LTC3728_15 Datasheet, PDF (19/36 Pages) Linear Technology – Dual, 550kHz, 2-Phase Synchronous Step-Down Switching Regulator

LTC3728
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
aforementioned 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 specific recommendations.
INTV_CC Regulator
An internal P-channel low dropout regulator produces 5V
at the INTV_CC pin from the V_IN supply pin. INTV_CC pow-
ers the drivers and internal circuitry within the LTC3728.
The INTV_CC pin regulator can supply a peak 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 di-
rectly adjacent to the INTV_CC and PGND IC pins is highly
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 MOS-
FETs are being driven at high frequencies may cause the
maximum 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_CC and 3.3V linear regulators also
needs to be taken into account for the power dissipation
calculations. The total INTV_CC current can be supplied by
either the 5V 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 5V linear regulator. Power dissipation for the
IC in this case is highest: (V_IN)(I_INTVCC), and overall ef-
ficiency is lowered. The gate charge current is dependent
on operating frequency, as discussed in the Efficiency
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_IN
current is limited to less than 24mA from a 24V supply
when not using the EXTV_CC pin, as follows:
T_J = 70°C + (24mA)(24V)(95°C/W) = 125°C
Use of the EXTV_CC input pin reduces the junction tem-
perature to:
T_J = 70°C + (24mA)(5V)(95°C/W) = 81°C
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_IN.
EXTV_CC Connection
The LTC3728 contains an internal P-channel MOSFET
switch connected between the EXTV_CC and INTV_CC pins.
When the voltage applied to EXTV_CC rises above 4.7V,
the internal regulator is turned off and the switch closes,
connecting the EXTV_CC pin to the INTV_CC pin thereby
supplying internal power. The switch remains closed as
long as the voltage applied to EXTV_CC remains above 4.5V.
This allows the MOSFET driver and control power to be
derived from the output during normal operation (4.7V
< V_OUT < 7V) and from the internal regulator when the
output is out of regulation (start-up, short-circuit). If more
current is required through the EXTV_CC switch than is
specified, an external Schottky diode can be added between
the EXTV_CC and INTV_CC pins. Do not apply greater than 7V
to the EXTV_CC pin and ensure that EXTV_CC < V_IN.
Significant efficiency gains can be realized by powering
INTV_CC from the output, since the V_IN current resulting
from the driver and control currents will be scaled by a
3728fg
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