LTC3717-1 View Datasheet(PDF) - Linear Technology
Part Name
Description
MFG CO.
LTC3717-1
Linear Technology
'LTC3717-1' PDF : 20 Pages View PDF
LTC3717-1
APPLICATIO S I FOR ATIO
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 guar-
antee 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. 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 6a is simple, but slightly
increases shutdown current. Connecting a resistor to
INTVCC as shown in Figure 6b 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 over-
come the 4µA maximum discharge current.
INTVCC Regulator
An internal P-channel low dropout regulator produces the
5V supply that powers the drivers and internal circuitry
within the LTC3717-1. The INTVCC pin can supply up to
50mA RMS and must be bypassed to ground with a
minimum of 4.7µF tantalum or other low ESR capacitor.
Good bypassing is necessary to supply the high transient
currents required by the MOSFET gate drivers. Applica-
tions using large MOSFETs with a high input voltage and
high frequency of operation may cause the LTC3717-1 to
exceed its maximum junction temperature rating or RMS
current rating. Most of the supply current drives the
MOSFET gates unless an external EXTVCC source is used.
In continuous mode operation, this current is IGATECHG =
f(Qg(TOP) + Qg(BOT)). The junction temperature can be
estimated from the equations given in Note 2 of the
Electrical Characteristics. For example, the LTC3717EUH-1
is limited to less than 14mA from a 30V supply:
TJ = 70°C + (14mA)(30V)(34°C/W) = 84.3°C
For larger currents, consider using an external supply with
the EXTVCC pin.
EXTVCC Connection
The EXTVCC pin can be used to provide MOSFET gate drive
and control power from the output or another external
source during normal operation. Whenever the EXTVCC
pin is above 4.7V the internal 5V regulator is shut off and
an internal 50mA P-channel switch connects the EXTVCC
pin to INTVCC. INTVCC power is supplied from EXTVCC until
this pin drops below 4.5V. Do not apply more than 7V to
the EXTVCC pin and ensure that EXTVCC ≤ VCC. The follow-
ing list summarizes the possible connections for EXTVCC:
1. EXTVCC grounded. INTVCC is always powered from the
internal 5V regulator.
2. EXTVCC connected to an external supply. A high effi-
ciency supply compatible with the MOSFET gate drive
requirements (typically 5V) can improve overall
efficiency.
3. EXTVCC connected to an output derived boost network.
The low voltage output can be boosted using a charge
pump or flyback winding to greater than 4.7V. The system
will start-up using the internal linear regulator until the
boosted output supply is available.
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
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