LTC3780EG Linear Technology, LTC3780EG Datasheet - Page 20

• Manufacturer Part Number: LTC3780EG
• Description: IC,SMPS CONTROLLER,CURRENT-MODE,CMOS,SSOP,24PIN,PLASTIC
• Manufacturer: Linear Technology

APPLICATIO S I FOR ATIO
LTC3780
EXTV
The LTC3780 contains an internal P-channel MOSFET
switch connected between the EXTV
When the voltage applied to EXTV
internal regulator is turned off and a switch connects the
EXTV
power. The switch remains closed as long as the voltage
applied to EXTV
MOSFET driver and control power to be derived from the
output when (5.7V < V
regulator when the output is out of regulation (start-up,
short-circuit). If more current is required through the
EXTV
diode can be interposed between the EXTV
pins. Ensure that EXTV
The following list summarizes the three possible connec-
tions for EXTV
1. EXTV
2. EXTV
3. EXTV
Output Voltage
The LTC3780 output voltage is set by an external feedback
resistive divider carefully placed across the output capaci-
tor. 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:
20
to be powered from the internal 6V regulator at the cost
of a small efficiency penalty.
This is the normal connection for a 6V regulator and
provides the highest efficiency.
supply is available in the 5.5V to 7V range, it may be
used to power EXTV
the MOSFET gate drive requirements.
V
OUT
CC
CC
CC
CC
CC
CC
Connection
pin to the INTV
switch than is specified, an external Schottky
=
left open (or grounded). This will cause INTV
connected directly to V
connected to an external supply. If an external
0 8
.
V
CC
CC
•
:
⎛
⎜
⎝
U
1
remains above 5.5V. This allows the
+
R
R
CC
CC
CC
OUT
2
1
U
⎞
⎟
⎠
≤ V
provided it is compatible with
pin thereby supplying internal
< 7V) and from the internal
IN
.
CC
OUT
W
rises above 5.7V, the
CC
(5.7V < V
and INTV
CC
and INTV
U
OUT
CC
< 7V).
pins.
CC
CC
Topside MOSFET Driver Supply (C
Referring to Figure 11, the external bootstrap capacitors
C
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
rises to approximately V
MOSFET Switch B turns on, the switch node SW2 drops to
low and the boost capacitor C
INTV
switch node SW1 rises to V
to approximately V
FET Switch C turns on, the switch node SW1 drops to low
and the boost capacitor C
INTV
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 equiva-
lent of V
An internal 1.2µA current source charges up the C
capacitor. As the voltage on SS increases from 0V to 2.4V,
the internal current limit rises from 0V/R
150mV/R
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.
Do not apply more than 6V to the SS pin.
A
T
and C
IRMP
CC
CC
. When the top MOSFET Switch D turns on, the
. The boost capacitors C
ITH
B
SENSE
=
connected to the BOOST1 and BOOST2 pins
).
1 2
2 4
.
.
. The output current limit ramps up slowly,
µ
V
A
OUT
•
C
SS
+ INTV
=
IN
A
(
OUT
+ INTVcc. When the bottom
1 5
is charged through D
B
. /
CC
is charged through D
s F C
and the BOOST1 pin rises
. When the bottom MOS-
A
µ
IN
and C
A
)
, D
and the BOOST2 pin
•
A
SS
, C
B
need to store
B
, D
SENSE
B
)
A
B
from
from
3780f
SS
to