LTC3839 Datasheet, PDF (25/50 Pages) Linear Technology – Fast, Accurate, 2-Phase, Single-Output Step-Down DC/DC Controller

LTC3839
APPLICATIONS INFORMATION
For high switching frequencies, reducing output ripple and
better EMI filtering may require small value capacitors that
have low ESL (and correspondingly higher self-resonant
frequencies) to be placed in parallel with larger value
capacitors that have higher ESL. This will ensure good
noise and EMI filtering in the entire frequency spectrum
of interest. Even though ceramic capacitors generally
have good high frequency performance, small ceramic
capacitors may still have to be parallel connected with
large ones to optimize performance.
High performance through-hole capacitors may also be
used, but an additional ceramic capacitor in parallel is
recommended to reduce the effect of their lead inductance.
Remember also to place high frequency decoupling capaci-
tors as close as possible to the power pins of the load.
Top MOSFET Driver Supply (C_B, D_B)
An external bootstrap capacitor, C_B, connected to the
BOOST pin supplies the gate drive voltage for the topside
MOSFET. This capacitor is charged through diode D_B from
DRV_CC when the switch node is low. When the top MOSFET
turns on, the switch node rises to V_IN and the BOOST pin
rises to approximately V_IN + INTV_CC. The boost capacitor
needs to store approximately 100 times the gate charge
required by the top MOSFET. In most applications a 0.1μF
to 0.47μF, X5R or X7R dielectric capacitor is adequate. It
is recommended that the BOOST capacitor be no larger
than 10% of the DRV_CC capacitor, C_DRVCC, to ensure that
the C_DRVCC can supply the upper MOSFET gate charge
and BOOST capacitor under all operating conditions. Vari-
able frequency in response to load steps offers superior
transient performance but requires higher instantaneous
gate drive. Gate charge demands are greatest in high
frequency low duty factor applications under high load
steps and at start-up.
DRV_CC Regulator and EXTV_CC Power
The LTC3839 features a PMOS low dropout (LDO) linear
regulator that supplies power to DRV_CC from the V_IN supply.
The LDO regulates its output at the DRV_CC1 pin to 5.3V.
The LDO can supply a maximum current of 100mA and
must be bypassed to ground with a minimum of 4.7μF
ceramic capacitor. Good bypassing is needed to supply
the high transient currents required by the MOSFET gate
drivers and to minimize interaction between the channels.
High input voltage applications in which large MOSFETs are
being driven at high frequencies may cause the maximum
junction temperature rating for the LTC3839 to be exceeded,
especially if the LDO is active and provides DRV_CC. Power
dissipation for the IC in this case is highest and is ap-
proximately equal to V_IN • I_DRVCC. 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 equation given in Note 2
of the Electrical Characteristics. For example, when using
the LDO, LTC3839’s DRV_CC current is limited to less than
32mA from a 38V supply at T_A = 70°C:
T_J = 70°C + (32mA)(38V)(44°C/W) = 125°C
To prevent the maximum junction temperature from being
exceeded, the input supply current must be checked while
operating in continuous conduction mode at maximum V_IN.
When the voltage applied to the EXTV_CC pin rises above
the switchover voltage (typically 4.6V), the V_IN LDO is
turned off and the EXTV_CC is connected to DRV_CC2 pin with
an internal switch. This switch remains on as long as the
voltage applied to EXTV_CC remains above the hysteresis
(around 200mV) below the switchover voltage. Using
EXTV_CC allows the MOSFET driver and control power to
be derived from the LTC3839’s switching regulator output
V_OUT during normal operation and from the LDO when the
output is out of regulation (e.g., start up, short circuit). If
more current is required through the EXTV_CC than is speci-
fied, an external Schottky diode can be added between the
EXTV_CC and DRV_CC pins. Do not apply more than 6V to the
EXTV_CC pin and make sure that EXTV_CC is less than V_IN.
Significant efficiency and thermal gains can be realized
by powering DRV_CC from the switching converter output,
since the V_IN current resulting from the driver and control
currents will be scaled by a factor of (Duty Cycle)/(Switcher
Efficiency).
Tying the EXTV_CC pin to a 5V supply reduces the junction
temperature in the previous example from 125°C to:
T_J = 70°C + (32mA)(5V)(44°C/W) = 77°C
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