LTC1876 View Datasheet(PDF) - Linear Technology
Part Name
Description
MFG CO.
LTC1876
Linear Technology
'LTC1876' PDF : 36 Pages View PDF
LTC1876
APPLICATIO S I FOR ATIO
VIN
+
CIN
VIN
LTC1876
TG1
N-CH
EXTVCC
SW
BG1
PGND
N-CH
+
1µF
BAT85
0.22µF
BAT85
VN2222LL
RSENSE
L1
BAT85
VOUT
+
COUT
1876 F06b
the internal precision 0.8V voltage reference by the error
amplifier. The output voltage is given by the equation:
VOUT
=
0.8V1+
R2
R1
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:
VOUTAUX
=
1.26V1+
R8
R7
Figure 6b. Capacitive Charge Pump for EXTVCC
Topside MOSFET Driver Supply (CB, DB)
External bootstrap capacitors CB connected to the BOOST
pins supply the gate drive voltages for the topside MOS-
FETs. Capacitor CB in the functional diagram is charged
though external diode DB from INTVCC when the SW pin is
low. When one of the topside MOSFETs is to be turned on,
the driver places the CB voltage across the gate-source of
the desired MOSFET. This enhances the MOSFET and
turns on the topside switch. The switch node voltage, SW,
rises to VIN and the BOOST pin follows. With the topside
MOSFET on, the boost voltage is above the input supply:
VBOOST = VIN + VINTVCC. The value of the boost capacitor
CB needs to be 100 times that of the total input capacitance
of the topside MOSFET(s). The reverse breakdown of the
external Schottky diode must be greater than VIN(MAX).
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
20
SENSE+/SENSE– Pins
The common mode input range of the current comparator
SENSE pins is from 0V to (1.1)INTVCC. Continuous linear
operation is guaranteed throughout this range allowing
output voltage setting from 0.8V to 7.7V, depending upon
the voltage applied to EXTVCC. A differential NPN input
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
VOUT resistive divider to compensate for the current
comparator’s negative input bias current. The maximum
current flowing out of each pair of SENSE pins is:
ISENSE+ + ISENSE– = (2.4V – VOUT)/24k
Since VOSENSE is servoed to the 0.8V reference voltage, we
can choose R1 in Figure 2 to have a maximum value to
absorb this current.
R1(MAX)
=
24k
0.8V
2.4V – VOUT
for VOUT < 2.4V
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.
1876fa
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