LTC1735 Linear Technology, LTC1735 Datasheet - Page 14

• Manufacturer Part Number: LTC1735
• Description: High Efficiency Synchronous Step-Down Switching Regulator
• Manufacturer: Linear Technology

LTC1735
APPLICATIO S I FOR ATIO
where I is the change in load current.
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
14
C
OUT
2
(
L I
( )
V V
)
OUT
2
U
U
W
U
as close as possible to the power pins of the load. Any
inductance present in the circuit board traces negates
their usefulness.
INTV
An internal P-channel low dropout regulator produces the
5.2V supply that powers the drivers and internal circuitry
within the LTC1735. The INTV
mum RMS current of 50mA and must be bypassed to
ground with a minimum of 4.7 F tantalum, 10 F special
polymer or low ESR type electrolytic capacitor. A 1 F
ceramic capacitor placed directly adjacent to the INTV
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
dissipation calculations. The total INTV
supplied by either the 5.2V internal linear regulator or by
the EXTV
EXTV
supplied by the internal 5.2V linear regulator. Power
dissipation for the IC in this case is highest: (V
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
Use of the EXTV
ture to:
To prevent maximum junction temperature from being
exceeded, the input supply current must be checked
operating in continuous mode at maximum V
T
T
J
J
CC
CC
CC
= 70 C + (17mA)(30V)(110 C/W) = 126 C
= 70 C + (17mA)(5V)(110 C/W) = 79 C
also needs to be taken into account for the power
Regulator
pin is less than 4.7V, all of the INTV
CC
input pin. When the voltage applied to the
CC
input pin reduces the junction tempera-
CC
CC
pin can supply a maxi-
pin as follows:
CC
current can be
CC
IN
IN
current is
)(I
.
INTVCC
CC
)