6
DEMO MANUAL DC252
DESIGN-READY SWITCHER
The VID codes (00000-11110) are engineered to be com-
patible with Intel Mobile Pentium
®
II
Processor specifica-
tions for output voltages from 0.925V to 2.00V.
The LSB (B0) represents 50mV increments in the upper
voltage range (2.00V to 1.30V) and 25mV increments in
the lower voltage range (1.275V to 0.925V). The MSB is
B4. When all bits are low or grounded, the output voltage
is 2.00V.
Each VID digital input is internally pulled up by a 40k
resistor in series with a diode from VIDV
CC
. Therefore,
digital inputs must be grounded to get a digital low input,
and digital inputs can be either left unconnected or con-
nected 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.
Maximum Input Voltage Considerations
The recommended maximum input voltage of this board
is 24V for nominal output voltages. The minimum on-time
for the LTC1736 is generally about 200ns and the operat-
ing frequency for this board is set to 270kHz. This imposes
a limit on the maximum input voltage when programming
low output voltages. For output voltages below 1.2V the
maximum input voltage is limited to:
V
IN(MAX)
< 20 (V
OUT
)
If a higher operating input voltage is required with V
OUT
<
1.2V, the operating frequency can be decreased by in-
creasing C
OSC1
. Refer to the LTC1736 data sheet for
details. If the duty cycle falls below what can be accommo-
dated by the minimum on-time, the LTC1736 will begin to
skip cycles. The output voltage will continue to be regu-
lated, but the ripple current and ripple voltage will in-
crease.
Power-Good Output
A window comparator monitors the output voltage and its
open-drain output (E1) is pulled low when the divided
output voltage is not within
±
7.5% of the reference voltage
of 0.8V. Jumper JP3 connects pull-up resistor R1 from
INTV
CC
to the power-good output, E1. This jumper is
provided to allow other pull-up voltages to be used. Make
OPERATIO
U
sure the maximum voltage on PGOOD is less than 7V.
During shutdown, the PGOOD output is pulled low.
INTV
CC
Regulator
An internal, P-channel, low dropout regulator produces
the 5.2V supply that powers the drivers and internal
circuitry within the LTC1736. The INTV
CC
pin can supply
up to 50mA (this includes the gate-drive currents). Exter-
nal loading of the INTV
CC
pin can be thermally limited
(allow 10mA to 20mA for gate-drive currents). At high
input voltages, the maximum junction temperature rating
for the LTC1736 may be exceeded if too large an external
load is placed on INTV
CC
. See the LTC1736 data sheet for
details.
EXTV
CC
Connection
The LTC1736 contains an internal P-channel MOSFET
switch connected between the EXTV
CC
and INTV
CC
pins.
The switch closes and supplies the INTV
CC
power when-
ever the EXTV
CC
pin is above 4.7V; it remains closed until
EXTV
CC
drops below 4.5V. This allows the MOSFET driver
and control power to be derived from the EXTV
CC
pin
instead of V
IN
. Do not apply greater than 7V to the EXTV
CC
pin and ensure that EXTV
CC
< V
IN
. Additional efficiency
gains can be realized by powering INTV
CC
from other high
efficiency sources, such as a 5V system power supply.
The following list describes the most common possible
connections for EXTV
CC
for low output voltage applica-
tions:
1. EXTV
CC
left open (or grounded); this will cause INTV
CC
to be powered from the internal 5.2V regulator resulting in
an efficiency penalty at low load currents and high input
voltages.
2. EXTV
CC
connected to an external supply ; if an external,
high efficiency supply is available in the 5V to 7V range
(EXTV
CC
< V
IN
), it may be used to power EXTV
CC
, providing
an efficiency boost. The typical connection in a notebook
CPU power solution is to connect it to the main 5V system
power supply.
Pentium is registered trademark of Intel Corportion.
