NCP1378 ON Semiconductor, NCP1378 Datasheet - Page 12

• Manufacturer Part Number: NCP1378
• Description: PWM Current-Mode Controller
• Manufacturer: ON Semiconductor

www.DataSheet4U.com
DataSheet U .com
4
Soft−Start
soften the constraints occurring in the power supply during
startup. It is activated during the power on sequence. As
soon as V
increased from nearly zero up to the maximum clamping
level (e.g. 1.0 V). The Soft−Start is also activated during
the overcurrent burst (OCP) sequence. Every restart
attempt is followed by a Soft−Start activation. Generally
speaking, the Soft−Start will be activated when V
up either from zero (fresh power–on sequence) or 5.5 V, the
latch–off voltage occurring during OCP.
Calculating the Vcc Capacitor
consumption as soon as V
a NCP1378 is used and drives a MOSFET with a 30 nC
total gate charge (Qg). The total average current is thus
made of ICC1 (1.0 mA) plus the driver current, Fsw x Qg
or 1.8 mA. The total current is therefore 2.8 mA. The DV
available to fully startup the circuit (e.g. never reach the
7.5 V UVLO during power on) is 8.4 – 7.5 = 0.9 V. We have
a capacitor that then needs to supply the NCP1378 with
2.8 mA during a given time until the auxiliary supply takes
The NCP1378 features an internal 1.0 ms Soft−Start to
The V
CC
CC
capacitor can be calculated knowing the IC
reaches VCC
CC
ON
Vcc
reaches VCC
, the peak current is gradually
Figure 27. Typical Waveforms in Short Circuit Conditions
ON
ON
. Suppose that
Vcc
min
CC
Vcc
http://onsemi.com
ramps
latch
NCP1378
12
over. Suppose that this time was measured at around 10 ms.
CV
31.1 mF. Select a 47 mF/25 V and this will fit. During the
latchoff phase, the current consumption drops to 220 mA.
We can now calculate how long this latchoff phase will last:
(7.5–5.5) x 47 m/220 u = 427 ms.
Protecting Pin 8 Against Negative Spikes
negative voltages that could appear on its pins. To avoid
any adverse latchup of the IC, we strongly recommend to
insert a resistor in series with pin8. This resistor prevents
from adversely latching the controller in case of negative
spikes appearing on the bulk capacitor during the
power−off sequence. A typical value of 6.8 kW/0.5 W is
suitable. This resistor does not dissipate any power since it
only sees current during the startup sequence and during
overload.
Operating Shots
120 VDC with a transformer featuring a 800 mH primary
inductance.
As any CMOS controller, NCP1378 is sensitive to
Below are some oscilloscope shots captured at Vin =
Driving Pulses
CC
Vcc
is calculated using the equation
C + Dt · i
DV
or C w