Datasheet

LTC3789
21
3789fa
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.
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.