NCP1442, NCP1443, NCP1444, NCP1445
The internal control circuitry, including the oscillator and
V
IN
linear regulator, requires a small amount of power even
when the switch is turned off. The specifications section of
V
V
CC
this datasheet reveals that the typical operating current, I ,
Q
SS
SS
due to this circuitry is 5.5 mA. Additional guidance can be
found in the graph of operating current vs. temperature. This
graph shows that IQ is strongly dependent on input voltage,
C
V , and the ambient temperature, T . Then:
IN
A
P
+ V
I
IN Q
BIAS
D2
D1
Since the onboard switch is an NPN transistor, the base
drive current must be factored in as well. This current is
R1
C1
drawn from the V pin, in addition to the control circuitry
IN
current. The base drive current is listed in the specifications
C2
C3
as DI /DI , or switch transconductance. As before, the
CC
SW
designer will find additional guidance in the graphs. With
that information, the designer can calculate:
I
CC
P
+ V
I
IN SW
D
DRIVER
DI
SW
Figure 39. Soft−Start
where:
= the current through the switch;
D = the duty cycle or percentage of switch on−time.
I
SW
Resistor R1 and capacitors C1 and C2 form the
compensation network. At turn on, the voltage at the V pin
C
starts to come up, charging capacitor C3 through Schottky
I
and D are dependent on the type of converter. In a
SW
diode D2, clamping the voltage at the V pin such that
boost converter,
C
switching begins when V reaches the V threshold,
typically 1.05 V (refer to graphs for detail over
temperature).
C
C
I
I
^ I D
LOAD
SW(AVG)
efficiency
V
* V
IN
OUT
D ^
V
+ V
)V
F(D2) C3
C
V
OUT
Therefore, C3 slows the startup of the circuit by limiting
In a flyback converter,
the voltage on the V pin. The soft−start time increases with
C
V
I
OUT LOAD
I
I
^
the size of C3.
SW(AVG)
V
efficiency
IN
Diode D1 discharges C3 when SS is low. If the shutdown
function is not used with this part, the cathode of D1 should
V
OUT
)
D ^
n
n
V
s V
IN
p
be connected to V .
OUT
IN
where:
n = number of turns in the transformer secondary winding.
Calculating Junction Temperature
To ensure safe operation of the NCP1442/3/4/5, the
designer must calculate the on−chip power dissipation and
determine its expected junction temperature. Internal
thermal protection circuitry will turn the part off once the
junction temperature exceeds 180°C ± 30°. However,
repeated operation at such high temperatures will ensure a
reduced operating life.
Calculation of the junction temperature is an imprecise
but simple task. First, the power losses must be quantified.
There are three major sources of power loss on the
NCP144X:
s
n = number of turns in the transformer primary winding.
p
The switch saturation voltage, V
, is the last major
(CE)SAT
source of on−chip power loss.
V
is the
(CE)SAT
collector−emitter voltage of the internal NPN transistor
when it is driven into saturation by its base drive current. The
value for V
can be obtained from the specifications
(CE)SAT
or from the graphs, as “Switch Saturation Voltage.” Thus,
P
^ V
I
D
SAT
(CE)SAT SW
Finally, the total on−chip power losses are:
• biasing of internal control circuitry, P
BIAS
P
+ P
)P
BIAS
)P
DRIVER SAT
D
• switch driver, P
DRIVER
• switch saturation, P
SAT
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