Datasheet

LTC3714
11
3714f
operaTion
Internal Op Amp
The internal op amp allows the user to program accurate
offsets to the output voltage during power saving modes.
By connecting the OP
+
pin to the output, the OPOUT pin
to the V
OSENSE
pin and an external resistor R1 between
the OP
–
and OPOUT pins, the op amp is hooked up as
a unity-gain feedback amplifier. Resistors R2 and R3,
together with series switches, can then be placed on the
OP
–
pin to allow negative offsets to be switched onto the
output voltage (see Figures 2a and 2b). The accuracy of
the offset will depend on the matching of the external
resistors R1 to R2 and R3.*
For applications that require less accurate output offsets,
or none at all, the user can use the internal op amp for
true differential remote sensing of the output voltage by
connecting OPOUT to V
OSENSE
and using OP
+
and OP
–
for
differential sensing across the output capacitor as shown
in Figure 2c.
Figure 2a Figure 2b
Figure 2c
INTV
CC
/EXTV
CC
Power
Power for the top and bottom MOSFET drivers and most of
the internal controller circuitry is derived from the INTV
CC
pin. The top MOSFET driver is powered from a floating
bootstrap capacitor C
B
. This capacitor is recharged from
INTV
CC
through an external Schottky diode D
B
when
the top MOSFET is turned off. When the EXTV
CC
pin is
grounded, an internal 5V low dropout regulator supplies
the INTV
CC
power from V
IN
. If EXTV
CC
rises above 4.7V,
the internal regulator is turned off, and an internal switch
connects EXTV
CC
to INTV
CC
. This allows a high efficiency
source connected to EXTV
CC
, such as an external 5V sup-
ply or a secondary output from the converter, to provide
the INTV
CC
power. Voltages up to 7V can be applied to
EXTV
CC
for additional gate drive. If the input voltage is
low and INTV
CC
drops below 3.5V, undervoltage lockout
circuitry prevents the power switches from turning on.
–
+
OP
+
17
V
OUT
OP
–
16
OPV
IN
15
OPOUT
R2
SLEEP
MODE
OFFSET
18
R1
R3
BATTERY
MODE
OFFSET
V
OSENSE
23
3714 F02b
V
FB
VID DAC
22
3714 F02b
–
+
OP
+
17
V
OUT
BATTERY
MODE
OFFSET
SLEEP
MODE
OFFSET
OP
–
16
OPV
IN
15
R2
18
R1
R3
R1
R2
V
FB
VID DAC
22
23
3714 F02c
–
+
OP
+
17
V
OUT
+
V
OUT
–
OP
–
16
OPV
IN
OPOUT V
OSENSE
15
R
R
18
R
23
R
*An alternate configuration, shown in Figure 2b, can be used to program
offsets as well. Either configuration can be used, depending upon the logic
of control signals. If offsets are not required, the op amp can be used to
remotely sense the output voltage, proving true differential sense.