LTC3374 Datasheet, PDF (15/24 Pages) Linear Technology – 8-Channel Parallelable 1A Buck DC/DCs

LTC3374
OPERATION
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 LTC3374 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).
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
=
V_TEMP + 19mV
6.75mV
•
1°C
(1)
If temperature monitoring functionality is not desired, then
the user may shut down the temperature monitor in order
to lower quiescent current (15µA typical) by tying TEMP to
V_CC. In this case all enabled buck switching regulators are
still shut down when the die temperature reaches 165°C
(typical) and remain in shutdown until the die tempera-
ture falls to 155°C (typical). If none of the buck switching
regulators are enabled, then the temperature monitor is
also 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
•
10¹¹ •
R_T
ΩHz
(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).
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3374f
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