NCP1217, NCP1217A
www.onsemi.com
10
Figure 18. Typical Waveforms in Short Circuit Conditions
VCC
ON
= 12.8 V
V
CC
DRIVING
PULSES
VCC
min
= 7.6 V
VCC
latch
= 5.6 V
Calculating the V
CC
Capacitor
The V
CC
capacitor can be calculated knowing the IC
consumption as soon as V
CC
reaches 12.8 V. Suppose that a
NCP1217P065 is used and drives a MOSFET with a 30 nC
total gate charge (Qg). The total average current is thus
made of ICC1 (750 A) plus the driver current,
Fsw * Qg + 1.95 mA
. The total current is therefore 2.7 mA.
The V available to fully startup the circuit (e.g. never reach
the 8.2 V VCC
min
during power on) is
13.7−8.2 + 5.5 V
best case or 4.9 V worse case
(11.9−7.0)
. We have a
capacitor that then needs to supply the NCP1217 with
2.7 mA during a given time until the auxiliary supply takes
over. Suppose that this time was measured at around 15 ms.
CV
CC
is calculated using the equation C +
t·i
V
or
C w 8.3 F.
Select a 22 F/25 V and this will fit.
Skipping Cycle Mode
The NCP1217 automatically skips switching cycles when
the output power demand drops below a given level. This is
accomplished by monitoring the FB pin. In normal
operation, pin 2 imposes a peak current accordingly to the
load value. If the load demand decreases, the internal loop
asks for less peak current. When this setpoint reaches a
determined level (Vpin 1), the IC prevents the current from
decreasing further down and starts to blank the output
pulses: the IC enters the so−called skip cycle mode, also
named controlled burst operation. The power transfer now
depends upon the width of the pulse bunches (Figure 20).
Suppose we have the following component values:
Lp, primary inductance = 350 H
Fsw, switching frequency = 65 kHz
Ip skip = 600 mA (or 333 mV/Rsense)
The theoretical power transfer is therefore:
1
2
·Lp·Ip
2
·Fsw+ 4.1 W. If this IC enters skip cycle
mode with a bunch length of 10 ms over a recurrent
period of 100 ms, then the total power transfer is:
4.1 * 0.1 + 410 mW
.
To better understand how this skip cycle mode takes place,
a look at the operation mode versus the FB level
immediately gives the necessary insight.
Figure 19.
SKIP CYCLE OPERATION
I
P(min)
= 333 mV/R
SENSE
NORMAL CURRENT
MODE OPERATION
FB
1 V
4.2 V, FB Pin Open
3.2 V, Upper
Dynamic Range
Time
When FB is above the skip cycle threshold (1.0 V by
default), the peak current cannot exceed 1.0 V/Rsense.
When the IC enters the skip cycle mode, the peak current
cannot go below Vpin1/3.3. The user still has the flexibility
to alter this 1.0 V by either shunting pin 1 to ground through
a resistor or raising it through a resistor up to the desired
level. In this later case, care must be taken to keep sufficient
margin between this pin 1 adjustment level and the latchoff
level. Grounding pin 1 permanently invalidates the skip
cycle operation.
