LTC3875_15 Datasheet, PDF (26/44 Pages) Linear Technology – Dual, 2-Phase, Synchronous Controller with Low Value DCR Sensing and Temperature Compensation

LTC3875
Applications Information
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 5.5V regulator resulting
in an efficiency penalty 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 below 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 11 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.
V_IN
LTC3875
INTV_CC
R_VIN
1Ω
5V
+ _CINTVCC
4.7µF
C_IN
3875 F11
Figure 11. Setup for a 5V Input
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
where V_DB is the diode forward voltage drop.
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.
Undervoltage Lockout
The LTC3875 has two functions that help protect the
controller in case of undervoltage conditions. A precision
UVLO comparator constantly monitors the INTV_CC voltage
to ensure that an adequate gate-drive voltage is present.
It locks out the switching action when INTV_CC is below
3.7V. To prevent oscillation when there is a disturbance
on the INTV_CC, the UVLO comparator has 500mV of preci-
sion hysteresis.
Another way to detect an undervoltage condition is to
monitor the V_IN supply. Because the RUN pins have a
precision turn-on reference of 1.22V, one can use a resistor
divider to V_IN to turn on the IC when V_IN is high enough.
An extra 4.5µA of current flows out of the RUN pin once
the RUN pin voltage passes 1.22V. One can program the
hysteresis of the run comparator by adjusting the values
of the resistive divider. For accurate V_IN undervoltage
detection, V_IN needs to be higher than 4.5V.
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 MOSFET.
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
C_IN and C_OUT Selection
The selection of C_IN is simplified by the 2-phase architec-
ture and its impact on the worst-case RMS current drawn
through the input network (battery/fuse/capacitor). It can
be shown that the worst-case capacitor RMS current oc-
curs when only one controller is operating. The controller
with the highest (V_OUT)(I_OUT) product needs to be used
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