LTC3890-2 Datasheet, PDF (23/40 Pages) Linear Technology – 60V Low IQ, Dual, 2-Phase Synchronous Step-Down DC/DC Controller

LTC3890-2
APPLICATIONS INFORMATION
High input voltage applications in which large MOSFETs
are being driven at high frequencies may cause the
maximum junction temperature rating for the LTC3890-2
to be exceeded. The INTV_CC current, which is dominated
by the gate charge current, may be supplied by either the
V_IN LDO or the EXTV_CC LDO. When the voltage on the
EXTV_CC pin is less than 4.7V, the V_IN LDO is enabled. Power
dissipation for the IC in this case is highest and is equal
to V_IN • I_INTVCC. 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 3 of the
Electrical Characteristics. For example, the LTC3890E-2
INTV_CC current is limited to less than 32mA from a 40V
supply when not using the EXTV_CC supply at a 70°C ambi-
ent temperature:
T_J = 70°C + (32mA)(40V)(43°C/W) = 125°C
To prevent the maximum junction temperature from be-
ing exceeded, the input supply current must be checked
while operating in forced continuous mode (PLLIN/MODE
= INTV_CC) at maximum V_IN.
When the voltage applied to EXTV_CC rises above 4.7V, the
V_IN LDO is turned off and the EXTV_CC LDO is enabled. The
EXTV_CC LDO remains on as long as the voltage applied to
EXTV_CC remains above 4.5V. The EXTV_CC LDO attempts
to regulate the INTV_CC voltage to 5.1V, so while EXTV_CC
is less than 5.1V, the LDO is in dropout and the INTV_CC
voltage is approximately equal to EXTV_CC. When EXTV_CC
is greater than 5.1V, up to an absolute maximum of 14V,
INTV_CC is regulated to 5.1V.
Using the EXTV_CC LDO allows the MOSFET driver and
control power to be derived from one of the LTC3890-2’s
switching regulator outputs (4.7V ≤ V_OUT ≤ 14V) during
normal operation and from the V_IN LDO when the output
is out of regulation (e.g., start-up, short-circuit). If more
current is required through the EXTV_CC LDO than is speci-
fied, an external Schottky diode can be added between the
EXTV_CC and INTV_CC pins. In this case, do not apply more
than 6V to the EXTV_CC pin and make sure that EXTV_CC ≤ V_IN.
Significant efficiency and thermal gains can be realized
by powering INTV_CC from the output, since the V_IN cur-
rent resulting from the driver and control currents will be
scaled by a factor of (Duty Cycle)/(Switcher Efficiency).
For 5V to 14V regulator outputs, this means connecting
the EXTV_CC pin directly to V_OUT. Tying the EXTV_CC pin to
an 8.5V supply reduces the junction temperature in the
previous example from 125°C to:
T_J = 70°C + (32mA)(8.5V)(43°C/W) = 82°C
However, for 3.3V and other low voltage outputs, additional
circuitry is required to derive INTV_CC power from the output.
The following list summarizes the four possible connec-
tions for EXTV_CC:
1. EXTV_CC Grounded. This will cause INTV_CC to be powered
from the internal 5.1V regulator resulting in an efficiency
penalty of up to 10% at high input voltages.
2. EXTV_CC Connected Directly to V_OUT. This is the normal
connection for a 5V to 14V regulator and provides the
highest efficiency.
3. EXTV_CC Connected to an External Supply. If an external
supply is available in the 5V to 14V range, it may be
used to power EXTV_CC providing it is compatible with the
MOSFET gate drive requirements. Ensure that EXTV_CC
< V_IN.
4. EXTV_CC Connected to an Output-Derived Boost Network.
For 3.3V and other low voltage regulators, efficiency
gains can still be realized by connecting EXTV_CC to an
output-derived voltage that has been boosted to greater
than 4.7V. This can be done with the capacitive charge
pump shown in Figure 9. Ensure that EXTV_CC < V_IN.
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