ncp1239f ON Semiconductor, ncp1239f Datasheet - Page 29

• Manufacturer Part Number: ncp1239f
• Description: Lowstandby High Performance Pwm Controller
• Manufacturer: ON Semiconductor

input voltage and Dt is the total delay in turning off the
power switch.
term in the Imax equation for a precise limitation of the peak
current. A current source (I
voltage flows out of Pin 9. Since Pin 5 receives a voltage
proportional to the input voltage for brown−out detection,
I
resistor Rcomp can be connected between Pin 9 and the
positive terminal of Rsense, so that Pin 9 monitors the
following voltage:
I
voltage simplifies as follows:
I
see parameters specification table) and V
the input voltage (V
The voltage V
Vocp. When V
threshold current (Ipth) is deducted from:
Vopl + (Rsense @ Ipth) ) (Rcomp @ 80 mA V @ k BO @ Vin)
The propagation delay (Dt) produces overcurrents (DI
voltage. As a consequence, the actual maximum current and then the power limit gets higher when the AC line increases.
pin9
pin9
pin9
The NCP1239 enables the compensation of the second
I max +
V pin9 + [Rsense @ (Ip ) I pin9 )] ) (Rcomp @ I pin9 )
being small compared to the inductor current, the Pin 9
is proportional to the Pin 5 voltage (80 mA/V*V
is proportional to the input voltage too. An external
V pin9 + (Rsense @ Ip) ) (Rcomp @ I pin9 )
Rsense
Vocp
I pin9 + 80 mA V @ k BO @ Vin
pin9
pin9
) Vin @ dt
is compared to the internal reference
pin5
reaches Vocp, the corresponding
= k
Lp
High Input
Voltage
BO
pin9
, where Vin is the converter
*Vin). Finally,
) proportional to the Pin 5
DI
HL
pin5
is a portion of
Dt
LL
http://onsemi.com
at low line, DI
pin5
Figure 49.
Low Input
Voltage
NCP1239
–
29
Dt
Then,
Taking into account the overcurrent resulting from the
propagation delays, the maximum current is finally:
the current limit is made constant in the whole input voltage
range (Imax = Vocp/Rsense).
As an example, let’s assume that:
− the minimum input voltage for operation is 100 V =>
k
− Rsense is 0.25 W,
− Lp=500 mH,
− The total propagation delays are Dt = 350 ns,
Then, the Rcomp resistor should be:
HL
BO
Actual Peak Current
I max +
Rcomp +
Choosing Rcomp so that
at high line in the figure) that are proportional to the input
=0.5/100=0.005,
Ipth +
DI
Rsense
LL
Vocp
80 m @ k BO @ Lp
Vopl * (Rcomp @ 80 mA V @ k BO @ Vin)
dt @ Rsense
Wished Maximum
*
Vopl/Rsense
Peak Current
Rcomp @ 80 mA V @ k BO @ Vin
+
Rcomp @ 80 mA V @ k BO
80 m @ 0.005 @ 500 m
Rsense
Rsense
350 n @ 0.25
Rsense
) Vin @ dt
[ 438 W
Lp
) dt
Lp
.
,