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threshold limit is passed. The discharge current is again
reduced to 250uA to slowly discharge the MOSFET gate. As
the drain voltage falls below the hysteresis threshold of the
drain-high threshold, the discharge current is increased to
8mA. This hysteretic cycling of discharge currents continues
?
PI2211
??
The start-up current level is set to approximately half the
circuit breaker threshold; (0.025V/0.052V) * circuit breaker
current. The start-up current limit is only in effect during
start-up, while the power-good signal is low, and acts to limit
the amount of current that the load can draw.
until the gate is completely discharged.
When start-up is completed, and power good is asserted
high, the current limit is no longer enabled and the circuit will
be allowed to draw current up to the circuit breaker
threshold or until an SOA fault is calculated. The circuit
breaker threshold is always enabled.
Another current threshold to consider is the maximum
operating current, IDC MAX . IDC MAX is calculated based on the
maximum rated junction temperature and the thermal and
resistive properties of the MOSFET. See the IDC MAX equation
in the Recommended Design Steps section for more details.
Operating above this current will result in an SOA shut down
and thermal cycling of the MOSFET when the PI2211 is
properly programmed.
The waveforms in Figure 11 are representative of a typical
Figure 10 - Glitch-Catcher response to shorted output.
Figure 10 shows the Glitch- Catcher? responding to a hard
short-circuit applied to its output. The BUS voltage (blue, Ch1)
starts to drop as the current though the FET (green, Ch4)
rises. The source voltage (purple, Ch3) separates from the
BUS voltage as the voltage drop across the sense resistor and
the FET increases. The gate voltage (cyan, Ch2) tracks the BUS
voltage until the over-current threshold is exceeded, which
starts the Glitch- Catcher? controlled gate discharge circuitry.
Current Limit:
The PI2211 has a start-up current limit and a circuit breaker
threshold, as shown in Figure 11 . The designer’s MOSFET
selection can be determined by the maximum load current,
acceptable power loss at max current and the maximum
ambient temperature.
The PI2211 has a current sense amplifier that uses an
external current sense resistor to monitor MOSFET current.
The circuit breaker current threshold is determined by
dividing the internal 52mV reference voltage by the desired
over-current threshold. Exceeding this threshold will initiate
the Glitch- Catcher? shut -down function, but the current is
not restricted. Since sense resistor value increments are
limited an additional resistor divider might be needed to
start-up sequence, followed by an over-current event, and
then a re-start into a shorted load, leading to SOA thermal
cycling.
As the BUS supply rises and clears the V CC POR and UV fault
thresholds, the programmable insertion delay timer starts.
After the insertion delay, the series MOSFET gate is charged
with a 25uA current, allowing the input current to gradually
increase until it reaches the start-up current threshold. The
gate will be regulated to maintain the start-up current until
either the output reaches the BUS voltage or the MOSFET is
turned off due to SOA. Here, the output voltage reaches the
BUS voltage and the current drops below the start-up current
threshold, stopping the regulation of the MOSFET V GS and
allowing it to increase to the full charge pump voltage level of
about 5V. The power good pin is de-asserted and allowed to
float once the V GS is above 4.4V.
Sometime after the normal start-up an over-current event
occurs, triggering the Glitch- Catcher? turn -off of the MOSFET
and the low assertion of the power good pin. See Figure 9 for
further details.
adjust for the desired
circuit breaker
threshold.
Picor Corporation · picorpower.com
PI2211
Rev 1.0, Page 13 of 26
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