15
LTC1736
APPLICATIO S I FOR ATIO
W
U
U
U
regulator shuts off, the switch closes and INTV
CC
power is
supplied via EXTV
CC
until EXTV
CC
drops below 4.5V. This
allows the MOSFET gate drive and control power to be
derived from the output or other external source during
normal operation. When the output is out of regulation
(start-up, short circuit) power is supplied from the internal
regulator. Do not apply greater than 7V to the EXTV
CC
pin
and ensure that EXTV
CC
< V
IN
.
Significant efficiency gains can be realized by powering
INTV
CC
from the output, since the V
IN
current resulting
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
CC
pin directly to V
OUT
.
However, for VID programmed regulators and other lower
voltage regulators, additional circuitry is required to de-
rive INTV
CC
power from the output.
The following list summarizes the three possible connec-
tions for EXTV
CC:
1. EXTV
CC
Left Open (or Grounded). This will cause INTV
CC
to be powered from the internal 5.2V regulator resulting
in a low current efficiency penalty of up to 10% at high
input voltages.
2. EXTV
CC
Connected to an External Supply (this option 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
CC
provid-
ing 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.
3. EXTV
CC
Connected to an Output-Derived Boost Net-
work. For this low output voltage regulator, efficiency
gains can still be realized by connecting EXTV
CC
to an
output-derived voltage that has been boosted to greater
than 4.7V. This can be done with either the inductive
boost winding or the capacitive charge pump circuits.
Refer to the LTC1735 data sheet for details. The charge
pump has the advantage of simple magnetics.
Output Voltage Programming
The output voltage is digitally set to levels between 0.925V
and 2.00V using the voltage identification (VID) inputs
VID0 to VID4. The internal 5-bit DAC configured as a
precision resistive voltage divider sets the output voltage
in 50mV or 25mV increments according to Table 1.
The VID codes (00000-11110) are engineered to be com-
patible with Intel Mobile Pentium II
and Pentium III pro-
cessor specifications for output voltages from 0.925V to
2.00V.
The LSB (VID0) represents 50mV increments in the upper
voltage range (1.30V to 2.00V) and 25mV increments in
the lower voltage range (0.925V to 1.275V). The MSB is
VID4. When all bits are low, or grounded, the output
voltage is 2.00V.
Between the V
FB
pin and ground is a variable resistor, R1,
whose value is controlled by the five input pins (VID0 to
VID4). Another resistor, R2, between the V
OSENSE
and the
V
FB
pins completes the resistive divider. The output volt-
age is thus set by the ratio of (R1 + R2) to R1.
The LTC1736 has remote sense capability. The top of the
internal resistive divider is connected to V
OSENSE
, and it is
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.
Each VID digital input is pulled up by a 40k resistor in
series with a diode from VIDV
CC
. Therefore, it must be
grounded to get a digital low input, and can be either
floated or connected to VIDV
CC
to get a digital high input.
The series diode is used to prevent the digital inputs from
being damaged or clamped if they are driven higher than
VIDV
CC
. The digital inputs accept CMOS voltage levels.
VIDV
CC
is the supply voltage for the VID section. It is
normally connected to INTV
CC
but can be driven from
other sources such as a 3.3V supply. If it is driven from
another source, that source MUST be in the range of 2.7V
to 5.5V and MUST be alive prior to enabling the LTC1736.
