LTC3866 Datasheet, PDF (23/36 Pages) Linear Technology – Current Mode Synchronous Controller for Sub Milliohm DCR Sensing

LTC3866
Applications Information
To prevent the maximum junction temperature from being
exceeded, the input supply current must be checked while
operating in continuous conduction mode (MODE/PLLIN
= SGND) at maximum V_IN. When the voltage applied to
EXTV_CC rises above 4.7V, the INTV_CC LDO is turned off
and the EXTV_CC is connected to the INTV_CC. The EXTV_CC
remains on as long as the voltage applied to EXTV_CC re-
mains above 4.5V. Using the EXTV_CC allows the MOSFET
driver and control power to be derived from an efficient
switching regulator output during normal operation. If more
current is required through the EXTV_CC than is specified,
an external Schottky diode can be added between the
EXTV_CC and INTV_CC pins. 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 EXTV_CC, 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 + (39mA)(5V)(37°C/W) = 77°C
However, for low voltage outputs, additional circuitry is
required to derive INTV_CC power from the output.
The following list summarizes the three possible connec-
tions for EXTV_CC:
1. EXTV_CC left open (or grounded). This will cause
INTV_CC to be powered from the internal LDO resulting
in an efficiency penalty of up to 10% at high input
voltages.
2. EXTV_CC connected to an external supply. If a 5V external
supply is available, it may be used to power EXTV_CC
providing it is compatible with the MOSFET gate drive
requirements.
3. 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.
For applications where the main input power is 5V, tie
the V_IN and INTV_CC pins together and tie the combined
pins to the 5V input with a 1Ω or 2.2Ω resistor as shown
in Figure 10 to minimize the voltage drop caused by the
gate charge current. This will override the INTV_CC linear
regulator and will prevent INTV_CC from dropping too low
due to the dropout voltage. Make sure the INTV_CC voltage
is at or exceeds the R_DS(ON) test voltage for the MOSFET
which is typically 4.5V for logic-level devices.
LTC3866
V_IN
INTV_CC
R_VIN
1Ω
+ _CINTVCC
4.7µF
5V
C_IN
3866 F10
Figure 10. Setup for a 5V Input
Topside MOSFET Driver Supply (C_B, D_B)
External bootstrap capacitor, C_B, connected to the BOOST
pin supplies the gate drive voltages for the topside MOS-
FET. Capacitor C_B in the Functional Diagram is charged
though external diode D_B from INTV_CC when the SW pin
is low. When the topside MOSFET is to be turned on, the
driver places the C_B voltage across the gate source of the
MOSFET. This enhances the MOSFET and turns on the
topside switch. The switch node voltage, SW, rises to V_IN
and the BOOST pin follows. With the topside MOSFET on,
the boost voltage is above the input supply:
V_BOOST = V_IN + V_INTVCC – V_DB
The value of the boost capacitor, C_B, needs to be 100 times
that of the total input capacitance of the topside MOSFET(s).
The reverse breakdown of the external Schottky diode
must be greater than V_IN(MAX). When adjusting the gate
drive level, the final arbiter is the total input current for
the regulator. If a change is made and the input current
decreases, then the efficiency has improved. If there is
no change in input current, then there is no change in
efficiency.
Setting Output Voltage
The LTC3866 output voltage is set by an external feedback
resistive divider carefully placed across the DIFFOUT pin,
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