LTC1735_15 Datasheet, PDF (15/32 Pages) Linear Technology – High Efficiency Synchronous Step-Down Switching Regulator

LTC1735
APPLICATIO S I FOR ATIO
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, ESR, RMS current han-
dling and load step specifications may require multiple
capacitors in parallel. Aluminum electrolytic, dry tantalum
and special polymer capacitors are available in surface
mount packages. Special polymer surface mount capaci-
tors offer very low ESR but have much lower capacitive
density 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 capaci-
tors 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 rang-
ing from 1.5mm to 4.1mm. Aluminum electrolytic capaci-
tors can be used in cost-driven applications, provided that
consideration is given to ripple current ratings, tempera-
ture and long-term reliability. A typical application will
require several to many aluminum electrolytic capacitors
in parallel. A combination of the above mentioned capaci-
tors will often result in maximizing performance and
minimizing overall cost. Other capacitor types include
Nichicon PL series, NEC Neocap, Panasonic SP and
Sprague 595D series. Consult manufacturers for other
specific recommendations.
Like all components, capacitors are not ideal. Each ca-
pacitor has its own benefits and limitations. Combina-
tions of different capacitor types have proven to be a very
cost effective solution. Remember also to include high
frequency decoupling capacitors. They should be placed
as close as possible to the power pins of the load. Any
inductance present in the circuit board traces negates
their usefulness.
INTV_CC Regulator
An internal P-channel low dropout regulator produces the
5.2V supply that powers the drivers and internal circuitry
within the LTC1735. The INTV_CC pin can supply a maxi-
mum RMS current of 50mA 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 directly adjacent to the INTV_CC
and PGND IC pins is highly recommended. Good bypass-
ing is required to supply the high transient currents
required by the MOSFET gate drivers.
Higher input voltage applications in which large MOSFETs
are being driven at high frequencies may cause the maxi-
mum junction temperature rating for the LTC1735 to be
exceeded. The system supply current is normally domi-
nated by the gate charge current. Additional loading of
INTV_CC also needs to be taken into account for the power
dissipation calculations. The total INTV_CC current can be
supplied by either the 5.2V 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 5.2V linear regulator. Power
dissipation for the IC in this case is highest: (V_IN)(I_INTVCC)
and overall efficiency is lowered. The gate charge is
dependent on operating frequency as discussed in the
Efficiency Considerations section. The junction tempera-
ture can be estimated by using the equations given in
Note␣ 2 of the Electrical Characteristics. For example, the
LTC1735CS is limited to less than 17mA from a 30V
supply when not using the EXTV_CC pin as follows:
T_J = 70°C + (17mA)(30V)(110°C/W) = 126°C
Use of the EXTV_CC input pin reduces the junction tempera-
ture to:
T_J = 70°C + (17mA)(5V)(110°C/W) = 79°C
To prevent maximum junction temperature from being
exceeded, the input supply current must be checked
operating in continuous mode at maximum V_IN.
EXTV_CC Connection
The LTC1735 contains an internal P-channel MOSFET
switch connected between the EXTV_CC and INTV_CC pins.
Whenever the EXTV_CC pin is above 4.7V, the internal 5.2V
regulator shuts off, the switch closes and INTV_CC power is
supplied via EXTV_CC until EXTV_CC drops below 4.5V. This
allows the MOSFET gate drive and control power to be
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