MB39C604 Datasheet, PDF (15/36 Pages) Cypress Semiconductor – PSR topology in an isolated flyback circuit

MB39C604
8.8 Protection Functions
Under Voltage Lockout Protection (UVLO)
The under voltage lockout protection (UVLO) prevents IC from a malfunction in the transient state during V_VDD startup and a
malfunction caused by a momentary drop of V_VDD, and protects the system from destruction/deterioration. An UVLO comparator
detects the voltage decrease below the UVLO threshold voltage on the VDD pin, and then the DRV pin is turned to “L” and the
switching stops. MB39C604 automatically returns to normal operation mode when V_VDD increases above the UVLO threshold
voltage.
Over Voltage Protection (OVP)
The over voltage protection (OVP) protects Secondary side components from an excessive voltage stress. If the LED is
disconnected, the output voltage of Secondary Winding rises up. The output overvoltage can be detected by monitoring the TZE pin.
During Secondary Winding energy discharge time, V_TZE is proportional to V_AUX and the voltage of Secondary Winding (refer to 8.1).
When V_TZE rises higher than the OVP threshold voltage for 3 continues switching cycles, the DRV pin is turned to “L”, and the
switching stops (latch off). When V_VDD drops below the UVLO threshold voltage, the latch is removed.
Over Current Protection (OCP)
The over current protection (OCP) prevents inductor or transformer from saturation. The drain current of the external switching
MOSFET is limited by OCP. When the voltage on the CS pin reaches the OCP threshold voltage, the DRV pin is turned to “L” and
the switching cycle ends. After zero crossing is detected on the TZE pin again, the DRV pin is turned to “H” and the next switching
cycle begins.
Short Circuit Protection (SCP)
The short circuit protection (SCP) protects the transformer and the Secondary side diode from an excessive current stress. When
the short circuit between LED terminals occurs, the output voltage decreases. If the voltage on the TZE pin falls below SCP
threshold voltage, V_COMP is discharged and fixed at 1.5V and then the switching enters a low frequency mode.(T_ON = 1.5 µs /T_OFF =
78 µs to 320 µs)
Over Temperature Protection (OTP)
The over temperature protection (OTP) protects IC from thermal destruction. When the junction temperature reaches +150°C, the
DRV pin is turned to “L”, and the switching stops. It automatically returns to normal operation mode if the junction temperature falls
back below +125°C.
Table 5. Protection Functions Table
Function
PIN Operation
DRV COMP ADJ
Detection Condition
Return
Condition
Remarks
Normal Operation
Active
Active
Active
-
-
-
Under Voltage Lockout
Protection (UVLO)
L
Over Voltage Protection (OVP) L
L
1.5V
fixed
L
Active
VDD < 7.9V
TZE > 4.3V
VDD > 13V
VDD < 7.9V
→ VDD > 13V
Auto Restart
Latch off
Over Current Protection (OCP) L
Active
Active
CS > 2V
Cycle by cycle
Auto Restart
Short Circuit Protection (SCP) Active
Over Temperature Protection
(OTP)
L
1.5V
fixed
1.5V
fixed
Active
Active
TZE (peak) < 0.7V
Tj > +150°C
TZE (peak) > 0.7V Auto Restart
Tj < +125°C
Auto Restart
Document Number: 002-08441 Rev.*A
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