LTC4280CUFD datasheet(15/28 Pages) LINER

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LTC4280

4280f

APPLICATIONS INFORMATION

it to INTVCC, the part is allowed to turn on again after an

internal 5 second timer has expired, in the same manner

as the TIMER pin passing its 0.2V threshold.

Overvoltage Fault

An overvoltage fault occurs when either the OV pin rises

above its 1.235V threshold, or the VDD pin rises above its

15.6V threshold, for more than 2μs. This shuts off the GATE

with a 1mA current to ground and sets the overvoltage

present bit C0 and the overvoltage fault bit D0. If the pin

subsequently falls back below the threshold for 100ms,

the GATE is allowed to turn on again unless overvoltage

auto-retry has been disabled by clearing bit A0.

Undervoltage Fault

An undervoltage fault occurs when the UV pin falls below

its 1.235V threshold for more than 2μs. This turns off the

GATE with a 1mA current to ground and sets undervoltage

present bit C1 and undervoltage fault bit D1. If the UV pin

subsequently rises above the threshold for 100ms, the

GATE is turned on again unless undervoltage auto-retry has

been disabled by clearing bit A1. When power is applied

to the device, if UV is below its 1.235V threshold after

INTVCC crosses its 2.64V undervoltage lockout threshold,

an undervoltage fault is logged in the fault register.

Board Present Change of State

Whenever the EN pin toggles, bit D4 is set to indicate a

change of state. When the EN pin goes high, indicating

board removal, the GATE turns off immediately (with a 1mA

current to ground) and clears the board present bit, C4. If

the EN pin is pulled low, indicating a board insertion, all

fault bits except D4 are cleared and enable bit, C4, is set.

If the EN pin remains low for 100ms the state of the ON

pin is captured in ‘FET On’ control bit A3. This turns the

switch on if the ON pin is tied high. There is an internal

10μA pull-up current source on the EN pin.

Figure 3. Short-Circuit Waveforms

VGATE

10V/DIV

VSOURCE

10V/DIV

VDD

10V/DIV

ILOAD

10A/DIV

5μs/DIV

4280 F03

RS = 5mΩ

CL = 0

RSHORT = 1Ω

R6 = 30k

C1 = 0.1μF

Overcurrent Fault

The LTC4280 features an adjustable current limit that

protects against short-circuits or excessive load current

until an overcurrent fault is generated. An overcurrent

fault can occur in two different manners. First, at the end of

start-up when the TIMER pin reaches its 1.235V threshold

or the internal 100ms start-up timer expires, if the part is

in current limit an overcurrent fault is generated. Second,

after start-up the FILTER pin ramps up with 10μA if the sense

voltage exceeds the 25mV circuit breaker threshold or the

current limit circuit regulates the sense voltage. When the

FILTER pin passes its 1.235V threshold the overcurrent

present bit C2 is set and an overcurrent fault is generated.

In both cases the external MOSFET is turned off and the

overcurrent fault bit is set.

After the MOSFET is turned off, the TIMER and FILTER

capacitors begins discharging with 2μA pulldown currents.

When the TIMER pin reaches its 0.2V threshold the MOSFET

is allowed to turn on again if the overcurrent fault has been

cleared. However, if the overcurrent auto-retry bit, A2 has

been set then the MOSFET turns on again automatically

without resetting the overcurrent fault. Use a minimum

value of 10nF for CT. If the TIMER pin is bypassed by tying

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