NCP1030, NCP1031
http://onsemi.com
16
D1
D2
COUT
CAUX1
Cin
R3
R4
CCT
CC
RC
CP
R1
R2
NCP1032
VDRAIN
VCC
OV
COMP
VFB
GND
CT
UV
Lsec
Lbias
Lpri
NCP1030/31
CVCC
RC
D2
CAUX2
Figure 39. Typical Application with the Series
Resistance Added to Control VCC
To reduce the problem, a series resistance can be added to
allow the part to clamp VCC with the characteristic current
draw of the regulator as the voltage increases. The resistor
value required is such that it will not implead normal
operation but will prevent damage to the device during
transients, startup, current limits, and over loads. The proper
sizing of the series resistance starts with an examination of
the current draw by the NCP1031 at the desired operating
frequency as shown in Figure 40. The resistor value should
be such that it does not exceed the VCC maximum voltage
of 16 V during the worst case overshoot. Further, the voltage
must not fall below the VCC minimum operating voltage of
7 V during heavy loading, transients, or line disturbances. A
series resistance calculated example of operation at 310 kHz
is shown in Equation 2. In this case, a 1.96 kW resistor can
be used to make the VCC node more robust.
Calculation of RC
16 V w V
OUTaux
* I
C_current
@ RC w 7.0 V
(eq. 2)
V
OUTaux
* 16 V
I
C_current
+ RC
24 V * 16 V
4.075 mA
+ 1.96 kW
12.5 V * 7.0 V
2.65 mA
+ 2.07 kW
Figure 40. NCP1031 Current Draw vs. Frequency and VCC Voltage
2
3
4
5
6
7
8
9
10
11
7
8
9
10
11
12
13
14
15
16
17
18
VCC Current Draw (mA)
VCC Voltage (V)
560 pF 310kHz
470 pF 350kHz
390 pF 390kHz
330 pF 450kHz
270 pF 500kHz
220 pF 573kHz
180 pF 635 kHz
150 pF 702kHz
100 pF 905kHz
82 pF 1MHz
The series resistor needs to be coupled with proper sizing
of the auxiliary winding and VCC capacitance. The CAUX1
and CAUX2 should be approximately the same size where
the CVCC should be between 1/10 to 1/100 the value of
CAUX2. The smaller size of CVCC serves to reduce the
amount of energy available to the internal clamping
structures in the event of a large unforeseen over voltage.
Proper sizing of capacitance and adding a series resistance
can reduce the likelihood of an over voltage on the VCC, but
cannot eliminate the possibility completely. A zener diode
can be added along with the series resistance value
calculated from Equation 2 which can be split into RC1 and
RC2 as shown in Figure 41. If the OV pin is not used, it can
be connected to the VCC node to monitor the voltage and
suspend switching if the voltage exceeds a predefined level.
The addition of the ROV1 and ROV2 will add a current draw
from VAUX and will increase the voltage drop across RC.
