LTC4280CUFD datasheet(12/28 Pages) LINER

12 / 28 page

LTC4280

4280f

The LTC4280 is designed to turn a board’s supply voltage

on and off in a controlled manner, allowing the board to be

safely inserted or removed from a live backplane. During

normal operation, the charge pump and gate driver turn

on an external N-channel MOSFET’s gate to pass power to

the load. The gate driver uses a charge pump that derives

its power from the VDD pin. Also included in the gate driver

is an internal 6.5V gate-to-source clamp. During start-up

the inrush current is tightly controlled by using current

limit foldback and output dV/dt limiting.

The current sense (CS) ampliﬁer monitors the load

current using the difference between the SENSE+ and

SENSE– pin voltages. The CS ampliﬁer limits the current

in the load by pulling back on the gate-to-source voltage

in an active control loop when the sense voltage exceeds

the commanded value. The CS ampliﬁer requires 20μA

input bias current from both the SENSE+ and the

SENSE– pins.

A short-circuit on the output to ground results in exces-

sive power dissipation during active current limiting. To

limit this power, the CS ampliﬁer regulates the voltage

between the SENSE+ and SENSE– pins at 26mV with

foldback to 10mV.

If an overcurrent condition persists, the internal circuit

breaker (CB) registers a fault when either the sense

voltage exceeds 25mV or the current sense ampliﬁer

is in regulation for more than the time limit set by the

capacitor on the FILTER pin. This indicates to the logic

that it is time to turn off the GATE to prevent overheating.

At this point the start-up TIMER pin voltage ramps down

using the 2μA current source until the voltage drops below

0.2V (comparator TM1) which tells the logic that the pass

transistor has cooled and it is safe to turn it on again if

overcurrent auto-retry is enabled. If the TIMER pin is tied

to INTVCC, the cool-down time defaults to 5 seconds on

an internal system timer in the logic.

The output voltage is monitored using the FB pin and the

Power Good (PG) comparator to determine if the power

is available for the load. The power good condition can be

signaled by the GPIO pin using an open-drain pulldown

transistor. The GPIO pin may also be conﬁgured to signal

power bad, or as a general purpose input (GP comparator),

or a general purpose open drain output.

The Functional Diagram shows the monitoring blocks of

the LTC4280. The group of comparators on the left side

includes the undervoltage (UV), overvoltage (OV), reset

(RST), enable (EN) and signal on (ON) comparators. These

comparators determine if the external conditions are valid

prior to turning on the GATE. But ﬁrst the two undervoltage

lockout circuits, UVLO1 and UVLO2, validate the input

supply and the internally generated 3.1V supply, INTVCC.

UVLO2 also generates the power-up initialization to the

logic circuits as INTVCC crosses this rising threshold. If the

ﬁxed internal overvoltage comparator, OV2, detects that

VDD is greater than 15.6V, the part immediately generates

an overvoltage fault and turns the GATE off.

Included in the LTC4280 is an 8-bit A/D converter. The

converter has a 3-input multiplexer to select between the

ADIN pin, the SOURCE pin and the SENSE+ – SENSE–

voltage.

An I2C interface is provided to read the A/D registers. It

also allows the host to poll the device and determine if

faults have occurred. If the ALERT line is conﬁgured as an

interrupt, the host is enabled to respond to faults in real

time. The typical SDA line is divided into an SDAI (input)

and SDAO (output). This simpliﬁes applications using an

optoisolator driven directly from the SDAO output. An

application which uses optoisolation is shown on the back

cover. The I2C device address is decoded using the ADR0,

ADR1 and ADR2 pins. These inputs have three states each

that decode into a total of 27 device addresses.

OPERATION

LTC4280CUFD датащи(PDF) 12 Page - Linear Technology

LTC4280CUFD датащи(HTML) 12 Page - Linear Technology

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28

датащи скачать

ссылки URL