NCP1212 ON Semiconductor, NCP1212 Datasheet - Page 17

• Manufacturer Part Number: NCP1212
• Description: Current Mode Pwm Controller For Both Forward And Flyback Converters
• Manufacturer: ON Semiconductor

I
20 mA typical.
The discharging time for 0.22 mF Soft−Start capacitor is:
Figure 31 to help the designers to determine the capacitance
for their selected switching frequency.
SD
T DIS +
The Switching Frequency against C
−I
2. Determine the PWM Switching Frequency
SS
The switching waveform is generated by the action
of charging and discharging by internal current
sources to a capacitor connected at C
relationship of the switching frequency and the value
of C
where:
I
D is the selected Maximum Duty Cycle, 48% or
82%;
C
V
Duty Cycle selection, 2.5 V for 48% Maximum
Duty Cycle and 3.8 V for 82% Maximum Duty
Cycle.
chg
is the internal current source for C
T
th
0.22 mF
is the capacitor connected to C
is the threshold voltage for different Maximum
T
is the charging current to C
is governed by the equation below:
F SW +
(5.0 V * 0.6 V * 0.5 V)
20 mA
C T
I chg
(V th * 1)
Figure 41. SS/DMAX Pin Discharging Waveform
T
D
, 278 mA typical;
T
T
SS
pin;
is shown in
T
+ 42.9 ms
discharging,
, pin 4. The
http://onsemi.com
36 ms
NCP1212
17
The discharging waveform on SS/DMAX pin is shown in
Figure 41. The discharging time from 4.0 V to 0.6 V is
measured as 36 ms from Figure 41. By interpolation,
discharging time can be estimated as about 41.3 ms when
output is overload which agreed with the calculated result.
3. Determine the BOK Thresholds
Brownout detect thresholds are determined by a
resistors network that monitors part of the bulk
capacitor voltage at BOK pin. Equations below
illustrate the calculation of the resistors value for the
network.
Where V
and lower bulk capacitor voltage for brownout
detection.
Assume V
select 3.3 kW for R
calculated to be 576 kW.
R Upper ) R Lower +
R Lower +
3.4 V
Bulk_H
Bulk_H
[1.21 V(V Bulk_H * V Bulk_L )]
and V
= 212 Vdc and V
(45 mA
Lower
Bulk_L
(V Bulk_H * V Bulk_L )
then R
are the desired upper
V Bulk_H )
45 mA
Bulk_L
Upper
= 186 Vdc,
can be