LTC3876_15 Datasheet, PDF (27/48 Pages) Linear Technology – Dual DC/DC Controller for DDR Power with Differential VDDQ Sensing and 50mA VTT Reference

LTC3876
APPLICATIONS INFORMATION
When the voltage applied to the EXTV_CC pin rises above
4.7V, 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 4.5V. Using EXTV_CC allows the MOSFET driver and
control power to be derived from the LTC3876’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 + (52mA)(5V)(28°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 > 4.7V. This provides the highest efficiency.
3. EXTV_CC connected to an external supply. If a 4.7V 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 4.7V.
For applications where the main input power never exceeds
5.3V, tie the DRV_CC1 and DRV_CC2 pins to the V_IN input
through a small resistor, (such as 1Ω to 2Ω) as shown
in Figure 7 to minimize the voltage drop caused by the
gate charge current. This will override the LDO and will
prevent DRV_CC from dropping too low due to the dropout
voltage. Make sure the DRV_CC voltage exceeds the R_DS(ON)
test voltage for the external MOSFET which is typically at
4.5V for logic-level devices.
LTC3876
DRV_CC2
DRV_CC1
R_DRVCC
C_DRVCC
V_IN
C_IN
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Figure 7. Setup for V_IN ≤ 5.3V
Input Undervoltage Lockout (UVLO)
The LTC3876 has two functions that help protect the con-
troller in case of input undervoltage conditions. An internal
UVLO comparator constantly monitors the INTV_CC and
DRV_CC voltages to ensure that adequate voltages are pres-
ent. The comparator enables internal UVLO signal, which
locks out the switching action of both channels, until the
INTV_CC and DR_VCC1,2 pins are all above their respective
UVLO thresholds. The rising threshold (to release UVLO)
of the INTV_CC is typically 4.2V, with 0.5V falling hysteresis
(to re-enable UVLO). The UVLO thresholds for DR_VCC1,2 are
lower than that of INTV_CC but higher than typical threshold
voltages of power MOSFETs, to prevent them from turning
on without sufficient gate drive voltages.
Generally for V_IN > 6V, a UVLO can be set through monitoring
the V_IN supply by using external voltage dividers at the RUN
pins from V_IN to SGND. To design the voltage divider, note
that both RUN pins have two levels of threshold voltages.
The precision gate-drive-enable threshold voltage of 1.2V
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