LTC3856 Datasheet, PDF (28/40 Pages) Linear Technology – 2-Phase Synchronous Step-Down DC/DC Controller with Diffamp

LTC3856
Applications Information
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 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 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).
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 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 left open (or grounded). This will cause INTV_CC
to be powered from the internal 5V LDO 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 regulator and provides the highest
efficiency.
3. 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.
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.
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 13) 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.
Topside MOSFET Driver Supply (C_B, D_B)
External bootstrap capacitors, C_B, connected to the
BOOST pins supply the gate drive voltages for the top-
side MOSFETs. Capacitor C_B in the Functional Diagram
is charged though external diode D_B from INTV_CC when
the SW pin is low. When one of the topside MOSFETs is
to be turned on, the driver places the C_B voltage across
the gate source of the desired 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
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.
LTC3856 ^VIN
INTV_CC
R_VIN
1Ω
5V
C_INTVCC
4.7µF
+
C_IN
3856 F13
Figure 13. Set-Up for a 5V Input
3856f
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