LTC1735I-1 Linear Technology, LTC1735I-1 Datasheet - Page 14

• Manufacturer Part Number: LTC1735I-1
• Description: High Efficiency Synchronous Step-Down Switching Regulator
• Manufacturer: Linear Technology

LTC1735-1
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. Special
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
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 Sanyo OS-CON, Nichicon PL
series and Sprague 595D series. Consult manufacturers
for other specific recommendations.
INTV
An internal P-channel low dropout regulator produces the
5.2V supply that powers the drivers and internal circuitry
within the LTC1735-1. The INTV
maximum 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
14
CC
Regulator
U
U
CC
W
pin can supply a
U
INTV
bypassing is required to supply the high transient cur-
rents 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 LTC1735-1 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 current
is dependant on operating frequency as discussed in the
Efficiency Consideration section. The junction tempera-
ture can be estimated by using the equations given in Note
2 of the Electrical Characteristics. For example, the
LTC1735CS-1 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
EXTV
The LTC1735-1 contains an internal P-channel MOSFET
switch connected between the EXTV
Whenever the EXTV
regulator shuts off, the switch closes and INTV
supplied via EXTV
allows the MOSFET gate drive and control power to be
derived from the output or other external source during
normal operation. When the output is out of regulation
(start-up, short circuit) power is supplied from the internal
regulator. Do not apply greater than 7V to the EXTV
and ensure that EXTV
T
T
J
J
= 70 C + (17mA)(30V)(110 C/W) = 126 C
CC
CC
CC
CC
= 70 C + (17mA)(5V)(110 C/W) = 79 C
and PGND IC pins is highly recommended. Good
also needs to be taken into account for the power
pin is less than 4.7V, all of the INTV
Connection
CC
input pin. When the voltage applied to the
CC
CC
input pin reduces the junction tempera-
CC
until EXTV
CC
pin is above 4.7V the internal 5.2V
< V
IN
.
CC
CC
drops below 4.5V. This
pin as follows:
CC
CC
and INTV
current can be
CC
IN
IN
CC
)(I
current is
.
power is
INTVCC
CC
CC
pins.
pin
),