LTC3611 Просмотр технического описания (PDF) - Linear Technology
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LTC3611 Datasheet PDF : 24 Pages
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LTC3611
APPLICATIONS INFORMATION
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/μF, during which the load current is folded
back until the output reaches 75% of its final value.
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 current then
begins discharging CSS. If the fault condition persists until
the RUN/SS pin drops to 3.5V, then the controller 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 de-
sired. 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 5a is simple, but slightly increases shutdown
current. Connecting a resistor to INTVCC as shown in
Figure 5b 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.
INTVCC
3.3V OR 5V
D1
VIN
RUN/SS
RSS*
CSS
RSS*
D2* RUN/SS
2N7002
CSS
(5a)
3611 F05
*OPTIONAL TO OVERRIDE
OVERCURRENT LATCHOFF
(5b)
Figure 5. RUN/SS Pin Interfacing with Latchoff Defeated
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 LTC3611 circuits:
1. DC I2R losses. These arise from the resistance of the
internal resistance 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 bot-
tom MOSFETs. If the two MOSFETs have approximately
the same RDS(ON), then the DC I2R loss for one MOSFET
can simply be determined by [RDS(ON) + RL] • IO.
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 sup-
plying INTVCC current through the EXTVCC pin from a
high efficiency source, such as an output derived boost
network or alternate supply if available.
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