LTC4217(RevC) Ver la hoja de datos (PDF) - Linear Technology

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Fabricante

LTC4217
(Rev.:RevC)

2A Integrated Hot Swap Controller

Linear Technology 

LTC4217

APPLICATIONS INFORMATION

LTC4217

GATE

CP

2.2nF

*OPTIONAL

RC TO LOWER

INRUSH CURRENT

4217 F03

Figure 3. Compensation for Small CLOAD

(OV pin), overcurrent circuit breaker (SENSE pin) or over

temperature. Normally the switch is turned off with a 250μA

current pulling down the GATE pin to ground. With the

switch turned off, the OUT voltage drops which pulls the

FB pin below its threshold. PG then pulls low to indicate

output power is no longer good.

If VDD drops below 2.65V for greater than 5μs or INTVCC

drops below 2.5V for greater than 1μs, a fast shutdown

of the switch is initiated. The GATE is pulled down with a

170mA current to the OUT pin.

Overcurrent Fault

The LTC4217 features an adjustable current limit with

foldback that protects against short-circuits or excessive

load current. To prevent excessive power dissipation in the

switch during active current limit, the available current is

reduced as a function of the output voltage sensed by the

FB pin. A graph in the Typical Performance Characteristics

curves shows the current limit versus FB voltage.

An overcurrent fault occurs when the current limit circuitry

has been engaged for longer than the timeout delay set

by the TIMER. Current limiting begins when the MOSFET

current reaches 0.5A to 2A (depending on the foldback).

The GATE pin is then brought down with a 140mA GATE-

to-OUT current. The voltage on the GATE is regulated in

order to limit the current to less than 2A. At this point, a

circuit breaker time delay starts by charging the external

timing capacitor from the TIMER pin with a 100μA pull-

up current. If the TIMER pin reaches its 1.2V threshold,

the internal switch turns off (with a 250μA current from

GATE to ground). Included in the Typical Performance

Characteristics curves is a graph of the Safe Operating

Area for the MOSFET. From this graph one can determine

the MOSFET’s maximum time in current limit for a given

output power.

Tying the TIMER pin to INTVCC will force the part to use

the internally generated (circuit breaker) delay of 2ms.

In either case the FLT pin is pulled low to indicate an

overcurrent fault has turned off the pass MOSFET. For a

given the circuit breaker time delay, the equation for setting

the timing capacitor’s value is as follows:

CT = tCB • 0.083(μF/ms)

After the switch is turned off, the TIMER pin begins dis-

charging the timing capacitor with a 2μA pull-down current.

When the TIMER pin reaches its 0.2V threshold, an internal

100ms timer is started. After the 100ms delay, the switch

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

cleared. Bringing the UV pin below 0.6V and then high will

clear the fault. If the TIMER pin is tied to INTVCC then the

switch is allowed to turn on again (after an internal 100ms

delay) if the overcurrent fault is cleared.

Tying the FLT pin to the UV pin allows the part to self-

clear the fault and turn the MOSFET on as soon as TIMER

pin has ramped below 0.2V. In this auto-retry mode the

LTC4217 repeatedly tries to turn on after an overcurrent

at a period determined by the capacitor on the TIMER pin.

The auto-retry mode also functions when the TIMER pin

is tied to INTVCC.

The waveform in Figure 4 shows how the output latches

off following a short-circuit. The current in the MOSFET

is 0.5A as the timer ramps up.

Current Limit Adjustment

The default value of the active current limit is 2A. The

current limit threshold can be adjusted lower by placing

VOUT

10V/DIV

IOUT

1A/DIV

ΔVGATE

10V/DIV

TIMER

2V/DIV

1ms/DIV

4217 F04

Figure 4. Short-Circuit Waveform

4217fc

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