LTC3707 Datasheet, PDF (17/32 Pages) Linear Technology – High Effi ciency, 2-Phase Synchronous Step-Down Switching Regulator

LTC3707
APPLICATIONS INFORMATION
that the output capacitance does not significantly discharge
during the operating frequency period due to ripple current.
The choice of using smaller output capacitance increases
the ripple voltage due to the discharging term but can be
compensated for by using capacitors of very low ESR to
maintain the ripple voltage at or below 50mV. The I_TH pin
OPTI-LOOP compensation components can be optimized
to provide stable, high performance transient response
regardless of the output capacitors selected.
Manufacturers such as Nichicon, United Chemicon and
Sanyo can be considered for high performance through-
hole capacitors. The OS-CON semiconductor dielectric
capacitor available from Sanyo has the lowest (ESR)(size)
product of any aluminum electrolytic at a somewhat
higher price. An additional ceramic capacitor in parallel
with OS-CON capacitors is recommended to reduce the
inductance effects.
In surface mount applications multiple capacitors may
need to be used in parallel to meet the ESR, RMS current
handling and load step requirements of the application.
Aluminum electrolytic, dry tantalum and special polymer
capacitors are available in surface mount packages. Special
polymer surface mount capacitors offer very low ESR but
have lower storage capacity per unit volume than other
capacitor types. These capacitors offer a very cost-effective
output capacitor solution and are an ideal choice when
combined with a controller having high loop bandwidth.
Tantalum capacitors offer the highest capacitance density
and are often used as output capacitors for switching
regulators having controlled soft-start. Several excellent
surge-tested choices are the AVX TPS, AVX TPSV or
the KEMET T510 series of surface mount tantalums,
available in case heights ranging from 2mm to 4mm.
Aluminum electrolytic capacitors can be used in cost-driven
applications providing that consideration is given to ripple
current ratings, temperature and long term reliability. A
typical application will require several to many aluminum
electrolytic capacitors in parallel. A combination of the
above mentioned capacitors will often result in maximizing
performance and minimizing overall cost. Other capacitor
types include Nichicon PL series, NEC Neocap, Pansonic
SP and Sprague 595D series. Consult manufacturers for
other specific recommendations.
INTV_CC Regulator
An internal P-channel low dropout regulator produces 5V
at the INTV_CC pin from the V_IN supply pin. INTV_CC pow-
ers the drivers and internal circuitry within the LTC3707.
The INTV_CC pin regulator can supply a peak current of
40mA and must be bypassed to ground with a minimum
of 4.7μF tantalum, 10μF special polymer, or low ESR type
electrolytic capacitor. A 1μF ceramic capacitor placed di-
rectly adjacent to the INTV_CC and PGND IC pins is highly
recommended. Good bypassing is necessary to supply
the high transient currents required by the MOSFET gate
drivers and to prevent interaction between channels.
Higher input voltage applications in which large MOSFETs
are being driven at high frequencies may cause the maxi-
mum junction temperature rating for the LTC3707 to be
exceeded. The system supply current is normally dominated
by the gate charge current. Additional external loading of
the INTV_CC and 3.3V linear regulators also needs to be
taken into account for the power dissipation calculations.
The total INTV_CC current can be supplied by either the 5V
internal linear regulator or by the EXTV_CC input pin. When
the voltage applied to the EXTV_CC pin is less than 4.7V, all
of the INTV_CC current is supplied by the internal 5V linear
regulator. Power dissipation for the IC in this case is high-
est: (V_IN)(I_INTVCC), and overall efficiency is lowered. The
gate charge current is dependent on operating frequency
as discussed in the Efficiency Considerations section.
The junction temperature can be estimated by using the
equations given in Note 2 of the Electrical Characteristics.
For example, the LTC3707 V_IN current is limited to less
than 24mA from a 24V supply when not using the EXTV_CC
pin as follows:
T_J = 70°C + (24mA)(24V)(95°C/W) = 125°C
Use of the EXTV_CC input pin reduces the junction tem-
perature to:
T_J = 70°C + (24mA)(5V)(95°C/W) = 81°C
Dissipation should be calculated to also include any added
current drawn from the internal 3.3V linear regulator.
To prevent maximum junction temperature from being
exceeded, the input supply current must be checked
operating in continuous mode at maximum V_IN.
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