LTC3729L-6 Datasheet, PDF(17/28 Page) Linear Technology – PolyPhase, Synchronous Step-Down Switching Regulator

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LTC3729L-6 Datasheet, PDF (17/28 Pages) Linear Technology – PolyPhase, Synchronous Step-Down Switching Regulator

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LTC3729L-6

APPLICATIO S I FOR ATIO

OPTIONAL EXTVCC CONNECTION

5V < VSEC < 7V

VIN

CIN

LTC3729L-6 VIN

EXTVCC

TG1

N-CH

SW1

BG1

1N4148

6.8V

VSEC

RSENSE

1ÂµF

VOUT

COUT

N-CH

PGND

3729L-6 F05a

Figure 5a. Secondary Output Loop and EXTVCC Connection

inductive boost winding as shown in Figure 5a or the

capacitive charge pump shown in Figure 5b. The charge

pump has the advantage of simple magnetics.

Topside MOSFET Driver Supply (CB,DB) (Refer to

Functional Diagram)

External bootstrap capacitors CB1 and CB2 connected to

the BOOST1 and BOOST2 pins supply the gate drive

voltages for the topside MOSFETs. Capacitor CB in the

Functional Diagram is charged though diode DB from

INTVCC when the SW pin is low. When the topside MOSFET

turns on, the driver places the CB voltage across the gate-

source of the desired MOSFET. This enhances the MOSFET

and turns on the topside switch. The switch node voltage,

SW, rises to VIN and the BOOST pin rises to VIN + VINTVCC.

The value of the boost capacitor CB needs to be 30 to 100

times that of the total input capacitance of the topside

MOSFET(s). The reverse breakdown of DB must be greater

than VIN(MAX).

The final arbiter when defining the best gate drive ampli-

tude level will be the input supply current. If a change is

made that decreases input current, the efficiency has

improved. If the input current does not change then the

efficiency has not changed either.

Output Voltage

The LTC3729L-6 has a true remote voltage sense capablity.

The sensing connections should be returned from the load

back to the differential amplifierâs inputs through a com-

mon, tightly coupled pair of PC traces. The differential

VIN

CIN

LTC3729L-6 VIN

TG1

N-CH

EXTVCC

SW1

BG1

BAT85

0.22ÂµF

BAT85

VN2222LL

RSENSE

BAT85

VOUT

COUT

N-CH

PGND

3729L-6 F05b

Figure 5b. Capacitive Charge Pump for EXTVCC

amplifier rejects common mode signals capacitively or

inductively radiated into the feedback PC traces as well

as ground loop disturbances. The differential amplifier

output signal is divided down and compared with the

internal precision 0.6V voltage reference by the error

amplifier.

The output is an NPN emitter follower without any internal

pull-down current. A DC resistive load to ground is re-

quired in order to sink current. The output will swing from

0V to 10V. (VIN â¥ VDIFFOUTâ£ +â£ 2V.)

Soft-Start/Run Function

The RUN/SS pin provides three functions: 1) Run/Shut-

down, 2) soft-start and 3) a defeatable short-circuit latchoff

timer. Soft-start reduces the input power sourcesâ surge

currents by gradually increasing the controllerâs current

limit ITH(MAX). The latchoff timer prevents very short,

extreme load transients from tripping the overcurrent

latch. A small pull-up current (>5ÂµA) supplied to the RUN/

SS pin will prevent the overcurrent latch from operating.

The following explanation describes how the functions

operate.

An internal 1.2ÂµA current source charges up the CSS

capacitor. When the voltage on RUN/SS reaches 1.5V, the

controller is permitted to start operating. As the voltage on

RUN/SS increases from 1.5V to 3.0V, the internal current

limit is increased from 25mV/RSENSE to 75mV/RSENSE.

The output current limit ramps up slowly, taking an

additional 1.4Âµs/ÂµF to reach full current. The output

current thus ramps up slowly, reducing the starting surge

sn3729l6 3729l6fs

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