LTC3838-1_15 Datasheet, PDF (27/52 Pages) Linear Technology – Dual, Fast, Accurate Step-Down DC/DC Controller with Dual Differential Output Sensing

LTC3838-1
APPLICATIONS INFORMATION
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 LTC3838-1 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 maxi-
mum junction temperature rating for the LTC3838-1 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 approximately equal to V_IN • I_DRVCC. The gate charge
current is dependent on operating frequency as discussed
in the Efficiency Considerations section. The junction tem-
perature can be estimated by using the equation given in
Note 2 of the Electrical Characteristics. For example, when
using the LDO, LTC3838-1’s DRV_CC current is limited to
less than 42mA from a 38V supply at T_A = 70°C:
T_J = 70°C + (42mA)(38V)(34°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 LTC3838-1’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 specified, 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 + (42mA)(5V)(34°C/W) = 77°C
However, for 3.3V and other low voltage outputs, ad-
ditional circuitry is required to derive DRV_CC power from
the converter output.
The following list summarizes the four possible connec-
tions for EXTV_CC:
1. EXTV_CC left open (or grounded). This will cause INTV_CC
to be powered from the internal 5.3V LDO resulting
in an efficiency penalty of up to 10% at high input
voltages.
2. EXTV_CC connected directly to switching converter output
V_OUT is higher than the switchover voltage’s higher limit
(4.8V). This provides the highest efficiency.
3. EXTV_CC connected to an external supply. If a 4.8V or
greater external supply is available, it may be used to
power EXTV_CC providing that the external supply is
sufficient for MOSFET gate drive requirements.
4. EXTV_CC connected to an output-derived boost network.
For 3.3V and other low voltage converters, efficiency
gains can still be realized by connecting EXTV_CC to an
output-derived voltage that has been boosted to greater
than 4.8V.
38381f
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