PI2211 datasheet(8/26 Pages) VICOR

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Picor Corporation · picorpower.com

PI2211

Rev 1.1, Page 8 of 26

PI2211

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PI2211 Introduction

The PI2211 limits the start-up current to a load, eliminating

the electrical disturbance or possible voltage sag imposed on

a backplane power supply. The PI2211 performs hot swap

protection during power-up or insertion and acts as a circuit

breaker during steady state operation. The PI2211 performs

these protection functions by controlling an external MOSFET

and limiting the MOSFET junction temperature rise to a safe

level, a key requirement for hot swap power managers

expected to operate over wide dynamic conditions.

Upon insertion, the PI2211 initiates a user programmable

turn-on delay where the gate of the MOSFET is held "off",

providing input BUS de-bounce. The PI2211 then turns "on"

the MOSFET pass element in a controlled manner, limiting

the current to a pre-defined level based on the value of a user

selected sense resistor. The PI2211 circuit breaker threshold

protects against over-current by comparing the voltage drop

across this sense resistor with a fixed internal reference

voltage. Once the load voltage has reached its steady-state

value, the Power-good pin is asserted "high" and the start-up

current limit is disabled. Under voltage (UV) and Over Voltage

(OV) trip points (user settable) ensure operation within a

defined operating range in addition to a Enable/Disable

feature shared with the UV input.

With

Power-good

established,

the

load

current

continuously monitored by the PI2211 with the MOSFET

operating in the low loss RDSON region. In this steady state

operation, the PI2211 now acts primarily as a circuit breaker.

An over-current threshold is fixed to be twice the start-up

current limit and sets an upper current boundary that

determines when a gross fault has occurred. Exceeding this

boundary will initiate the PI2211 Glitch-Catcher™ circuitry

and assert the power good pin low.

Glitch-Catcher™

The Glitch-Catcher™ feature of the PI2211 prevents

overvoltage events caused by the energy stored in the

parasitic inductance of the input power path in response to a

rapid interruption of the forward current during an

overcurrent fault event. Acting as an active snubber, this

circuitry mitigates the need for large external protection

components by shunting the energy through the MOSFET to

the low impedance load.

True-SOA™

The Picor PI2211 ensures efficient operation within the

MOSFET SOA by emulating the MOSFET junction temperature

rise via a internal digital processor. The PI2211, with True-

SOA™, constantly monitors MOSFET power to calculate the

MOSFET junction temperature rise and determine the proper

operation regardless of load conditions. The amount of time

that the PI2211 will turn a MOSFET on during SOA is

dependent on the calculated temperature rise, not a fixed

time, making the pulse width dynamic with varying line

voltages. The PI2211 will keep the MOSFET on until it predicts

an absolute 60°C junction temperature rise. Selecting 60°C as

the maximum junction temperature rise allows for the use of

the MOSFET at ambient temperatures approaching 90°C and

prevents exceeding the MOSFET’s maximum junction

temperature, typically at 150°C.

Once the junction temperature rise has been calculated to be

60°C, the PI2211 will shut down the MOSFET and allow for

thermal cooling. While in True-SOA™ protection mode, the

PI2211 will attempt to start the MOSFET when the True-

SOA™ emulator has calculated that the junction temperature

has dropped by 39°C. The 39°C thermal cycling range will

retry start-up for a total of 16 pulses before the range is

extended to 57°C, where the thermal cycling will go on

indefinitely or until the low impedance load is removed.

The PI2211 can also protect the MOSFET when it is operating

at a higher than anticipated load current, but still below the

circuit breaker threshold. True-SOA™ constantly calculates

junction temperature rise as a function of power dissipation

and can shut down the MOSFET preventing damage. A typical

hot-swap controller will only fault when a threshold is

exceeded and cannot continuously protect the MOSFET

during operation.

Emulation of the MOSFET thermal performance is possible

with the use of the MOSFET manufacturers’ transient thermal

impedance curves. The Picor True-SOA™ digital algorithm

ensures maintaining a MOSFET within the actual SOA of the

device, optimizing the size of the device without the need to

oversize the MOSFET. The PI2211 True-SOA™ is programmed

for specific MOSFET thermal characteristics by the setting of

three resistors. The SOAS resistor determines the magnitude

and scaling of the current through the MOSFET. The SOAR

and SOAT resistors program the transient thermal parameters

of the MOSFET's junction to case characteristics. By

programming the MOSFET thermal characteristics, the PI2211

adapts its control and start-up or thermal cycling to that

specific MOSFET as shown in Figure 5.

PI2211 Datasheet(PDF) 8 Page - Vicor Corporation

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