Cirrus Logic AN368 - Manual (Page 66)

AN368
66 AN368REV2
Design Tip - Sense resistor is open circuited or unusually large
Check:
Determine if the power supply is operating in either auto-restart or latch mode when
load is open and output voltage reaches the threshold.
Test Stimulus: Connect a resistor with a high impedance in series with the original current-sensing
resistor, and then connect a switch in parallel with the high-resistance resistor.
Manually switch on/off. The CS voltage should rise to above 1.69V (OCP
threshold) after the set OCP blanking period is over and after the gate drive turns
on.
Performance: Once the voltage across the current-sensing resistor reaches the OCP threshold,
the power supply is protected in either auto-restart or latch mode, depending on the
OTP setting.
c. Open Loop Protection
The OTP settings are configured using the following:
1. Set bit OLP in register Config47 at Address 79 to ‘0’ to enable open loop protection
2. Set bit OLP_LAT in register Config49 at Address 81 to ‘0’
to configure the OLP fault type as unlatched
3. Set bits OLP_BLANK[2:0] in register Config48 at Address 80 to ‘010’ to configure the blanking time
interval t
OLP
to 2 s
4. Set bits OLP_CNT[2:0] in register Config49 at Address 81 to ‘101’ to declare an OLP fault after 5
consecutive OLP events
Design Tip - Power supply produces high peak current when the sense resistor is short circuited
Check:
Determine if the power supply is operating in either auto-restart or latch mode when
the current-sensing resistor is shorted.
Test Stimulus: Use a switch to connect a resistor with a low impedance in parallel with the original
current-sensing resistor or just short the current-sensing resistor. The CS voltage
should not rise above the 200mV OLP threshold in the set OLP blanking window
after the gate-drive turns on.
Performance: Once the current-sensing resistor is shorted or reduced, the power supply is
protected in either auto-restart or latch mode, depending on the OTP setting.
d. Overvoltage Protection
The overvoltage protection is configured to accommodate both LED strings. The OTP settings are configured
using the following:
1. Set bit OVP in register Config47 at Address 79 to ‘0’ to enable overvoltage protection
2. Set bit OVP_LAT in register Config50 at Address 82 to ‘0’ to configure the OVP fault type as unlatched
3. Set bits OVP_BLANK[2:0] in register Config50 at Address 82 to ‘011’ to configure the blanking time
interval t
OVP
to 2.5 s
Set bit OVP_TYPE in register Config50 at Address 82 to ‘1’ to set the T2 offset blanking mode so that
the blanking time is always equal to the corresponding channel’s previous T2 switching cycle period
minus an offset of 500ns
4. Set bits OVP_CNT[2:0] in register Config50 at Address 82 to ‘101’ to declare an OVP fault after 5
consecutive OVP events