DC2027A-B Datasheet DC2027A-B | P10-P12

LTC4364-1/LTC4364-2

436412f

OPERATION

The LTC4364 is designed to suppress high voltage surges

and limit the output voltage to protect load circuitry and

ensure normal operation in high availability power systems.

It features an overvoltage protection regulator that drives

an external N-channel MOSFET (M1) as the pass device

and an ideal diode controller that drives a second external

N-channel MOSFET (M2) for reverse input protection and

output voltage holdup.

The LTC4364 operates from a wide range of supply voltage,

from 4V to 80V. With a clamp limiting the V

supply, the

input voltage may be higher than 80V. The input supply

can also be pulled below ground potential by up to 40V

without damaging the LTC4364. The low power supply

requirement of 4V allows it to operate even during cold

cranking conditions in automotive applications.

Normally, the pass device M1 is fully on, supplying current

to the load with very little power loss. If the input voltage

surges too high, the voltage amplifier (VA) controls the gate

of M1 and regulates the voltage at the OUT pin to a level

that is set by an external resistive divider from the OUT pin

to ground and the internal 1.25V reference. The LTC4364

also detects an overcurrent condition by monitoring the

voltage across an external sense resistor placed between

the SENSE and OUT pins. An active current limit circuit

(IA) controls the gate of M1 to limit the sense voltage to

50mV if OUT is above 2.5V. In the case of a severe output

short that brings OUT below 1.5V, the sense voltage is

reduced to 25mV to reduce the stress on M1.

During an overvoltage or overcurrent event, a current

source starts charging up the capacitor connected at

the TMR pin to ground. The pull-up current source in

overcurrent condition is 5 times of that in overvoltage to

accelerate turn-off. When TMR reaches 1.25V, the F LT pin

pulls low to warn of impending turn-off. The pass device

M1 stays on and the TMR pin is further charged up until it

reaches 1.35V, at which point the HGATE pin pulls low and

turns off M1. The fault timer allows the load to continue

functioning during brief transient events while protecting

the MOSFET from being damaged by a long period of input

overvoltage, such as load dump in vehicles. The fault timer

period decreases with the voltage across the MOSFET,

to help keep the MOSFET within its safe operating area

(SOA). The LTC4364-1 latches off M1 and keeps F LT low

after a fault timeout. The LTC4364-2 allows M1 to turn

back on and F LT to go high impedance after a cool down

timer cycle, provided the OV pin is below its threshold.

After the HGATE pin is latched low following fault, mo-

mentarily pulling the SHDN pin below 0.5V resets the fault

and allows HGATE to pull high for both LTC4364-1 and

LTC4364-2. In addition, momentarily pulling the UV pin

below 0.6V allows HGATE to pull high after the cool down

timer delay for LTC4364-1, but has no effect on LTC4364-2.

The source and drain of MOSFET M2 serve as the anode

and cathode of the ideal diode. The LTC4364 controls the

DGATE pin to maintain a 30mV forward voltage across the

drain and source terminals of M2. It reduces the power

dissipation and increases the available supply voltage to

the load, as compared to using a discrete blocking diode.

If M2 is driven fully on and the load current results in

more than 30mV of forward voltage, the forward voltage

is equal to R

DS(ON)

• I

LOAD

In the event of an input short or a power supply failure,

reverse current temporarily flows through the MOSFET

M2 that is on. If the reverse voltage exceeds –30mV, the

LTC4364 pulls the DGATE pin low strongly and turns off

M2, minimizing the disturbance at the output.

If the input supply drops below the GND pin voltage, the

DGATE pin is pulled to the SOURCE pin voltage, keeping

M2 off. When the HGATE pin pulls low in any fault condi-

tion, the DGATE pin also pulls low, so both pass devices

are turned off.

If the output (and so the SOURCE pin, through the body

diode of M2) drops below GND, the HGATE pin is pulled

to the SOURCE pin voltage, turning M1 off and shutting

down the forward current path.

