LTC4210 -1/LTC4210 -2
9
421012fa
For more information www.linear.com/LTC4210-1
APPLICATIONS INFORMATION
Hot Circuit Insertion
When circuit boards are inserted into live backplanes,
the supply bypass capacitors can draw large transient
currents from the backplane power bus as they charge.
Such transient currents can cause permanent damage to
connector pins, glitches on the system supply or reset
other boards in the system.
The LTC4210 is designed to turn a printed circuit board’s
supply voltage ON and OFF in a controlled manner, allow
-
ing the circuit board to be safely inserted into or removed
from a live backplane. The LTC4210 can reside either on
the backplane or on the daughter board for hot circuit
insertion applications.
Overview
The LTC4210 is designed to operate over a range of sup
-
plies from 2.7V to 16.5V. Upon insertion, an undervoltage
lockout circuit determines if sufficient supply voltage is
present. When the ON pin goes high an initial timing cycle
assures that the board is fully seated in the backplane
before the MOSFET is turned on. A single timer capacitor
sets the periods for all of the timer functions. After the
initial timing cycle the LTC4210 can either start up in cur
-
rent limit or with a lower load current. Once the external
MOSFET is fully enhanced and the supply has ramped
up, the LTC4210
monitors the load current through an
external sense resistor. Overcurrent faults are actively
limited to 50mV/R
SENSE
for a specified circuit breaker
timer limit. The LTC4210-1 will automatically retry after
a current limit fault while the LTC4210-2 latches off. The
LTC4210-1 timer function limits the retry duty cycle to
3.8% for MOSFET cooling.
Undervoltage Lockout
An internal undervoltage lockout (UVLO) circuit resets the
LTC4210 if the V
CC
supply is too low for normal operation.
The UVLO has a low-to-high threshold of 2.5V, a 100mV
hysteresis and a high-to-low glitch filter of 30µs. Above
2.5V supply voltage, the LTC4210 will start if the ON pin
conditions are met. A short supply dip below 2.4V for less
than 30µs is ignored to allow for bus supply transients.
ON Function
The ON pin is the input to a comparator which has a
low-to-high threshold of 1.3V, an 80mV hysteresis and a
high-to-low glitch filter of 30µs. A low input on the ON pin
resets the LTC4210 TIMER status and turns off the external
MOSFET by pulling the GATE pin to ground. A low-to-high
transition on the ON pin starts an initial cycle followed by
a start-up cycle. A 10k pull-up resistor connecting the ON
pin to the supply is recommended. The 10k resistor shunts
any potential static charge on the backplane and reduces
the overvoltage stress at the ON pin during live insertion.
Alternatively, an external resistor divider at the ON pin can
be used to program an undervoltage lockout value higher
than the internal UVLO circuit. An RC filter can be added
at the ON pin to increase the delay time at card insertion
if the internal glitch filter delay is insufficient.
GATE Function
During hot insertion of the PCB, an abrupt application of
supply voltage charges the external MOSFET drain/gate
capacitance. This can cause an unwanted gate voltage
spike. An internal proprietary circuit holds GATE low before
the internal circuitry wakes up. This reduces the MOSFET
current surges substantially at insertion. The GATE pin is
held low in reset mode and during the initial timing cycle.
In the start-up cycle the GATE pin is pulled up by a 10µA
current source. During an overcurrent fault condition, the
error amplifier servoes the GATE pin to maintain a constant
current to the load until the circuit breaker trips. When the
circuit breaker trips, the GATE pin shuts down abruptly.
