LTC1735I-1 Linear Technology, LTC1735I-1 Datasheet - Page 15

• Manufacturer Part Number: LTC1735I-1
• Description: High Efficiency Synchronous Step-Down Switching Regulator
• Manufacturer: Linear Technology

APPLICATIO S I FOR ATIO
Significant efficiency gains can be realized by powering
INTV
from the driver and control currents will be scaled by a
factor of (Duty Cycle)/(Efficiency). For 5V regulators this
simply means connecting the EXTV
However, for dynamic (VID-like) programmed regulators
and other lower voltage regulators, additional circuitry is
required to derive INTV
The following list summarizes the four possible connec-
tions for EXTV
1. EXTV
2. EXTV
3. EXTV
4. EXTV
Output Voltage Programming
The output voltage is set by an external resistive divider
according to the following formula:
to be powered from the internal 5.2V regulator resulting
in an efficiency penalty of up to 10% at high input
voltages.
connection for a 5V to 7V output regulator and provides
the highest efficiency. For output voltages > 5V, EXTV
is required to connect to V
absolute maximum ratings are not exceeded.
is the Most Likely Used). If an external supply is
available in the 5V to 7V range, such as notebook main
5V system power, it may be used to power EXTV
providing it is compatible with the MOSFET gate drive
requirements. This is the typical case as the 5V power
is almost always present and is derived by another high
efficiency regulator.
work. For low output voltage regulators, efficiency
gains can still be realized by connecting EXTV
output-derived voltage that has been boosted to greater
than 4.7V. This can be done with either the inductive
boost winding or capacitive charge pump circuits.
Refer to the LTC1735 data sheet for details. The charge
pump has the advantage of simple magnetics.
V
OUT
CC
from the output, since the V
CC
CC
CC
CC
Left Open (or Grounded). This will cause INTV
connected directly to V
Connected to an External Supply (This Option
0 8 1
Connected to an Output-Derived Boost Net-
.
V
CC:
U
R
R
2
1
CC
U
power from the output.
OUT
OUT
CC
W
so the SENSE pins
IN
pin directly to V
. This is the normal
current resulting
U
CC
to an
OUT
CC
CC
CC
.
The resistive divider is connected to the output as shown
in Figure 3 allowing remote voltage sensing.
The output voltage can be digitally set to voltages between
any two levels with the addition of a resistor and small
signal N-channel MOSFET as shown in the circuit of
Figure 1. Dynamic output voltage selection can be accom-
plished with this technique. Output voltages of 1.30V and
1.55V are set by the resistors R1 to R3. With the gate of
the MOSFET low, (V
ratio of R1 to R2. When the MOSFET is on (V
output voltage is the ratio of R1 to the parallel combina-
tion of R2 and R3. With the available power good output
(PGOOD), the circuit in Figure 1 creates a low cost Intel
Pentium III mobile processor compliant supply.
The LTC1735-1 has remote sense capability. The top of the
internal resistive divider is connected to V
referenced to the SGND pin. This allows a kelvin connec-
tion for remotely sensing the output voltage directly across
the load, eliminating any PC board trace resistance errors.
Topside MOSFET Driver Supply (C
An external bootstrap capacitor C
pin supplies the gate drive voltage for the topside
MOSFET. Capacitor C
though external diode D
low. Note that the voltage across C
below INTV
on, the driver places the C
of the MOSFET. This enhances the MOSFET and turns on
the topside switch. The switch node voltage SW rises to
V
the boost capacitor C
the total input capacitance of the topside MOSFET. In most
applications 0.1 F to 0.33 F is adequate. The reverse
breakdown on D
IN
and the BOOST pin rises to V
Figure 3. Setting the LTC1735-1 Output Voltage
CC
. When the topside MOSFET is to be turned
B
LTC1735-1
must be greater than V
G
B
B
= 0), the output voltage is set by the
in the Functional Diagram is charged
V
needs to be 100 times greater than
OSENSE
B
SGND
from INTV
B
voltage across the gate-source
1735-1 F03
B
IN
connected to the BOOST
47pF
B
+ INTV
CC
B
LTC1735-1
is about a diode drop
, D
V
when the SW pin is
OUT
B
R2
R1
)
CC
IN(MAX) .
OSENSE
. The value of
G
= high), the
15
and is