Datasheet

LTC3780
21
3780ff
For more information www.linear.com/LTC3780
applicaTions inForMaTion
EXTV
CC
Connection
The LTC3780 contains an internal P-channel MOSFET
switch connected between the EXTV
CC
and INTV
CC
pins.
When the voltage applied to EXTV
CC
rises above 5.7V, the
internal regulator is turned off and a switch connects the
EXTV
CC
pin to the INTV
CC
pin thereby supplying internal
power. The switch remains closed as long as the voltage
applied to EXTV
CC
remains above 5.5V. This allows the
MOSFET driver and control power to be derived from the
output when (5.7V < V
OUT
< 7V) and from the internal
regulator when the output is out of regulation (start-up,
short-circuit). If more current is required through the
EXTV
CC
switch than is specified, an external Schottky
diode can be interposed between the EXTV
CC
and INTV
CC
pins. Ensure that EXTV
CC
≤ V
IN
.
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 6V regulator at the cost
of a small efficiency penalty.
2. EXTV
CC
connected directly to V
OUT
(5.7V < V
OUT
< 7V).
This is the normal connection for a 6V regulator and
provides the highest efficiency.
3. EXTV
CC
connected to an external supply. If an external
supply is available in the 5.5V to 7V range, it may be
used to power EXTV
CC
provided it is compatible with
the MOSFET gate drive requirements.
Output Voltage
The LTC3780 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 0.800V voltage reference by the error amplifier.
The output voltage is given by the equation:
VV
R
R
OUT
=+






08 1
2
1
.•
Topside MOSFET Driver Supply (C
A
, D
A
, C
B
, D
B
)
Referring to Figure 11, 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 MOSFET switch A turns
on, the switch node SW2 rises to V
IN
and the BOOST2
pin rises to approximately V
IN
+ INTV
CC
. When the bottom
MOSFET switch B turns on, the switch node SW2 drops
to low and the boost capacitor C
B
is charged through D
B
from INTV
CC
. When the top MOSFET switch D turns on,
the switch node SW1 rises to V
OUT
and the BOOST1 pin
rises to approximately V
OUT
+ INTV
CC
. When the bottom
MOSFET switch C turns on, the switch node SW1 drops
to low and the boost capacitor C
A
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
MOSFET switch A and D. In most applications a 0.1µF to
0.47µF, X5R or X7R dielectric capacitor is adequate.
Run Function
The RUN pin provides simple ON/OFF control for the
LTC3780. Driving the RUN pin above 1.5V permits the
controller to start operating. Pulling RUN below 1.5V puts
the LTC3780 into low current shutdown. Do not apply more
than 6V to the RUN pin.
Soft-Start Function
Soft-start reduces the input power sources’ surge cur
-
rents by gradually increasing the controller’s current
limit (proportional to an internally buffered and clamped
equivalent of V
ITH
).
An internal 1.2µA current source charges up the C
SS
ca-
pacitor. As the voltage on SS increases from 0V to 2.4V,
the internal current limit rises from 0V/R
SENSE
to 150mV/
R
SENSE
. The output current limit ramps up slowly, taking
1.5s/µF to reach full current. The output current thus ramps
up slowly, eliminating the starting surge current required
from the input power supply.
T
IRMP
=
2.4V
1.2µA
• C
SS
= 1.5s/µF
(
)
• C
SS
Do not apply more than 6V to the SS pin.
Current foldback is disabled during soft-start until the
voltage on C
SS
reaches 2V. Make sure C
SS
is large enough
when there is loading during start-up.