ltc1876eg-trpbf Linear Technology Corporation, ltc1876eg-trpbf Datasheet - Page 20

• Manufacturer Part Number: ltc1876eg-trpbf
• Description: High Efficiency, 2-phase, Dual Synchronous Step-down Switching Controller And Step-up Regulator
• Manufacturer: Linear Technology Corporation

LTC1876
APPLICATIO S I FOR ATIO
20
Topside MOSFET Driver Supply (C
External bootstrap capacitors C
pins supply the gate drive voltages for the topside MOS-
FETs. Capacitor C
though external diode D
low. When one of the topside MOSFETs is to be turned on,
the driver places the C
the desired MOSFET. This enhances the MOSFET and
turns on the topside switch. The switch node voltage, SW,
rises to V
MOSFET on, the boost voltage is above the input supply:
V
C
of the topside MOSFET(s). The reverse breakdown of the
external Schottky diode must be greater than V
When adjusting the gate drive level, the final arbiter is the
total input current for the regulator. If a change is made
and the input current decreases, then the efficiency has
improved. If there is no change in input current, then there
is no change in efficiency.
Output Voltage
The LTC1876 output voltages are each set by an external
feedback resistive divider carefully placed across the
output capacitor as shown in Figure 2. For the step-down
controller, the resultant feedback signal is compared with
BOOST
B
needs to be 100 times that of the total input capacitance
EXTV
CC
Figure 6b. Capacitive Charge Pump for EXTV
= V
LTC1876
IN
IN
and the BOOST pin follows. With the topside
PGND
+ V
TG1
BG1
SW
V
IN
INTVCC
C
B
IN
U
in the functional diagram is charged
N-CH
N-CH
+
B
. The value of the boost capacitor
B
voltage across the gate-source of
V
from INTV
IN
U
BAT85
B
connected to the BOOST
L1
W
CC
B
, D
VN2222LL
when the SW pin is
0.22 F
B
R
SENSE
)
+
+
CC
U
1876 F06b
BAT85
BAT85
1 F
C
V
IN(MAX)
OUT
OUT
.
the internal precision 0.8V voltage reference by the error
amplifier. The output voltage is given by the equation:
For the auxillary boost regulator, the resultant feedback
signal is compared with the internal precision 1.26V
voltage reference by the error amplifier. The output volt-
age is given by the equation:
SENSE
The common mode input range of the current comparator
SENSE pins is from 0V to (1.1)INTV
operation is guaranteed throughout this range allowing
output voltage setting from 0.8V to 7.7V, depending upon
the voltage applied to EXTV
stage is biased with internal resistors from an internal 2.4V
source as shown in the Functional Diagram. This requires
that current either be sourced or sunk from the SENSE
pins depending on the output voltage. If the output voltage
is below 2.4V current will flow out of both SENSE pins to
the main output. The output can be easily preloaded by the
V
comparator’s negative input bias current. The maximum
current flowing out of each pair of SENSE pins is:
Since V
can choose R1 in Figure 2 to have a maximum value to
absorb this current.
Regulating an output voltage of 1.8V, the minimum value
of R1 should be 32k. Note that for an output voltage above
2.4V, R1 has no maximum value since the SENSE pins
load the output.
OUT
I
for V
R
V
V
SENSE
OUT
OUTAUX
1
(
MAX
resistive divider to compensate for the current
+
OSENSE
OUT
/SENSE
+
)
+ I
0 8 1
< 2.4V
.
SENSE
24
V
1 26 1
is servoed to the 0.8V reference voltage, we
–
.
k
Pins
–
V
2 4
R
= (2.4V – V
.
R
2
1
V V
0 8
.
–
R
R
V
8
7
OUT
CC
. A differential NPN input
OUT
)/24k
CC
. Continuous linear
1876fa