Datasheet

LTC3375
24
3375fc
For more information www.linear.com/3375
operaTion
Table 2. Die Temperature Warning Thresholds
DT[1], DT[0] DIE TEMPERATURE WARNING THRESHOLD
00 (Default) Inactive
01 140°C
10 125°C
11 110°C
A die temperature warning is reported to the user by pulling
the IRQ pin low. This warning can be read back on the LSB
of the Temp_Monitor register. The die temperature warning
flag is disabled when the DT bits are set to 00 (default).
The temperature may be read back by the user either
digitally through the I
2
C Temp_Monitor Register or 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)
The analog voltage can be digitally polled using an internal
A/D converter. In order to digitally read the temperature
voltage the user should first issue a RD_TEMP I
2
C com-
mand to tell the A/D converter to poll the TEMP voltage.
At least 2ms after this command has been written the user
may then poll the TEMP bits in the Temp_Monitor register
.
The TEMP bits are related to the TEMP voltage as follows:
V
TEMP
= 1.3V • (0.08333 + 0.007161 • D) (2)
where D corresponds to the bit weight of the digital code.
Combining Equation 1 and Equation 2 yields:
T = 18.86°C + 1.379°C • D (3)
If die temperature warning and temperature read back
functionality are not desired, then the user may shut
down the temperature monitor in order to lower quies
-
cent 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 temperature 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.
RESET_ALL Functionality
The RESET_ALL bit shuts down all enabled regulators
(enabled either via its enable pin or I
2
C) for one second.
The RESET_ALL bit is self clearing, and all other I
2
C bits
(besides the enable bits, which are set low) will remain in
their previous states. The RESET_ALL bit will also reset
the pushbutton to the powered-down state.
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 LTC3375 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
T
(4)
While the LTC3375 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 LTC3375’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 any
buck switching regulators operating at 0° phase are locked
to the rising edge of the external frequency source. All
other buck switching regulators are locked to the appro
-
priate phase of the external frequency source (see Buck
Switching Regulators). The synchronization frequency
range is 1MHz to 3MHz.