LTC3720_15 Datasheet, PDF(16/24 Page) Linear Technology – Single Phase VRM8.5 Current Mode Step-Down Controller

English

English German Russian Spanish Italian Polish Chinese Japanese Korean French Portuguese	Language :

LTC3720_15 Datasheet, PDF (16/24 Pages) Linear Technology – Single Phase VRM8.5 Current Mode Step-Down Controller

◁

LTC3720

APPLICATIO S I FOR ATIO

External Gate Drive Buffers

The LTC3720 drivers are adequate for driving up to about

60nC into MOSFET switches with RMS currents of 50mA.

Applications with larger MOSFET switches or operating at

higher frequencies requiring greater RMS currents will

benefit from using external gate drive buffers such as the

LTC1693. Alternately, the external buffer circuit shown in

Figure 5 can be used. Note that the bipolar devices reduce

the signal swing at the MOSFET gate and benefit from an

increased EXTVCC voltage of about 6V.

VIN

3.3V OR 5V

RUN/SS

RSS*

CSS

(6a)

INTVCC

RSS*

D2* RUN/SS

CSS

3720 F06

*OPTIONAL TO OVERRIDE

OVERCURRENT LATCHOFF

(6b)

Figure 6. RUN/SS Pin Interfacing with Latchoff Defeated

BOOST

INTVCC

FMMT619

10â¦

GATE

OF M1

10â¦

GATE

OF M2

FMMT720

PGND

3720 F05

Figure 5. Optional External Gate Driver

Soft-Start and Latchoff with the RUN/SS Pin

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

LTC3720 as well as a timer for soft-start and overcurrent

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

LTC3720 into a low quiescent current shutdown

(IQ < 30ÂµA). Releasing the pin allows an internal 1.2ÂµA

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

1.2ÂµA

C SS

1.3s/ÂµF CSS

When the voltage on RUN/SS reaches 1.5V, the LTC3720

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/ÂµF, during which the load current is folded

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

can be driven from logic as shown in Figure 6. Diode D1

reduces the start delay while allowing CSS to charge up

slowly for the soft-start function.

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 1.8ÂµA cur-

rent then begins discharging CSS. If the fault condition

persists until the RUN/SS pin drops to 3.5V, then the con-

troller 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 guar-

antee 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 â 4 [F/V s])

Generally 0.1ÂµF 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

3720f

▷