ADP3212/NCP3218
pin voltage drops to less than −300 mV, the low-side MOSFETs
is turned off. DRVL outputs are allowed to turn back on when
the CSREF voltage recovers to greater than −100 mV.
OUTPUT CROWBAR
To prevent the CPU and other external components from
damage due to overvoltage, the ADP3212/NCP3218 turns off
the DRVH1 and DRVH2 outputs and turns on the DRVL1 and
DRVL2 outputs when the output voltage exceeds the OVP
threshold (1.55 V typical).
OUTPUT ENABLE AND UVLO
For the ADP3212/NCP3218 to begin switching, the VCC supply
voltage to the controller must be greater than the VCCOK
threshold and the EN pin must be driven high. If the VCC
voltage is less than the VCCUVLO threshold or the EN pin is a logic
low, the ADP3212/NCP3218 shuts off. In shutdown mode, the
controller holds the PWM outputs low, shorts the capacitors of
the SS and PGDELAY pins to ground, and drives the DRVH
and DRVL outputs low.
Turning on the low-side MOSFETs forces the output capacitor
to discharge and the current to reverse due to current build up
in the inductors. If the output overvoltage is due to a drain-
source short of the high-side MOSFET, turning on the low-side
MOSFET results in a crowbar across the input voltage rail. The
crowbar action blows the fuse of the input rail, breaking the
circuit and thus protecting the microprocessor from
destruction.
The user must adhere to proper power-supply sequencing during
startup and shutdown of the ADP3212/NCP3218. All input pins
must be at ground prior to removing or applying VCC, and all
output pins should be left in high impedance state while VCC is
off.
When the OVP feature is triggered, the ADP3212/NCP3218 is
latched off. The latch-off function can be reset by removing and
reapplying VCC to the ADP3212/NCP3218 or by briefly pulling
the EN pin low.
THERMAL THROTTLING CONTROL
The ADP3212/NCP3218 includes a thermal monitoring circuit
to detect whether the temperature of the VR has exceeded a
user-defined thermal throttling threshold. The thermal
monitoring circuit requires an external resistor divider
connected between the VCC pin and GND. The divider consists
of an NTC thermistor and a resistor. To generate a voltage that
is proportional to temperature, the midpoint of the divider is
connected to the TTSNS pin. An internal comparator circuit
compares the TTSNS voltage to half the VCC threshold and
outputs a logic level signal at the VRTT output when the
temperature trips the user-set alarm threshold. The VRTT
output is designed to drive an external transistor that in turn
provides the high current, open-drain VRTT signal required by
the IMVP-6.5 specification. The internal VRTT comparator has a
hysteresis of approximately 100 mV to prevent high frequency
oscillation of VRTT when the temperature approaches the set
alarm point.
Pulling TTSNS to less than 1 V disables the overvoltage
protection function. In this configuration, VRTT should be tied
to ground.
REVERSE VOLTAGE PROTECTION
Very large reverse current in inductors can cause negative VCORE
voltage, which is harmful to the CPU and other output
components. The ADP3212/NCP3218 provides a reverse
voltage protection (RVP) function without additional system
cost. The VCORE voltage is monitored through the CSREF pin.
When the CSREF pin voltage drops to less than −300 mV, the
ADP3212/NCP3218 triggers the RVP function by disabling all
PWM outputs and driving DRVL1 and DRVL2 low, thus
turning off all MOSFETs. The reverse inductor currents can be
quickly reset to 0 by discharging the built-up energy in the
inductor into the input dc voltage source via the forward-biased
body diode of the high-side MOSFETs. The RVP function is
terminated when the CSREF pin voltage returns to greater than
−100 mV.
OUTPUT CURRENT MONITOR
The ADP3212/NCP3218 has an output current monitor. The
IMON pin sources a current proportional to the inductor
current. A resistor from IMON pin to FBRTN sets the gain. A
0.1 μF is added in parallel with RMON to filter the inductor
ripple. The IMON pin is clamped to prevent it from going
above 1.15V.
Sometimes the crowbar feature inadvertently causes output
reverse voltage because turning on the low-side MOSFETs
results in a very large reverse inductor current. To prevent
damage to the CPU caused from negative voltage, the
ADP3212/NCP3218 maintains its RVP monitoring function
even after OVP latch-off. During OVP latch-off, if the CSREF
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