LED6001 Datasheet, PDF (20/26 Pages) STMicroelectronics – Switching controller section

Device description
LED6001
7.4
7.4.1
7.4.2
7.4.3
Device protections
Linear regulators undervoltage lockout
Both the 5 V and 3.3 V linear regulators of the LED6001 are equipped with an undervoltage
lockout (UVLO) protection. The UVLO protections avoid improper device operation in case
at least one of the two outputs (VDR and LDO3) is below the allowed level. In particular, the
LED6001 performs the soft-start sequence only after both VDR and LDO3 cross their
respective upper UVLO threshold.
Power switch overcurrent
The current flowing through the external power MOSFET is monitored, cycle-by-cycle, by
sensing the voltage across the shunt resistor in series with its source. If the voltage drop
exceeds the overcurrent protection (OCP) level, the ongoing switching cycle is suddenly
terminated (cycle-by-cycle power MOSFET OCP). Normal operation is automatically
resumed once the root cause has been removed. The XFAULT pin is not affected by OCP.
As explained in Section 7.2 on page 12 the slope compensation is added by injecting
a sawtooth current at the CSNS pin. As a consequence, the OCP threshold depends on
both the slope compensation amount and the boost converter's operating point:
Equation 6
I_{MOS OCP} = ^V-----^C----^S---^N----^S---^--^O----^C----^P----–-_R----^D-_S----_N-^--_S--^I--^S---^L-----^----^R----^S----^L---^O----^P---^E--
Where V_CSNS,OCP = 360 mV (typ.), I_SL = 50 µA (typ.) and D is the switching duty cycle.
Output overvoltage and OVFB pin disconnection
The output overvoltage fault detection is achieved by comparing the voltage at the OVFB
pin with an internal threshold. Because of this fault can potentially damage both the device
and the external components, a latched turn off condition is triggered once this event has
been detected. A resistor divider connected to the output of the boost converter sets the
desired OVP threshold.
The OVFB is internally pulled-up in order to protect the device against an OVFB pin
disconnection fault: if the pin is left floating, the OVP is suddenly triggered regardless of the
output voltage level. This small pull-up current (I_OVFB,PU) must be taken into account when
designing an OVP output divider involving high resistance values. Equation 7 allows setting
the desired output OVP level (R_OVPH and R_OVPL are the two resistors of the output divider
whose central tap is connected to the OVFB pin of LED6001):
Equation 7
V_{OUT OVP} = ^R-----^O----^V----^P_R---^H--_O---+-_V---_P-^R---_L-^O----^V----^P---^L- V_{TH OVFB} – R_OVPL  I_{OVFB PU}
Where V_TH,OVFB = 1.2 V (typ.) and I_OVFB,PU = 1 µA (typ.).
Once the OVP faulty condition is detected, the LED6001 device suddenly stops switching.
Both GATE and PWMO are forced low and the XFAULT pin is lowered. The condition is
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