the
CS pin on a 1351 demoboard, whereas Figure 7 zooms on
the sense resistor voltage captured by respect to the
controller ground.
The choice of these two elements is simple. Suppose you
want to develop 1 V across the sense resistor. You would
select the offset resistor via the following formula:
1
ICS
270
If you need a peak current of 2 A, then, simply apply the
ohm law to obtain the sense resistor value:
1
Ipeak_max
Due to the circuit flexibility, suppose you only have access
to a 0.33 resistor. In that case, the peak current will exceed
the 2 A limit. Why not changing the offset resistor value
then To obtain 2 A from the 0.33 resistor, you should
develop:
The offset resistor is thus derived by:
Vsense
RsenseIpeak_max
Roffset
1
3.7 k
(eq. 1)
Rsense
1
2
0.5
(eq. 2)
0.33
2
660 mV
(eq. 3)
Roffset
0.66
ICS
0.66
270
2.44 k
(eq. 4)
If reducing the sense resistor is of good practice to
improve the efficiency, we recommend to adopt sense values
between 0.5 V and 1 V. Reducing the voltage below these
levels will degrade the noise immunity.
Figure 4. Positive CurrentSense Technique
C
Bulk
+
+
I
Lp
I
Lp
I
Lp
Reset
Peak
Setpoint
V
gs
I
Lp
V
sense
L
P
DRV
CS
R
sense
GND
Figure 5. A Simplified Circuit of the Negative Sense
Implementation
C
Bulk
+
+
I
Lp
I
Lp
Reset
V
sense
V
th
I
Lp
V
DD
L
P
DRV
CS
R
offset
GND
V
offset
+
ICS
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| NCP1351ADAPGEVB | 功能描述:電源管理IC開發(fā)工具 NCP1351 57 W ADAPTER EVB RoHS:否 制造商:Maxim Integrated 產(chǎn)品:Evaluation Kits 類型:Battery Management 工具用于評估:MAX17710GB 輸入電壓: 輸出電壓:1.8 V |
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| NCP1351BDR2G | 功能描述:電流型 PWM 控制器 OSPI PWM CONTROLLER RoHS:否 制造商:Texas Instruments 開關頻率:27 KHz 上升時間: 下降時間: 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 安裝風格:SMD/SMT 封裝 / 箱體:TSSOP-14 |
| NCP1351BPG | 功能描述:電流型 PWM 控制器 VRIABLE OFF TM CNTRL RoHS:否 制造商:Texas Instruments 開關頻率:27 KHz 上升時間: 下降時間: 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 安裝風格:SMD/SMT 封裝 / 箱體:TSSOP-14 |
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