LTC3374
15
3374fc
For more information www.linear.com/LTC3374
operation of the combined bucks; the enable of the slave
regulator (EN2) must be tied to ground.
Any combination of 2, 3, or 4 adjacent buck regulators
may be combined to provide either 2A, 3A, or 4A of aver-
age output load current. For example, buck regulator 1
and buck regulator 2 may run independently, while buck
regulators 3 and 4 may be combined to provide 2A, while
buck regulators 5 through 8 may be combined
to provide
4A. Buck regulator 1 is never a slave, and buck regulator
8 is never a master. 15 unique output power stage con-
figurations are possible to maximize application flexibility.
Power Failure Reporting Via PGOOD_ALL Pin
Power failure conditions are reported back via the
PGOOD_ALL pin. All buck switching regulators have an
internal power good (PGOOD) signal. When the regulated
output voltage of an enabled switcher rises
above 93.5%
of its programmed value, the PGOOD signal will transition
high. When the regulated output voltage falls below
92.5% of its programmed value, the PGOOD signal is
pulled low. If any internal PGOOD signal stays low for
greater than 100µs, then the PGOOD_ALL pin is pulled
low, indicating to a microprocessor that a power failure
fault has occurred. The 100µs filter time prevents the pin
from
being pulled low due to a transient.
An error condition that pulls the PGOOD_ALL pin low
is not latched. When the error condition goes away, the
PGOOD_ALL pin is released and is pulled high if no other
error condition exists. If no buck switching regulators are
enabled, then PGOOD_ALL will be pulled low.
Temperature Monitoring and Overtemperature
Protection
To prevent thermal damage to the LTC
3374 and its sur-
rounding components, the LTC3374 incorporates an
overtemperature (OT) function. When the LTC3374 die
temperature reaches 165°C (typical) all enabled buck
switching regulators are shut down and remain in shutdown
until the die temperature falls to 155°C (typical).
OPERATION
The temperature may be read back by the user by sampling
the TEMP pin analog voltage. The temperature, T, indicated
by the TEMP pin voltage
is given by:
T =
TEMP
•1°C
(1)
If none of the buck switching regulators are enabled, then
the temperature monitor is shut down to further reduce
quiescent current.
Programming the Operating Frequency
Selection of the operating frequency is a trade-off between
efficiency and component size. High frequency operation
allows the use of smaller inductor and capacitor values.
Operation at lower frequencies improves efficiency by
reducing internal gate charge losses but requires larger
inductance
values and/or capacitance to maintain low
output voltage ripple.
The operating frequency for all of the LTC3374 regulators
is determined by an external resistor that is connected
between the RT pin and ground. The operating frequency
can be calculated by using the following equation:
f
OSC
=
8 •10
11
•ΩHz
R
(2)
While the LTC3374 is designed to function with operat-
ing frequencies between 1MHz and 3MHz, it has safety
clamps that will prevent the oscillator from running faster
than 4MHz (typical) or slower than 250kHz (typical). Tying
the RT pin to V
CC
sets the oscillator to the default internal
operating frequency of 2MHz (typical).
The LTC3374’s internal oscillator can be synchronized
through an internal PLL
circuit, to an external frequency
by applying a square wave clock signal to the SYNC pin.
During synchronization, the top MOSFET turn-on of buck
switching regulators 1 and 2 are locked to the rising edge
of the external frequency source. All other buck switching
