LTC3785 LINER [Linear Technology], LTC3785 Datasheet - Page 20

• Manufacturer Part Number: LTC3785
• Description: High Efficiency, Synchronous, 4-Switch Buck-Boost Controller
• Manufacturer: LINER [Linear Technology]

LTC3789
applicaTions inForMaTion
Significant efficiency and thermal gains can be realized by
powering INTV
resulting from the driver and control currents will be scaled
by a factor of (Duty Cycle)/(Switcher Efficiency).
Tying the EXTV
tion temperature in the previous example from 125°C
to 97°C:
Powering INTV
enough gate drive when V
a wider operating range for V
into regulation.
The following list summarizes the three possible connec-
tions for EXTV
1. EXTV
2. EXTV
3. EXTV
Note that there is an internal body diode from INTV
V
lower than 4.5V, the diode will create a back-feeding path
from EXTV
10Ω ~ 15Ω resistor is recommended between the system
V
Output Voltage
The LTC3789 output voltage is set by an external feedback
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:
where R1 and R2 are defined in Figure 13.
20
IN
IN
T
to be powered from the internal 5.5V regulator at the
cost of a small efficiency penalty.
This is the normal connection for the 5.5V regulator and
provides the highest efficiency.
supply is available in the 4.7V to 14V range, it may be
used to power EXTV
the MOSFET gate drive requirements.
V
. When INTV
voltage and the chip V
J
OUT
= 70°C + (28mA)(12V)(80°C/W) = 97°C
CC
CC
CC
= 0.8V • 1+
left open (or grounded). This will cause INTV
connected directly to V
connected to an external supply. If an external
CC
to V
CC
CC
CC
CC
CC
:
IN



from the output, since the V
pin to a 12V output reduces the junc-
is powered from EXTV
. To limit this back-feeding current, a
from the output can also provide
R2
R1
CC



IN
IN
provided it is compatible with
drops below 5V. This allows
pin.
IN
after the controller start
OUT
(4.7V < V
CC
and V
OUT
IN
IN
current
< 14V).
drops
CC
CC
to
Topside MOSFET Driver Supply (C
Referring to Figure 13, the external bootstrap capacitors
C
supply the gate drive voltage for the topside MOSFET
switches A and D. When the top switch A turns on, the
switch node SW2 rises to V
to approximately V
B turns on, the switch node SW2 drops to low and the
boost capacitor C
When the top switch D turns on, the switch node SW1
rises to V
V
switch node SW1 drops to low and the boost capacitor C
is charged through D
C
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
to ensure that an adequate gate-drive voltage is present. It
locks out the switching action when INTV
To prevent oscillation when there is a disturbance on the
INTV
hysteresis.
Another way to detect an undervoltage condition is to moni-
tor the V
turn-on reference of 1.22V, one can use a resistor divider
to V
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.
Soft-Start Function
When a capacitor is connected to the SS pin, a soft-start
current of 3µA starts to charge the capacitor. A soft-start
function is achieved by controlling the output ramp volt-
age according to the ramp rate on the SS pin. Current
foldback is disabled during this phase to ensure smooth
soft-start. When the chip is in the shutdown state with its
RUN pin voltage below 1.22V, the SS pin is actively pulled
A
OUT
A
and C
and C
IN
CC
+ INTV
to turn on the IC when V
, the UVLO comparator has 400mV of precision
IN
B
B
OUT
need to store about 100 times the gate charge
connected to the BOOST1 and BOOST2 pins
supply. Because the RUN pin has a precision
CC
and the BOOST1 pin rises to approximately
. When the bottom switch C turns on, the
B
IN
is charged through D
A
+ INTV
from INTV
IN
CC
IN
and the BOOST2 pin rises
. When the bottom switch
CC
is high enough. An extra
. The boost capacitors
A
, D
A
CC
, C
B
from INTV
is below 3.4V.
B
, D
CC
B
)
voltage
3789f
CC
A
.