An input undervoltage condition is accurately detected

using the UV pin. The HGATE and DGATE pins remain low

if UV is below its 1.25V threshold. The SHDN pin not only

turns off the pass devices but also shuts down the internal

circuitry, reducing the supply current to 10µA.

LTC4364-1/LTC4364-2

436412f

APPLICATIONS INFORMATION

Some power systems must cope with high voltage surges

of short duration such as those in automobiles. Load

circuitry must be protected from these transients, yet

critical systems may need to continue operating during

these events.

The LTC4364 drives an N-channel MOSFET (M1) at the

HGATE pin to limit the voltage and current to the load cir-

cuitry during supply transients or overcurrent events. The

selection of M1 is critical for this application. It must stay

on and provide a low impedance path from the input sup-

ply to the load during normal operation and then dissipate

power during overvoltage or overcurrent conditions. The

LTC4364 also drives a second N-channel MOSFET (M2) at

the DGATE pin as an ideal diode to protect the load from

damage during reverse polarity input conditions, and to

block reverse current flow in the event the input collapses.

A typical application circuit using the LTC4364 to regulate

the output at 27V during input surges with reverse input

protection is shown in Figure 1.

Overvoltage Fault

The LTC4364 limits the voltage at the OUT pin during an

overvoltage situation. An internal voltage amplifier regu-

lates the HGATE pin voltage to maintain 1.25V at the FB

pin. During this period of time, the N-channel MOSFET

M1 remains on and supplies current to the load. This

allows uninterrupted operation during brief overvoltage

transient events.

If the voltage regulation loop is engaged for longer than

the timeout period, set by the timer capacitor, an overvolt-

age fault is detected. The HGATE pin is pulled down to the

SOURCE pin by a 130mA current, turning M1 off. This

prevents M1 from being damaged during a long period

of overvoltage, such as during load dump in automobiles.

After the fault condition has disappeared and a cool down

period has transpired, the HGATE pin starts to pull high

again (LTC4364-2). The LTC4364-1 latches the HGATE pin

low after an overvoltage fault timeout and can be reset

using the SHDN or UV pin (see Resetting Faults).

Overcurrent Fault

The LTC4364 features an adjustable current limit that

protects against short circuits and excessive load current.

During an overcurrent event, the HGATE pin is regulated

to limit the current sense voltage across the SENSE and

OUT pins (∆V

SNS

) to 50mV when OUT is above 2.5V. The

current limit sense voltage is reduced to 25mV when OUT is

below 1.5V for additional protection during an output short.

A current sense resistor is placed between SENSE and

OUT and its value (R

SNS

) is determined by:

SNS

∆V

SNS

LIM

where I

LIM

is the desired current limit.

Figure 1. 4A, 12V Overvoltage Output Regulator with Reverse Current Protection

OUTSENSEDGATESOURCEHGATE

TMRGND

TMR

47nF

UV = 6V

CMZ5945B

68V

1.5KE200A

MAX DC:

100V/–24V

MAX 1ms

TRANSIENT:

200V

SMAJ24A

0.1µF

2.2k

0.5W

OV = 60V

ENOUT

FAULT

ENABLE

436412 F01

F LT

SHDN FB

10Ω

100Ω

1N4148W

12V

90.9k

383k

10k

102k

4.99k

OUT

22µF

OUT

CLAMPED AT 27V

FDB33N25

FDB3682

SNS

10mΩ

LTC4364

LTC4364-1/LTC4364-2

436412f

APPLICATIONS INFORMATION

An overcurrent fault occurs when the current limit circuitry

has been engaged for longer than the timeout delay set

by the timer capacitor. The HGATE pin is then immediately

pulled low by 130mA to the SOURCE pin, turning off the

MOSFET M1. After the fault condition has disappeared

and a cool down period has transpired, the HGATE pin

is allowed to pull back up and turn on the pass device

(LTC4364-2). The LTC4364-1 latches the HGATE pin low

after the overcurrent fault timeout and can be reset using

the SHDN or UV pin (see Resetting Faults).

