ltc3728legn-trpbf Linear Technology Corporation, ltc3728legn-trpbf Datasheet - Page 19

• Manufacturer Part Number: ltc3728legn-trpbf
• Description: Dual, 550khz, 2-phase Synchronous Step-down Switching Regulator
• Manufacturer: Linear Technology Corporation

APPLICATIONS INFORMATION
have lower storage capacity per unit volume than other
capacitor types. These capacitors offer a very cost-effec-
tive 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 ap-
plications 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, Cornell
Dubilier ESRE and Sprague 595D series. Consult manu-
facturers for other specifi c recommendations.
INTV
An internal P-channel low dropout regulator produces 5V
at the INTV
ers the drivers and internal circuitry within the LTC3728.
The INTV
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 di-
rectly adjacent to the INTV
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 LTC3728 to be
exceeded. The system supply current is normally dominated
by the gate charge current. Additional external loading of
the INTV
taken into account for the power dissipation calculations.
The total INTV
internal linear regulator or by the EXTV
CC
Regulator
CC
CC
CC
and 3.3V linear regulators also needs to be
pin regulator can supply a peak current of
CC
pin from the V
current can be supplied by either the 5V
CC
IN
and PGND IC pins is highly
supply pin. INTV
CC
input pin. When
CC
pow-
the voltage applied to the EXTV
of the INTV
regulator. Power dissipation for the IC in this case is high-
est: (V
gate charge current is dependent on operating frequency
as discussed in the Effi ciency Considerations section.
The junction temperature can be estimated by using the
equations given in Note 2 of the Electrical Characteristics.
For example, the LTC3728 V
than 24mA from a 24V supply when not using the EXTV
pin as follows:
Use of the EXTV
perature to:
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 op-
erating in continuous mode at maximum V
EXTV
The LTC3728 contains an internal P-channel MOSFET
switch connected between the EXTV
When the voltage applied to EXTV
the internal regulator is turned off and the switch closes,
connecting the EXTV
supplying internal power. The switch remains closed as
long as the voltage applied to EXTV
This allows the MOSFET driver and control power to be
derived from the output during normal operation (4.7V
< V
output is out of regulation (start-up, short-circuit). If more
current is required through the EXTV
specifi ed, an external Schottky diode can be added between
the EXTV
to the EXTV
Signifi cant effi ciency gains can be realized by powering
INTV
from the driver and control currents will be scaled by a
T
T
OUT
J
J
CC
CC
= 70°C + (24mA)(24V)(95°C/W) = 125°C
= 70°C + (24mA)(5V)(95°C/W) = 81°C
IN
< 7V) and from the internal regulator when the
from the output, since the V
Connection
)(I
CC
INTVCC
CC
and INTV
CC
current is supplied by the internal 5V linear
pin and ensure that EXTV
), and overall effi ciency is lowered. The
CC
input pin reduces the junction tem-
CC
CC
pins. Do not apply greater than 7V
pin to the INTV
IN
CC
current is limited to less
pin is less than 4.7V, all
CC
CC
CC
remains above 4.5V.
IN
CC
rises above 4.7V,
LTC3728
and INTV
current resulting
CC
CC
switch than is
< V
IN
.
pin thereby
IN
.
CC
19
pins.
3728fd
CC