LTC1629C datasheet(17/28 Pages) LINER

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LTC1629/LTC1629-PG

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 LTC1629 has a true remote voltage sense capablity.

The sensing connections should be returned from the load

back to the differential amplifier’s inputs through a com-

mon, tightly coupled pair of PC traces. The differential

amplifier rejects common mode signals capacitively or

inductively radiated into the feedback PC traces as well as

ground loop disturbances. The differential amplifier out-

put signal is divided down and compared with the internal

precision 0.8V voltage reference by the error amplifier.

The differential amplifier can be used in either of two

configurations according to the voltage applied to the

AMPMD pin. The first configuration, with the connections

illustrated in the Functional Diagram, utilizes a set of

internal precision resistors to enable precision instrumen-

tation-type measurement of the output voltage. This con-

figuration is activated when the AMPMD pin is tied to

ground and is the only configuration available for the

LTC1629-PG. When the AMPMD pin is tied to INTVCC, the

resistors are disconnected and the amplifier inputs are

made directly available. The amplifier can then be used as

a general purpose op amp. The amplifier has a 0V to 3V

common mode input range limitation due to the internal

switching of its inputs. The output is an NPN emitter

follower without any internal pull-down current. A DC

resistive load to ground is required in order to sink current.

The output will swing from 0V to 10V. (VIN≥VDIFFOUT + 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 sources’ surge

currents by gradually increasing the controller’s current

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

extreme load transients from tripping the overcurrent

latch. A small pull-up current (>5

µA) supplied to the RUN/

SS pin will prevent the overcurrent latch from operating.

The following explanation describes how the functions

operate.

An internal 1.2

µA 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 1.4

µs/µF to reach full current. The output cur-

APPLICATIO S I FOR ATIO

Figure 5a. Secondary Output Loop and EXTVCC Connection

Figure 5b. Capacitive Charge Pump for EXTVCC

1629 F05a

VIN

TG1

N-CH

1N4148

N-CH

BG1

PGND

LTC1629

SW1

EXTVCC

OPTIONAL EXTVCC CONNECTION

5V < VSEC < 7V

RSENSE

VSEC

6.8V

VOUT

VIN

CIN

µF

COUT

1629 F05b

VIN

TG1

N-CH

BG1

PGND

LTC1629

SW1

EXTVCC

RSENSE

BAT85

0.22

µF

VOUT

VIN

CIN

COUT

VN2222LL

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