Datasheet

NCP1028
http://onsemi.com
16
Fault Condition – Low Input Voltage
The NCP1028 includes a brown-out circuitry able to
protect the power supply in case of low input voltage
conditions. Figure 29 shows how internally the NCP1028
monitors the voltage image of the bulk capacitor. Below a
given level, the controller blocks the driving pulses, above
it, it authorizes them. The internal circuitry, depicted by
Figure 29a, offers a way to observe the high-voltage (HV)
rail. A resistive divider made of R
upper
and R
lower
, brings
a portion of the HV rail on pin 3. Below the turn-on level,
the 10 mA current source IBO is off. Therefore, the turn-on
level solely depends on the division ratio brought by the
resistive divider.
+
-
BO
ON/OFF
V
DD
IBO
BO
Vbulk
Rupper
Rlower
+
VBO
1 vin 2 vcmp
20.0u 60.0u 100u 140u 180u
Time in Seconds
0
40.0
80.0
120
160
0
4.00
8.00
12.0
16.0
Plot1
2
1
Vcmp Volts
Vin in Volts
Vbulk = 100 V
Vbulk = 70 V
Figure 29a. The internal brown-out
configuration with an offset current source.
Figure 29b. Simulation results for 100/70 ON/OFF levels.
Figure 29.
To the contrary, when the internal BO signal is high, the IBO source is activated and creates an hysteresis. As a result,
it becomes possible to select the turn-on and turn-off levels via a few lines of algebra.
IBOisOff
(eq. 1)
V()) + V
bulk1
R
lower
R
lower
) R
upper
IBOisOn
(eq. 2)
V()) + V
bulk2
R
lower
R
lower
) R
upper
) IBO
ǒ
R
lower
R
upper
R
lower
) R
upper
Ǔ
We can now extract R
lower
from Equation 1 and plug it into Equation 2, then solve for R
upper
:
R
upper
+ R
lower
V
bulk1
-VBO
VBO
R
lower
+ VBO
V
bulk1
-V
bulk2
IBO (V
bulk1
-VBO)
If we decide to turn-on our converter for Vbulk1 equals 100 V and turn it off for Vbulk2 equals 70 V, then we obtain:
R
upper
= 3.0 MW
R
lower
= 18 kW