LTC3727LX-1_15 Datasheet, PDF (16/28 Pages) Linear Technology – High Efficiency, 2-Phase Synchronous Step-Down Switching Regulator

LTC3727LX-1
APPLICATIO S I FOR ATIO
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 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 TPS Series III
or the KEMET T510 series of surface mount tantalums,
available in case heights ranging from 1.2mm to 4.1mm.
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 Nichicon PL
series, NEC Neocap, Cornell Dubilier ESRE and Sprague
595D series. Consult manufacturers for other specific
recommendations.
INTV_CC Regulator
An internal P-channel low dropout regulator produces
7.5V at the INTV_CC pin from the V_IN supply pin. INTV_CC
powers the drivers and internal circuitry within the
LTC3727LX-1. 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 directly adjacent to the INTV_CC and
PGND IC pins is highly recommended. Good bypassing is
necessary to supply the high transient currents required
16
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 LTC3727LX-1 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 7.5V internal linear regulator or by the EXTV_CC
input pin. When the voltage applied to the EXTV_CC pin is
less than 7.3V, all of the INTV_CC current is supplied by the
internal 7.5V linear regulator. Power dissipation for the IC
in this case is highest: (V_IN)(I_INTVCC), and overall efficiency
is lowered. The gate charge current is dependent on
operating frequency as discussed in the Efficiency Consid-
erations section. The junction temperature can be esti-
mated by using the equations given in Note 2 of the
Electrical Characteristics. For example, the LTC3727LX-1
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 tempera-
ture to:
T_J = 70°C + (24mA)(7.5V)(95°C/W) = 87°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 ex-
ceeded, the input supply current must be checked operat-
ing in continuous mode at maximum V_IN.
EXTV_CC Connection
The LTC3727LX-1 contains an internal P-channel MOS-
FET switch connected between the EXTV_CC and INTV_CC
pins. When the voltage applied to EXTV_CC rises above
7.3V, 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 7.0V. This allows the MOSFET driver and control
power to be derived from the output during normal opera-
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