NCP1445 ONSEMI [ON Semiconductor], NCP1445 Datasheet - Page 16

• Manufacturer Part Number: NCP1445
• Description: 4.0 A 280 kHz/560 kHz Boost Regulators
• Manufacturer: ONSEMI [ON Semiconductor]

V
compensation network. At turn on, the voltage at the V
starts to come up, charging capacitor C3 through Schottky
diode D2, clamping the voltage at the V
switching begins when V
typically 1.05 V (refer to graphs for detail over
temperature).
the voltage on the V
the size of C3.
function is not used with this part, the cathode of D1 should
be connected to V
Calculating Junction Temperature
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
repeated operation at such high temperatures will ensure a
reduced operating life.
but simple task. First, the power losses must be quantified.
There are three major sources of power loss on the
NCP144X:
SS
IN
Resistor R1 and capacitors C1 and C2 form the
Therefore, C3 slows the startup of the circuit by limiting
Diode D1 discharges C3 when SS is low. If the shutdown
To ensure safe operation of the NCP1442/3/4/5, the
Calculation of the junction temperature is an imprecise
biasing of internal control circuitry, P
switch driver, P
switch saturation, P
D1
IN
DRIVER
V C + V F(D2) )V C3
C
Figure 39. Soft−Start
.
pin. The soft−start time increases with
SAT
SS
D2
C3
C
V
V
reaches the V
CC
C
NCP1442, NCP1443, NCP1444, NCP1445
C1
BIAS
C
30 . However,
R1
C2
pin such that
C
threshold,
http://onsemi.com
C
pin
16
linear regulator, requires a small amount of power even
when the switch is turned off. The specifications section of
this datasheet reveals that the typical operating current, I
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,
V
drive current must be factored in as well. This current is
drawn from the V
current. The base drive current is listed in the specifications
as DI
designer will find additional guidance in the graphs. With
that information, the designer can calculate:
where:
boost converter,
where:
source of on−chip power loss. V
collector−emitter voltage of the internal NPN transistor
when it is driven into saturation by its base drive current. The
value for V
or from the graphs, as “Switch Saturation Voltage.” Thus,
Finally, the total on−chip power losses are:
IN
The internal control circuitry, including the oscillator and
Since the onboard switch is an NPN transistor, the base
I
D = the duty cycle or percentage of switch on−time.
I
In a flyback converter,
n
n
The switch saturation voltage, V
SW
SW
s
p
, and the ambient temperature, T
= number of turns in the transformer secondary winding.
= number of turns in the transformer primary winding.
CC
= the current through the switch;
and D are dependent on the type of converter. In a
/DI
I SW(AVG) ^
I SW(AVG) ^ I LOAD
SW
P DRIVER + V IN I SW
(CE)SAT
P D + P BIAS )P DRIVER )P SAT
, or switch transconductance. As before, the
P SAT ^ V (CE)SAT I SW
IN
D ^
can be obtained from the specifications
D ^
pin, in addition to the control circuitry
P BIAS + V IN I Q
V OUT I LOAD
V OUT )
V OUT * V IN
V IN
V OUT
V OUT
n s
n p
D
(CE)SAT
V IN
A
DI SW
I CC
. Then:
efficiency
efficiency
(CE)SAT
, is the last major
D
I
I
D
is the
Q
,