LTC1778 datasheet(16/24 Pages) LINER

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LTC1778/LTC1778-1

1778fb

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 1.8µA 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 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

operation 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

current prevents the discharge of CSS during a fault and

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

as shown in Figure 8a is simple, but slightly increases

shutdown current. Connecting a resistor to INTVCC as

shown in Figure 8b 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.

Efficiency Considerations

The percent efficiency of a switching regulator is equal to

the output power divided by the input power times 100%.

It is often useful to analyze individual losses to determine

what is limiting the efficiency and which change would

produce the most improvement. Although all dissipative

elements in the circuit produce losses, four main sources

account for most of the losses in LTC1778 circuits:

1. DC I2R losses. These arise from the resistances of the

MOSFETs, inductor and PC board traces and cause the

efficiency to drop at high output currents. In continuous

mode the average output current flows through L, but is

chopped between the top and bottom MOSFETs. If the two

MOSFETs have approximately the same RDS(ON), then the

resistance of one MOSFET can simply be summed with the

resistances of L and the board traces to obtain the DC I2R

loss. For example, if RDS(ON) = 0.01Ω and RL = 0.005Ω, the

loss will range from 15mW to 1.5W as the output current

varies from 1A to 10A.

2. Transition loss. This loss arises from the brief amount

of time the top MOSFET spends in the saturated region

during switch node transitions. It depends upon the input

voltage, load current, driver strength and MOSFET

capacitance, among other factors. The loss is significant

at input voltages above 20V and can be estimated from:

Transition Loss ≅ (1.7A–1) VIN2 IOUT CRSS f

3. INTVCC current. This is the sum of the MOSFET driver

and control currents. This loss can be reduced by supply-

ing INTVCC current through the EXTVCC pin from a high

efficiency source, such as an output derived boost net-

work or alternate supply if available.

4. CIN loss. The input capacitor has the difficult job of

filtering the large RMS input current to the regulator. It

must have a very low ESR to minimize the AC I2R loss and

sufficient capacitance to prevent the RMS current from

causing additional upstream losses in fuses or batteries.

APPLICATIO S I FOR ATIO

Figure 8. RUN/SS Pin Interfacing with Latchoff Defeated

3.3V OR 5V

RUN/SS

VIN

INTVCC

RUN/SS

(8a)

(8b)

D2*

CSS

RSS*

CSS

*OPTIONAL TO OVERRIDE

OVERCURRENT LATCHOFF

RSS*

1778 F08

2N7002

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