LTC3819 Datasheet, PDF (17/32 Pages) Linear Technology – 2-Phase, High Efficiency, Step-Down Controller for Sun Server CPUs

LTC3819
APPLICATIO S I FOR ATIO
INTV_CC Regulator
An internal P-channel low dropout regulator produces 5V
at the INTV_CC pin from the V_IN supply pin. The INTV_CC
regulator powers the drivers and internal circuitry of the
LTC3819. The INTV_CC pin regulator can supply up to 50mA
peak and must be bypassed to power ground with a
minimum of 4.7µF tantalum or electrolytic capacitor. An
additional 1µF ceramic capacitor placed very close to the
IC is recommended due to the extremely high instanta-
neous currents required by the MOSFET gate drivers.
High input voltage applications in which large MOSFETs
are being driven at high frequencies may cause the maxi-
mum junction temperature rating for the LTC3819 to be
exceeded. The supply current is dominated by the gate
charge supply current, in addition to the current drawn
from the differential amplifier output. The gate charge is
dependent on operating frequency as discussed in the
Efficiency Considerations section. The supply current can
either be supplied by the internal 5V regulator or via the
EXTV_CC pin. When the voltage applied to the EXTV_CC pin
is less than 4.7V, all of the INTV_CC load current is supplied
by the internal 5V linear regulator. Power dissipation for
the IC is higher in this case by (I_IN)(V_IN – INTV_CC) and
efficiency is lowered. The junction temperature can be
estimated by using the equations given in Note 1 of the
Electrical Characteristics. For example, the LTC3819 V_IN
current is limited to less than 24mA from a 24V supply:
T_J = 70°C + (24mA)(24V)(85°C/W) = 119°C
Use of the EXTV_CC pin reduces the junction temperature to:
T_J = 70°C + (24mA)(5V)(85°C/W) = 80.2°C
The input supply current should be measured while the
controller is operating in continuous mode at maximum
V_IN and the power dissipation calculated in order to
prevent the maximum junction temperature from being
exceeded.
EXTV_CC Connection
The LTC3819 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 an internal switch
closes, connecting the EXTV_CC pin to the INTV_CC pin
thereby supplying internal and MOSFET gate driving power
to the IC. 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 a
separate 5V supply during normal operation (4.7V <
V_EXTVCC < 7V) and from the internal regulator when the
external 5V supply is not available. Do not apply greater
than 7V to the EXTV_CC pin and ensure that EXTV_CC < V_IN +
0.3V when using the application circuits shown. If an
external voltage source is applied to the EXTV_CC pin when
the V_IN supply is not present, a diode can be placed in
series with the LTC3819’s V_IN pin and a Schottky diode
between the EXTV_CC and the V_IN pin, to prevent current
from backfeeding V_IN.
Topside MOSFET Driver Supply (C_B,D_B) (Refer to
Functional Diagram)
External bootstrap capacitors C_B1 and C_B2 connected to
the BOOST1 and BOOST2 pins supply the gate drive
voltages for the topside MOSFETs. Capacitor C_B in the
Functional Diagram is charged though diode D_B from
INTV_CC when the SW pin is low. When the topside MOSFET
turns on, the driver places the C_B voltage across the gate-
source of the desired MOSFET. This enhances the MOSFET
and turns on the topside switch. The switch node voltage,
SW, rises to V_IN and the BOOST pin rises to V_IN + V_INTVCC.
The value of the boost capacitor C_B needs to be 30 to 100
times that of the total input capacitance of the topside
MOSFET(s). The reverse breakdown of D_B must be greater
than V_IN(MAX).
The final arbiter when defining the best gate drive ampli-
tude level will be the input supply current. If a change is
made that decreases input current, the efficiency has
improved. If the input current does not change then the
efficiency has not changed either.
3819f
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