LTC1929C datasheet(13/24 Pages) LINER

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LTC1929

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. 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 1.4s/

µF to reach full current. The output current

thus ramps up slowly, reducing the starting surge current

required from the input power supply. If RUN/SS has been

pulled all the way to ground there is a delay before starting

of approximately:

F C

DELAY

=µ

()

125

The time for the output current to ramp up is then:

F C

IRAMP

−

=µ

()

31 5

125

By pulling both RUN/SS controller pins below 0.8V the

LTC1929 is put into low current shutdown (IQ < 40µA). The

RUN/SS pins can be driven directly from logic as shown in

Figure 6. Diode D1 in Figure 6 reduces the start delay but

allows CSS to ramp up slowly providing the soft-start

function. The RUN/SS pin has an internal 6V zener clamp

(see Functional Diagram).

Fault Conditions: Overcurrent Latchoff

The RUN/SS pin also provides the ability to latch off the

controllers when an overcurrent condition is detected. The

RUN/SS capacitor, CSS, is used initially to limit the inrush

current of both controllers. After the controllers have been

started and been given adequate time to charge up the

output capacitors and provide full load current, the RUN/

SS capacitor is used for a short-circuit timer. If the output

voltage falls to less than 70% of its nominal value, after

CSS reaches 4.1V, CSS begins discharging on the assump-

tion that the output is in an overcurrent condition. If the

condition lasts for a long enough period as determined by

the size of CSS, the controller will be shut down until the

RUN/SS pin voltage is recycled. If the overload occurs

during start-up, the time can be approximated by:

TLO1 ≈ (CSS • 0.6V)/(1.2µA) = 5 • 105 (CSS)

If the overload occurs after start-up the voltage on the

RUN/SS capacitor will continue charging and will provide

additional time before latching off:

TLO2 ≈ (CSS • 3V)/(1.2µA) = 2.5 • 106 (CSS)

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