NCP1203
12.8 V
7.8 V
V
CC
4.9 V
DRIVING PULSES
Figure 15. Typical Waveforms in Short Circuit Conditions
Calculating the V
CC
Capacitor
The theoretical power transfer is therefore:
1 · Lp · Ip2 · Fsw
+
3.8 W
2
The V
CC
capacitor can be calculated knowing the IC
consumption as soon as V
CC
reaches 12.8 V. Suppose that a
NCP1203P60 is used and drives a MOSFET with a 30 nC
total gate charge (Qg). The total average current is thus made
of ICC1 (700
mA)
plus the driver current, Fsw x Qg or
1.8 mA. The total current is therefore 2.5 mA. The
DV
available to fully startup the circuit (e.g. never reach the
7.8 V UVLO during power on) is 12.8–7.8 = 5 V. We have
a capacitor who then needs to supply the NCP1203 with
2.5 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
+
Dt
· i
or
C
w
7.5
mF
. Select a 22
mF/16
V and this will fit.
Skipping Cycle Mode
DV
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:
3.8 . 0.1
+
380 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:
FB
4.2 V, FB Pin Open
NORMAL CURRENT
MODE OPERATION
3.2 V, Upper
Dynamic Range
The NCP1203 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 17).
Suppose we have the following component values:
Lp, primary inductance = 350
mH
Fsw , switching frequency = 61 kHz
Ip skip = 600 mA (or 333 mV/Rsense)
SKIP CYCLE OPERATION
I
P(min)
= 333 mV/R
SENSE
1V
Figure 16.
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/Rsense. 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. Grounding pin 1 permanently
invalidates the skip cycle operation. However, given the
extremely low standby power the controller can reach, the
PWM in no−load conditions can quickly enter the minimum
t
on
and still transfer too much power. An instability can take
place. We recommend in that case to leave a little bit of skip
level to always allow 0% duty cycle.
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