LTC3780
APPLICATIONS INFORMATION
EXTV Connection
supply the gate drive voltage for the topside MOSFET
switches A and D. When the top MOSFET switch A turns
CC
The LTC3780 contains an internal P-channel MOSFET
switch connected between the EXTVCC and INTVCC pins.
When the voltage applied to EXTVCC rises above 5.7V, the
internal regulator is turned off and a switch connects the
EXTVCC pin to the INTVCC pin thereby supplying internal
power. The switch remains closed as long as the voltage
applied to EXTVCC remains above 5.5V. This allows the
MOSFET driver and control power to be derived from the
output when (5.7V < VOUT < 7V) and from the internal
regulator when the output is out of regulation (start-up,
short-circuit). If more current is required through the
EXTVCC switch than is specified, an external Schottky
diode can be interposed between the EXTVCC and INTVCC
pins. Ensure that EXTVCC ≤ VIN.
on, the switch node SW2 rises to V and the BOOST2
IN
pin rises to approximately V + INTV . When the bottom
IN
CC
MOSFET switch B turns on, the switch node SW2 drops
to low and the boost capacitor C is charged through D
B
B
from INTV . When the top MOSFET switch D turns on,
CC
the switch node SW1 rises to V
rises to approximately V
and the BOOST1 pin
CC
OUT
+ INTV . When the bottom
OUT
MOSFET switch C turns on, the switch node SW1 drops
to low and the boost capacitor C is charged through D
A
A
from INTV . The boost capacitors C and C need to
CC
A
B
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.
The following list summarizes the three possible connec-
Run Function
tions for EXTV :
CC
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
theLTC3780intolowcurrentshutdown.Donotapplymore
than 6V to the RUN pin.
1. EXTV left open (or grounded). This will cause INTV
CC
CC
to be powered from the internal 6V regulator at the cost
of a small efficiency penalty.
2. EXTV connected directly to V
(5.7V < V
< 7V).
CC
OUT
OUT
This is the normal connection for a 6V regulator and
provides the highest efficiency.
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
3. EXTV connected to an external supply. If an external
CC
supply is available in the 5.5V to 7V range, it may be
used to power EXTV provided it is compatible with
CC
equivalent of V ).
ITH
the MOSFET gate drive requirements.
An internal 1.2μA current source charges up the C
SS
Output Voltage
capacitor. As the voltage on SS increases from 0V to
2.4V, the internal current limit rises from 0V/R
to
SENSE
The LTC3780 output voltage is set by an external feedback
resistivedividercarefullyplacedacrosstheoutputcapacitor.
Theresultantfeedbacksignaliscomparedwiththeinternal
precision 0.800V voltage reference by the error amplifier.
The output voltage is given by the equation:
150mV/R
. The output current limit ramps up slowly,
SENSE
taking1.5s/μFtoreachfullcurrent.Theoutputcurrentthus
ramps up slowly, eliminating the starting surge current
required from the input power supply.
2.4V
1.2µA
T
=
•CSS = 1.5s/µF •C
SS
(
)
IRMP
R2
R1
⎛
⎝
⎞
VOUT = 0ꢀ8V s 1+
⎜
⎟
⎠
Do not apply more than 6V to the SS pin.
Topside MOSFET Driver Supply (C , D , C , D )
A
A
B
B
Current foldback is disabled during soft-start until the
voltage on C reaches 2V. Make sure C is large enough
SS
SS
Referring to Figure 11, the external bootstrap capacitors
when there is loading during start-up.
C and C connected to the BOOST1 and BOOST2 pins
A
B
3780fe
20
Linear Systems [ Linear Systems ]
