LTC4352 datasheet(13/22 Pages) LINER

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LTC4235

4235f

For more information www.linear.com/LTC4235

applicaTions inForMaTion

Overcurrent Fault

The LTC4235 features an adjustable current limit with

foldback that protects the external MOSFET against short

circuits or excessive load current. The voltage across the

external sense resistor RS is monitored by an active cur-

rent limit amplifier. The amplifier controls the gate of the

Hot Swap MOSFET to reduce the load current as a func-

tion of the output voltage sensed by the OUT pin during

active current limit. A graph in the Typical Performance

Characteristics shows the current limit sense voltage

versus OUT voltage.

An overcurrent fault occurs when the output has been in

currentlimitforlongerthanthefaulttimerperiodconfigured

at the FTMR pin. Current limiting begins when the sense

voltage between the SENSE+ and SENSE– pins reaches

8.3mV to 25mV depending on the OUT pin voltage. The

gate of the Hot Swap MOSFET is brought under control by

the current limit amplifier and the output current is regu-

lated to limit the sense voltage to less than 25mV. At this

point, the fault timer starts with a 100µA current charging

the FTMR pin capacitor. If the FTMR pin voltage exceeds

its 1.235V threshold, the external MOSFET turns off with

HGATE pulled to ground by 2mA and FAULT pulls low.

After the Hot Swap MOSFET turns off, the FTMR pin ca-

pacitor is discharged with a 2µA pull-down current until

its threshold reaches 0.2V. This is followed by a cool-off

period of 14 timing cycles as described in the FTMR Pin

Functions. Figure 2 shows an overcurrent fault on the

12V output.

200µs/DIV

OUT

10V/DIV

HGATE

10V/DIV

ILOAD

20A/DIV

4235 F02

Figure 2. Overcurrent Fault on 12V Output

5µs/DIV

OUT

10V/DIV

HGATE

10V/DIV

ILOAD

20A/DIV

4235 F03

Figure 3. Severe Short-Circuit on 12V Output

Intheeventofasevereshort-circuitfaultonthe12Voutput

as shown in Figure 3, the output current can surge to tens

ofamperes.TheLTC4235respondswithin1µstobringthe

current under control by pulling the HGATE to OUT voltage

downtozerovolts.Almostimmediately,thegateoftheHot

Swap MOSFET recovers rapidly due to the charge stored

in the RHG and CHG network and current is actively limited

until the fault timer expires. Due to parasitic supply lead

inductance, an input supply without any bypass capaci-

tor may collapse during the high current surge and then

spike upwards when the current is interrupted. Figure 10

shows the input supply transient suppressors comprising

of Z1, RSNUB1, CSNUB1 and Z2, RSNUB2, CSNUB2 for the two

supplies if there is no input capacitance.

FTMR Pin Functions

An external capacitor CFT connected from the FTMR pin

to GND serves as fault timing when the supply output is

in active current limit. When the voltage across the sense

resistorexceedsthefoldbackcurrentlimitthreshold(from

25mV to 8.3mV), FTMR pulls up with 100µA. Otherwise,

it pulls down with 2µA. The fault timer expires when the

1.235V FTMR threshold is exceeded, causing the FAULT

pin to pull low. For a given fault timer period, the equation

for setting the external capacitor CFT value is:

CFT = tFT • 0.083 [µF/ms]

After the fault timer expires, the FTMR pin capacitor pulls

down with 2µA from the 1.235V FTMR threshold until it

reaches0.2V.Then,itcompletes14coolingcyclesconsist-

ing of the FTMR pin capacitor charging to 1.235V with a

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