ncp1927 ON Semiconductor, ncp1927 Datasheet - Page 20

• Manufacturer Part Number: ncp1927
• Description: Combination Power Factor Correction Controller And Flyback Controller For Flat Panel Tvs
• Manufacturer: ON Semiconductor

voltage falls below the UVP limit. During a UVP fault, the
drive output and error amplifier (EA) are disabled, and
C
stage does not start if V
protects the application when there is a problem with the
PControl
The NCP1927 detects a UVP fault when the output
V out(nom)
V out
R POVUV2
R POVUV1
is discharged. It is important to note that the PFC
PDRV
V out
OVP
Figure 25. OVP Timing Diagram
Figure 26. POVUV Pin Block
POVUV
POVUV
R
PCS
is lower than V
R
I
I
PCS
sense
L
Figure 27. Current Sense Block
OVP
UVP
PCS
UVP
http://onsemi.com
time
time
time
. This
I
PCS
V
DD
20
−
+
power path to the bulk capacitor (i.e. the capacitor is unable
to charge up) or if the controller is unable to sense the output
voltage (i.e. the POVUV Pin is floating). The output voltage
that causes a UVP fault is calculated using Equation 14.
Overcurrent Protection (OCP)
stage by limiting the coil current. A current sense resistor
(R
voltage proportional to the coil current (V
portrayed by Figure 27. The circuit uses V
when the coil current exceeds its maximum permissible
level. To do so, the circuit incorporates an operational
amplifier that sources the current necessary to maintain the
PCS pin at zero volts. A resistor (R
PCS pin and R
pin (I
where I
using Equation 16.
condition is detected and the driver is turned off. The driver
remains off until I
transition occurs or the watchdog timer expires. The
maximum coil current (I
Equation 17.
where I
V
The NCP1927 contains an OCP circuit to protect the PFC
Rearranging Equation 15 allows I
If I
sense
out(UVP)
PCS
PCS
I
) is inserted in the return path to generate a negative
PCS
L
OCP
) to be adjusted via Equation 15.
is the current flowing through the boost inductor.
+ V
> I
exceeds I
− R
is the OCP threshold current.
OCP
UVP
sense
sense
I
L(MAX)
@
PCS
To PDRV disable
R
allows the current sourced by the PCS
I
@ I
PCS
POVUV1
OCP
falls below I
L
+
+
) R
R
(typically 250 mA), an OCP
POVUV2
R
R
R
R
L(MAX)
) R
sense
sense
PCS
PCS
PCS
POVUV2
PCS
@ I
@ I
@ I
OCP
) is calculated with
OCP
) inserted between the
L
PCS
PCS
, and the next ZCD
) I
to be calculated
+ 0
Rsense
UVP
@ R
Rsense
to detect
(eq. 14)
(eq. 15)
(eq. 16)
(eq. 17)
POVUV1
) as