LTC1736_15 Datasheet, PDF (14/28 Pages) Linear Technology – 5-Bit Adjustable High Efficiency Synchronous Step-Down Switching Regulator

LTC1736
APPLICATIO S I FOR ATIO
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. Spe-
cial polymer surface mount capacitors 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 capacitors offer the highest capaci-
tance 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 combina-
tion of the above mentioned capacitors 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 LTC1736. 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. Good
bypassing 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
maximum junction temperature rating for the LTC1736 to
be exceeded. The system supply current is normally
dominated 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
LTC1736G 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 LTC1736 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
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