NCP1351BPG ON Semiconductor, NCP1351BPG Datasheet - Page 10

• Manufacturer Part Number: NCP1351BPG
• Description: IC CTRLR PWM PROG CM OTP 8DIP
• Manufacturer: ON Semiconductor

Specifications of NCP1351BPG

• Output Isolation: Isolated
• Frequency Range: Adjusting
• Voltage - Input: 9.5 ~ 28 V
• Operating Temperature: -25°C ~ 125°C
• Package / Case: 8-DIP (0.300", 7.62mm)
• Number Of Outputs: 1
• Output Voltage: - 0.3 V to + 20 V
• Output Current: 400 mA
• Mounting Style: Through Hole
• Maximum Operating Temperature: + 150 C
• Fall Time: 100 ns
• Rise Time: 90 ns
• Synchronous Pin: No
• Topology: Flyback
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

The Negative Sensing Technique
sensing technique as portrayed by Figure 6. 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:
In this simplified example, the source directly connects to
the controller ground. Hence, if V
gate-source 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
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
is negative. Thus, the CS pin voltage goes low as the primary
offset
Standard current-mode controllers use the positive
Figure 7 represents the negative current sense technique.
offset
Figure 6. Positive Current-Sense Technique
1. Difficulties to precisely adjust the peak current. If
2. The voltage developed across the sense resistor
3. The current in the sense resistor also includes the
+
C
x I
I
1 V is the maximum sense level, you must
combine low valued resistors to reach the exact
limit you need.
subtracts from the gate voltage. If your VCC
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.
C
disturbs the controller and requires adequate
treatment through a LEB circuitry for instance.
x I
Bulk
Lp
iss
CS
CS
. Let us assume that these elements lead to have
current at turn-on. This narrow spike often
) plus the voltage across the sense resistor which
Reset
I
Lp
I
Lp
+
-
Peak
Setpoint
DRV
CS
CC
R
GND
is 8 V, the effective
sense
V
gs
L
P
APPLICATION INFORMATION
I
Lp
V
sense
http://onsemi.com
(min)
NCP1351
10
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 9 zooms on
the sense resistor voltage captured by respect to the
controller ground.
want to develop 1 V across the sense resistor. You would
select the offset resistor via the following formula:
ohm law to obtain the sense resistor value:
to a 0.33 W 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 W resistor, you should
develop:
The offset resistor is thus derived by:
V sense + R sense I peak_max + 0.33
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.
V
+
C
Figure 7. A Simplified Circuit of the Negative Sense
The choice of these two elements is simple. Suppose you
If you need a peak current of 2 A, then, simply apply the
Due to the circuit flexibility, suppose you only have access
If reducing the sense resistor is of good practice to
I
offset
Bulk
Lp
R offset +
R sense +
R offset +
CS
V
R
sense
offset
V
DD
ICS
I CS
0.66
I CS
I peak_max
+
1
V
Implementation
th
+
+
-
+
1
270 m
270 m
0.66
1
+
Reset
+ 3.7 kW
+ 2.44 kW
I
1
2
Lp
+ 0.5 W
DRV
GND
2 + 660 mV
L
P
I
Lp
(eq. 1)
(eq. 2)
(eq. 3)
(eq. 4)