NCP1212
http://onsemi.com
16
APPLICATION INFORMATION AND TYPICAL WAVEFORMS
The NCP1212 is an ideal choice for next generation
isolated fix switching frequency forward mode converters
that only need few external components to complete the
system. Converting your existing application from using
UC384X controllers to NCP1212 is easy and simple. In
below is a description on how to determine external
components value for a typical application example. For the
schematic of the application, please refer to Figure 1 in this
data sheet.
Finding the external component values can be broken
down into several steps as introduced below:
1. Select the maximum Duty Cycle for forward mode
operation and calculate the Soft−Start time.
Select the system, operate in forward mode with
82% maximum Duty Cycle. Only a capacitor is
required at SS/DMAX pin and the Soft−Start time is
determined by the capacitor, C
SS
. Its value is given
by the equation below:
C
SS
+
I T
SS
V
1
* V
2
where:
I is an 8.0 mA constant current source flow out of the
SS/DMAX pin;
T
SS
is the required Soft−Start time;
V
1
is the Upper Threshold Voltage in the oscillator
block and which is effectively controlling the PWM
maximum Duty Cycle at gate driver output.
Soft−Start block will have no effect to the PWM
operation once SS/DMAX pin voltage reaches this
threshold. This threshold voltage is 2.5 V with 48%
maximum Duty Cycle;
V
2
is about 0.4 V (1.0 V minus one diode drop)
which is the Lower Voltage Threshold for the PWM
operation. There will be no PWM gate driver output
before SS/DMAX pin voltage attains this threshold.
For example, the required Soft−Start time is 50 ms,
the timing capacitor, C
SS
can be calculated as:
C
SS
+
8 mA 50 ms
2.5 V * 0.4 V
+ 0.182 mF
In this case, a 0.22 mF capacitor is used for this
application and the Soft−Start time is calculated as
57.75 ms. The charging waveform at SS/DMAX pin
is shown in Figure 40. From the captured waveform,
the charge time from 0 V to 4.0 V is 115.2 ms and for
the voltage charging up to 2.5 V, i.e. hitting the
Upper Threshold Voltage, the elapsed time is about
70 ms that matched with the theoretical calculation
closely.
Figure 40. SS/DMAX Pin Charging Waveform
115.2 ms
4.0 V
Overload condition is signified by current sense input
voltage hitting the Maximum Current Sense Threshold,
V
CS
. To avoid false trigger that may happen during transient
load change, C
SS
starts to discharge by an internal current
source of 20 mA, I
SD
−I
SS
and the overload protection will
only be issued until the voltage at SS/DMAX pin falls below
0.5 V. The discharging time, T
DIS
for 0.22 mF Soft−Start
capacitor is given by:
T
DIS
+
C
SS
(V
ref
* V
D
* V
OL
)
I
SD
* I
SS
where:
C
SS
is the Soft−Start timing capacitor;
V
ref
is the internal reference voltage, 5.0 V typical;
V
D
is the internal diode forward voltage on between the
reference voltage and SS/DMAX pin in IC internal, is 0.6 V
typical;
V
OL
is the overload threshold voltage. Refer to Figure 39
Overload Detection Block Diagram, the overload threshold
voltage is 0.5 V typical;