LTC3608(RevC) Просмотр технического описания (PDF) - Linear Technology

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LTC3608
(Rev.:RevC)

18V, 8A Monolithic Synchronous Step-Down DC/DC Converter

Linear Technology 

LTC3608

Applications Information

The EXTVCC pin can be used to provide MOSFET gate drive

and control power from the output or another external

source during normal operation. Whenever the EXTVCC

pin is above 4.7V the internal 5V regulator is shut off and

an internal 50mA P-channel switch connects the EXTVCC

pin to INTVCC. INTVCC power is supplied from EXTVCC

until this pin drops below 4.5V. Do not apply more than

7V to the EXTVCC pin and ensure that EXTVCC ≤ VIN. The

following list summarizes the possible connections for

EXTVCC:

1. EXTVCC grounded. INTVCC is always powered from the

internal 5V regulator.

2. EXTVCC connected to an external supply. A high efficiency

supply compatible with the MOSFET gate drive require-

ments (typically 5V) can improve overall efficiency.

3. EXTVCC connected to an output derived boost network.

The low voltage output can be boosted using a charge

pump or flyback winding to greater than 4.7V. The

system will start-up using the internal linear regulator

until the boosted output supply is available.

Soft-Start and Latchoff with the RUN/SS Pin

The RUN/SS pin provides a means to shut down the

LTC3608 as well as a timer for soft-start and overcurrent

latchoff. Pulling the RUN/SS pin below 0.8V puts the

LTC3608 into a low quiescent current shutdown (IQ <

30µA). Releasing the pin allows an internal 1.2µA current

source to charge up the external timing capacitor CSS. If

RUN/SS has been pulled all the way to ground, there is a

delay before starting of about:

( ) tDELAY

=

1.5V

1.2µA

CSS

=

1.3s/µF

CSS

When the voltage on RUN/SS reaches 1.5V, the LTC3608

begins operating with a clamp on ITH of approximately

0.9V. As the RUN/SS voltage rises to 3V, the clamp on ITH

is raised until its full 2.4V range is available. This takes an

additional 1.3s/µF, during which the load current is folded

back until the output reaches 75% of its final value.

After the controller has been started and given adequate

time to charge up the output capacitor, CSS is used as a

short-circuit timer. After the RUN/SS pin charges above 4V,

if the output voltage falls below 75% of its regulated value,

then a short-circuit fault is assumed. A 1.8µA current then

begins discharging CSS. If the fault condition persists until

the RUN/SS pin drops to 3.5V, then the controller turns

off both power MOSFETs, shutting down the converter

permanently. The RUN/SS pin must be actively pulled

down to ground in order to restart operation.

The overcurrent protection timer requires that the soft‑start

timing capacitor, CSS, be made large enough to guarantee

that the output is in regulation by the time CSS has reached

the 4V threshold. In general, this will depend upon the

size of the output capacitance, output voltage and load

current characteristic. A minimum soft-start capacitor

can be estimated from:

CSS > COUT VOUT RSENSE (10 –4 [F/V s])

Generally 0.1µF is more than sufficient.

Overcurrent latchoff operation is not always needed or

desired. Load current is already limited during a short

circuit by the current foldback circuitry and latchoff op-

eration can prove annoying during troubleshooting. The

feature can be overridden by adding a pull-up current

greater than 5µA to the RUN/SS pin. The additional cur-

rent prevents the discharge of CSS during a fault and also

shortens the soft-start period. Using a resistor to VIN as

shown in Figure 5a is simple, but slightly increases shut-

down current. Connecting a resistor to INTVCC as shown

in Figure 5b eliminates the additional shutdown current,

but requires a diode to isolate CSS. Any pull-up network

must be able to pull RUN/SS above the 4.2V maximum

threshold of the latchoff circuit and overcome the 4µA

maximum discharge current.

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