ncp1239f ON Semiconductor, ncp1239f Datasheet - Page 24

• Manufacturer Part Number: ncp1239f
• Description: Lowstandby High Performance Pwm Controller
• Manufacturer: ON Semiconductor

becomes easy to evaluate the burst period and its duty−cycle.
This can be done by properly identifying all time events on
Figure 42 and applying the classical formula: t = C * DV / i.
To simplify, let’s consider t1 starts while V
Then:
•
•
•
•
•
which leads to Tfault = 1793 ms. If Tpulse = 100 ms, then
our burst duty−cycle equals 100/(1869 + 100) ≈ 5%, which
is excellent.
V
calculated in the case where Tpulse exactly equals the active
phase duration (switching period when V
V
CC
CCON
If by design we have selected a 47 mF V
The total period duration is thus the sum of all these events
In fact, the calculation assumption, t1 starts while
t1: I = I
t1 = 505 ms
t2: I = 3.6 mA, DV= 16.4 – 6.9 = 9.5 V ! t2 = 124 ms
t3: I = 400 mA, DV= 16.4 – 11.2 = 5.2 V ! t3 = 611 ms
t’1 = t1= 505 ms
t’2 = t2 = 124 ms
= V
Pin 3
Drv
to V
V
CCOFF
CC
CC
CCOFF
3 = 400 mA, DV= 11.2 – 6.9 = 3 V !
When Vpin3 exceeds 2.4 V, NCP1239 permanently latches−off the output pulses0until its V
illustrate a case where a thermistor supplied by REF5V is connected to Pin 3 to detect excessive temperatures of the application
(refer to application schematic).
, gives the worse case since the duty cycle is
).
V
CCOFF
CC
Latch−off phase level
Logic reset level
CC
CC
decreases from
= V
capacitor, it
Stop!
V
CCOFF
CCON
http://onsemi.com
Figure 43.
NCP1239
.
24
2.4 V
In fact, Tpulse is generally:
(Major) Fault Detection and Latched Off Mode
permanently monitors the “Fault Detect” pin level. If for any
reason this level exceeds 2.4 V (typical), the part
immediately stops oscillating and stays latched off until the
user cycles down the power supply. This enables the SMPS
designer to externally shut down the part in particular when
a major default occurs, e.g. an Overvoltage Protection
(OVP). Figure 43 shows what happens when the part is
latched:
The user has unplugged, reset!
The NCP1239 features a fast comparator that
− shorter than the switching phase period. In this
case, t1 is longer since the latched off phase starts
earlier (at a V
consequence, the final duty cycle is lower than
previously estimated,
− longer than the switching phase period. In this case,
the circuit detects an overload condition simply
because V
the fault timer has elapsed. Tpulse is lower than 100
ms and as a result the duty cycle is also lower.
CC
drops below V
CC
CC
goes below 4 V. The figure can
higher than V
CCOFF
(11.2 V) before
CCOFF
). As a