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LTC3375

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Table 2. Die Temperature Warning Thresholds

DT[1], DT[0]

DIE TEMPERATURE WARNING THRESHOLD

00 (Default)

Inactive

140°C

125°C

110°C

Adietemperaturewarningisreportedtotheuserbypulling

the IRQ pin low. This warning can be read back on the LSB

oftheTemp_Monitorregister.Thedietemperaturewarning

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 I2C Temp_Monitor Register or by

sampling the TEMP pin analog voltage. The temperature,

T, indicated by the TEMP pin voltage is given by:

VTEMP +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 I2C 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:

VTEMP = 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 VCC. 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 I2C) for one second.

The RESET_ALL bit is self clearing, and all other I2C 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:

fOSC =

8 • 1011 •

ΩHz

(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 VCC 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

buckswitchingregulatorsoperatingat0°phasearelocked

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.

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