PI2211 datasheet(13/26 Pages) VICOR

13 / 26 page

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Picor Corporation · picorpower.com

PI2211

Rev 1.1, Page 13 of 26

PI2211

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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

until the gate is completely discharged.

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

adjust

for

the

desired

circuit

breaker

threshold.

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.

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, IDCMAX. IDCMAX is calculated based on the

maximum rated junction temperature and the thermal and

resistive properties of the MOSFET. See the IDCMAX 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

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 VCC 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 VGS 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 VGS 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.

PI2211 Datasheet(PDF) 13 Page - Vicor Corporation

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