LTC1871-1 LINER [Linear Technology], LTC1871-1 Datasheet - Page 20

• Manufacturer Part Number: LTC1871-1
• Description: High Effi ciency, Synchronous, 4-Switch Buck-Boost Controller
• Manufacturer: LINER [Linear Technology]

APPLICATIONS INFORMATION
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 capacitor.
The resultant feedback signal is compared with the internal
precision 0.800V voltage reference by the error amplifi er.
The output voltage is given by the equation:
Topside MOSFET Driver Supply (C
Referring to Figure 11, the external bootstrap capacitors
C
20
A
to be powered from the internal 6V regulator at the cost
of a small effi ciency penalty.
This is the normal connection for a 6V regulator and
provides the highest effi ciency.
supply is available in the 5.5V to 7V range, it may be
used to power EXTV
the MOSFET gate drive requirements.
V
and C
OUT
CC
CC
CC
CC
CC
CC
=
Connection
pin to the INTV
switch than is specifi ed, an external Schottky
B
0 8
left open (or grounded). This will cause INTV
connected directly to V
connected to an external supply. If an external
connected to the BOOST1 and BOOST2 pins
V
CC
CC
:
⎛
⎝ ⎜
1
+
remains above 5.5V. This allows the
R
R
2
CC
1
CC
CC
OUT
⎞
⎠ ⎟
≤ V
pin thereby supplying internal
provided it is compatible with
< 7V) and from the internal
IN
.
CC
OUT
A
rises above 5.7V, the
, D
CC
(5.7V < V
and INTV
A
, C
CC
B
, D
and INTV
OUT
B
CC
)
< 7V).
pins.
CC
CC
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
pin rises to approximately V
MOSFET switch B turns on, the switch node SW2 drops
to low and the boost capacitor C
from INTV
the switch node SW1 rises to V
rises to approximately V
MOSFET switch C turns on, the switch node SW1 drops
to low and the boost capacitor C
from INTV
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
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.
Current foldback is disabled during soft-start until the
voltage on C
when there is loading during start-up.
T
IRMP
=
SENSE
CC
CC
1.2µA
2.4V
SS
. When the top MOSFET switch D turns on,
. The boost capacitors C
ITH
reaches 2V. Make sure C
. The output current limit ramps up slowly,
).
• C
SS
= 1.5s/µF
OUT
(
IN
+ INTV
+ INTV
OUT
B
A
is charged through D
is charged through D
)
CC
and the BOOST1 pin
• C
CC
IN
. When the bottom
. When the bottom
A
SS
SS
and the BOOST2
and C
is large enough
B
SENSE
need to
3780fe
SS
to
B
A