LTC3789_15 Datasheet, PDF (21/30 Pages) Linear Technology – High Efficiency, Synchronous, 4-Switch Buck-Boost Controller

LTC3789
APPLICATIONS INFORMATION
Significant efficiency and thermal gains can be realized
by powering EXTV_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 12V output reduces the junction
temperature in the previous example from 125°C to 97°C:
T_J = 70°C + (28mA)(12V)(80°C/W) = 97°C
Powering EXTV_CC from the output can also provide
enough gate drive when V_IN drops below 5V. This allows
a wider operating range for V_IN after the controller start
into regulation.
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 5.5V regulator at the
cost of a small efficiency penalty.
2. EXTV_CC connected directly to V_OUT (4.7V < V_OUT < 14V).
This is the normal connection for the 5.5V regulator and
provides the highest efficiency.
3. EXTV_CC connected to an external supply. If an external
supply is available in the 4.7V to 14V range, it may be
used to power EXTV_CC provided it is compatible with
the MOSFET gate drive requirements.
Note that there is an internal body diode from INTV_CC to
V_IN. When INTV_CC is powered from EXTV_CC and V_IN drops
lower than 4.5V, the diode will create a back-feeding path
from EXTV_CC to V_IN. To limit this back-feeding current, a
10Ω ~ 15Ω resistor is recommended between the system
V_IN voltage and the chip V_IN pin. To truly eliminate this
back-feeding current, a blocking Schottky diode should
be connected between the system V_IN and the chip V_IN.
Output Voltage
The LTC3789 output voltage is set by an external feed-
back resistive divider carefully placed across the output
capacitor. The resultant feedback signal is compared with
the internal precision 0.8V voltage reference by the error
amplifier. The output voltage is given by the equation:
V_OUT
=
0.8V
•
₁₊
R2 
R1
where R1 and R2 are defined in Figure 13.
Topside MOSFET Driver Supply (C_A, D_A, C_B, D_B)
Referring to Figure 13, the external bootstrap capacitors
C_A and C_B connected to the BOOST1 and BOOST2 pins
supply the gate drive voltage for the topside MOSFET
switches A and D. When the top switch A turns on, the
switch node SW1 rises to V_IN and the BOOST1 pin rises
to approximately V_IN + INTV_CC. When the bottom switch
B turns on, the switch node SW1 drops to low and the
boost capacitor C_A is charged through D_A from INTV_CC.
When the top switch D turns on, the switch node SW2
rises to V_OUT and the BOOST2 pin rises to approximately
V_OUT + INTV_CC. When the bottom switch C turns on, the
switch node SW2 drops to low and the boost capacitor C_B
is charged through D_A from INTV_CC. The boost capacitors
C_A and C_B need to store about 100 times the gate charge
required by the top switches A and D. In most applica-
tions, a 0.1µF to 0.47µF, X5R or X7R dielectric capacitor
is adequate.
Undervoltage Lockout
The LTC3789 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
3.4V. To prevent oscillation when there is a disturbance
on the INTV_CC, the UVLO comparator has 400mV of preci-
sion hysteresis.
Another way to detect an undervoltage condition is to moni-
tor the V_IN supply. Because the RUN pin has 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
5µA of current flows out of the RUN pin once its voltage
passes 1.22V. One can program the hysteresis of the run
comparator by adjusting the values of the resistive divider.
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