Input Overvoltage Comparator

Input overvoltage is detected with the OV pin and an ex-

ternal resistive divider connected to the input (Figure 1).

At power-up, if the OV pin voltage is higher than its 1.25V

threshold before the 100μs internal power-on-reset expires,

or before the input undervoltage condition is cleared at

the UV pin, the HGATE pin will be held low until the OV

pin voltage drops below its threshold. To prevent start-up

in the event the board is hot swapped into an overvoltage

supply, separate resistive dividers with filtering capacitors

can be used for the OV and UV pins (Figure 2). The RC

constants should be skewed so that τ

/τ

> 50. In Fig-

ure2, If the board is plugged into a supply that is higher

than 60V, the LTC4364 will not turn on the pass devices

until the supply voltage drops below 60V.

Once the HGATE pin begins pulling high, an input overvolt-

age condition detected by OV will not turn off the pass

device. Instead, OV prevents the LTC4364 from restarting

following a fault (see Cool Down Period and Restart). This

prevents the pass device from cycling between ON and OFF

states when the input voltage stays at an elevated level for

a long period of time, reducing the stress on the MOSFET.

Input Undervoltage Comparator

The LTC4364 detects input undervoltage conditions such

as low battery using the UV pin. When the voltage at the

UV pin is below its 1.25V threshold, the HGATE pin pulls

low to keep the pass device off. Once the UV pin voltage

rises above the UV threshold plus the UV hysteresis (50mV

typical), the HGATE pin is allowed to pull up without go-

ing through a timer cycle. In Figure 1 and Figure 2, the

input UV threshold is set by the resistive dividers to 6V.

An undervoltage condition does not produce an output

at the F LT pin.

Fault Timer

The LTC4364 includes an adjustable fault timer. Con-

necting a capacitor from the TMR pin to ground sets the

delay period before the MOSFET M1 is turned off during

an overvoltage or overcurrent fault condition. The same

capacitor also sets the cool down period before M1 is

allowed to turn back on after the fault condition has

disappeared. Once a fault condition is detected, a current

source charges up the TMR pin. The current level varies

depending on the voltage drop across the V

pin and the

OUT pin, corresponding to the MOSFET V

. The on time

is inversely proportional to the voltage drop across the

MOSFET. This scheme therefore takes better advantage

of the available safe operating area (SOA) of the MOSFET

than would a fixed timer current.

The timer current starts at around 2μA with 0.5V or less

of V

– V

OUT

, increasing linearly to 50μA with 75V of

– V

OUT

during an overvoltage fault (Figure 3a):

TMR(UP)OV

= 2μA + 0.644[μA/V] • (V

– V

OUT

– 0.5V)

During an overcurrent fault, the timer current starts at

10μA with 0.5V or less of V

– V

OUT

and increases to

260μA with 75V of V

– V

OUT

(Figure 3b):

TMR(UP)OC

= 10μA + 3.36[μA/V] • (V

– V

OUT

– 0.5V)

This arrangement allows the pass device to turn off faster

during an overcurrent event, since more power is dissipated

under this condition. Refer to the Typical Performance

Characteristics section for the timer current at different

– V

OUT

in both overvoltage and overcurrent events.

475k

UV = 6V

0V = 60V

436412 F02

10k

383k

100k

= (383k||100k) • 10nF

= (475k||10k) •1nF

10nF

1nF

LTC4364

Figure 2. External UV and OV Configuration Blocks Start-Up Into

an Overvoltage Condition

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DC2027A-B

Mfr. #:

Buy DC2027A-B

Manufacturer:

Analog Devices Inc.

Description:

Power Management IC Development Tools LTC4364DE-2 Demoboard: 12V Surge Stopper

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DC2027A-B