NCP1217A ON Semiconductor, NCP1217A Datasheet - Page 12

• Manufacturer Part Number: NCP1217A
• Description: Enhanced PWM Current-Mode Controller
• Manufacturer: ON Semiconductor

Figure 22. Inserting a Resistor in Series with the Current
a duty cycle max at 74%. Over a 65 kHz frequency, for
instance, it corresponds to a 254 mV/ms ramp. In our
FLYBACK design, let’s suppose that our primary
inductance Lp is 350 mH, delivering 12 V with a Np:Ns ratio
of 1:0.1. The OFF time primary current slope is thus given
by:
projected over an Rsense of 0.1 W, for instance. If we select
75% of the downslope as the required amount of ramp
compensation, then we shall inject 27 mV/ms. Our
internal compensation being of 254 mV/ms, the divider
ratio (divratio) between Rcomp and the 19 kW is 0.106.
A
19 k · divratio
(1−divratio)
In the NCP1217, the ramp features a swing of 2.9 V with
Sense Information Brings Ramp Compensation
few
(Vout ) Vf) ·
−
+
Setpoint
Figure 24. When Vadj is Pulled Above 3.1 V, NCP1217 Permanently Latches−Off the Output Pulses
From
lines
Lp
+ 2.26 kW
L.E.B.
VCC
VCC
of
Duty Cycle Typ = 74%
Np
Ns
VCC
+ 371 mA ms
ON
latch
algebra
min
.
= 12.8 V
= 7.6 V
= 5.6 V
19 k
CS
2.9 V
0 V
V
Drv
Adj
to
CC
Rcomp
or
determine
37 mV ms
adj level
Fault brings adj above latching level
Default
NCP1217, NCP1217A
Rsense
Driver
Pulses
Rcomp:
http://onsemi.com
when
Reset level
12
The startup current source keeps the
device latched until reset occurs.
Latched−off
Latching Off the NCP1217
through a simple PNP bipolar transistor as depicted by
Figure 23. When OFF, Q1 is transparent to the operation.
When forward biased, the transistor pulls the Adj pin toward
V
above the latching level (typical 3.1 V). Figure 23 shows
how to wire the bipolar transistor to activate the latchoff. A
typical candidate for Q1 could be an MMBT3906 from
ON Semiconductor.
CC
Total latched shutdown can easily be implemented
Off
Figure 23. A Simple Bipolar Transistor Totally
and permanently latches−off the IC as soon Vadj goes
Disables the IC
Q1
Time
Time
Time
Rlimit
1
2
3
4
V
CC
8
7
6
5
CV
CC