IR3502B
Page 24 of 38 V3.2
is set. A high level at the PGOOD pin indicates that the converter is in operation and has no fault. The PGOOD
stays high as long as the output voltage is within 300 mV of the programmed VID. During start-up, it is pulled low
with an input voltage as low as 2 V. It stays low until the startup sequence has completed, and the output voltage
has moved to the programmed VID.
Open Voltage Loop Detection
The output voltage range of error amplifier is detected all the time to ensure the voltage loop is in regulation. If any
fault condition forces the error amplifier output above VCCL-1.08V for 8 switching cycles, the fault latch is set. The
fault latch can only be cleared by cycling power to VCCL.
Open Remote Sense Line Protection
If either remote sense line VOSEN+ or VOSEN- or both are open, the output of remote sense amplifier (VO) drops.
The IR3502B monitors VO pin voltage continuously. If VO voltage is lower than 200 mV, two separate pulse
currents are applied to VOSEN+ and VOSEN- pins respectively to check if the sense lines are open. If VOSEN+ is
open, a voltage higher than 90% of V(VCCL) will be present at VOSEN+ pin and the output of open line detect
comparator will be high. If VOSEN- is open, a voltage higher than 700mV will be present at VOSEN- pin and the
output of open-line-detect comparator will be high. The open sense line fault latch is set, which pulls error amplifier
output low immediately and shut down the converter. The SS/DEL voltage is discharged and the fault latch can only
be reset by cycling VCCL power. During dynamic VID down, OVP may be triggered when output voltage can not
follow VDAC voltage change at light load with large output capacitance. Therefore, over-voltage threshold is raised
to 1.73V from VDAC+130mV whenever dynamic VID is detected and the difference between output voltage and
VDAC is more than 50mV, as shown in Figure 17. The over-voltage threshold is changed back to VDAC+130mV if
the difference is smaller than 50mV.
Open Daisy Chain Protection
IR3502B checks the daisy chain every time it powers up. It starts a daisy chain pulse on the PHSOUT pin and
detects the feedback at PHSIN pin. If no pulse comes back after 32 CLKOUT pulses, the pulse is restarted again. If
the pulse fails to come back the second time, the open daisy chain fault is registered, and SS/DEL is not allowed to
charge. The fault latch can only be reset by cycling the power to VCCL.
After powering up, the IR3502B monitors PHSIN pin for a phase input pulse equal or less than the number of
phases detected. If PHSIN pulse does not return within the number of phases in the converter, another pulse is
started on PHSOUT pin. If the second started PHSOUT pulse does not return on PHSIN, an open daisy chain fault
is registered.
Enable Input
The ENABLE pin below 0.8V sets the Fault Latch and a voltage above 0.85V enables the soft start of the converter.
Thermal Monitoring (VRHOT)
A resistor divider including a thermistor at HOTSET pin sets the VRHOT threshold. The thermistor is usually placed
at the temperature sensitive region of the converter, and is linearized by a series resistor. The IR3502B compare
HOTSET pin voltage with a reference voltage of 1.6V. The VRHOT pin is an open-collector output and should be
pulled up to a voltage source through a resistor. If the thermal trip point is reached the VRHOT output drives low.
The hysteresis of the VRHOT comparator helps eliminate toggling of VRHOT output.
The overall system must be considered when designing for OVP. In many cases the over-current protection of the
AC-DC or DC-DC converter supplying the multiphase converter will be triggered and provide effective protection
without damage as long as all PCB traces and components are sized to handle the worst-case maximum current. If
this is not possible, a fuse can be added in the input supply to the multiphase converter.
