LTC3883 Datasheet, PDF (22/112 Pages) Linear Technology – Single Phase Step-Down DC/DC Controller with Digital Power System Management

LTC3883/LTC3883-1
Operation
TSNS
LTC3883
GND
GND
10nF
MMBT3906
3883 F04
Figure 4. Temperature Sense Circuit
temperature is calculated from the ∆V_BE measurement. The
external transistor temperature is digitized by the telemetry
ADC, and the value is returned by the PMBus READ_
TEMPERATURE_1 command.
The READ_TEMPERATURE_2 command returns the
junction temperature of the LTC3883 using an on-chip
diode. The slope of the external temperature sensor can be
modified with the temperature slope coefficient stored in
MFR_TEMP_1_GAIN. Typical PNPs require temperature
slope adjustments slightly less than 1. The MMBT3906 has
a recommended value in this command of approximately
MFR_TEMP_1_GAIN = 0.991 based on the ideality factor
of 1.01. Simply invert the ideality factor to calculate the
MFR_TEMP_1_GAIN. Different manufacturers and differ-
ent lots may have different ideality factors. Consult with
the manufacturer to set this value.
The offset of the external temperature sense can be adjusted
by MFR_TEMP_1_OFFSET. A value of 0 in this command
sets the temperature offset to –273.15°C.
If the PNP cannot be placed in direct contact with the
inductor, the slope or offset can be increased to account for
temperature mismatches. If the user is adjusting the slope,
the intercept point is at absolute zero, –273.15°C, so small
adjustments in slope can change the apparent measured
temperature significantly. Another way to artificially
increase the slope of the temperature term is to increase
the MFR_IOUT_CAL_GAIN_TC term. This will modify the
temperature slope with respect to room temperature.
If an external temperature sense element is not used, the
TSNS pin must be shorted to GND. The UT_FAULT_LIMIT
must be set to –275°C, and the UT_FAULT_RESPONSE
must be set to ignore. The user also needs to set the
IOUT_CAL_GAIN_TC to a value of 0.
22
RCONFIG (Resistor Configuration) Pins
The pins FREQ_CFG, VOUT_CFG and VTRIM_CFG can be
used to select important operating parameters without
programming the configuration EEPROM. Connecting
these pins to external resistor dividers selects the switching
frequency, output voltage and basic power management
supervisor parameters. The ASEL pin is used to select the
unique device bus address. Connect this pin to an external
resistor divider to select the device address. Always use
a resistor divider to select the device address. Setting
the device address in EEPROM is allowed, but can create
problems if the device address is somehow lost by the host.
It is safe and prudent to use the ASEL pin to set the device
address. If RCONFIG pins are floated, the value stored in
the corresponding NVM command is used. If bit 6 of the
MFR_CONFIG_ALL_LTC3883 configuration command is
asserted in NVM, the resistor inputs are ignored upon
power-up except for ASEL which is always respected.
The resistor configuration pins are only measured during
a power-up reset or after an MFR_RESET command is
executed.
The V_{OUT_CFG} and V_TRIM pin settings are described in Tables
12 and 13. These pins select the output voltage for the
LTC3883’s analog PWM controller. If both pins are open,
the VOUT_COMMAND command is loaded from NVM to
determine the output voltage.
The following parameters are set as a percentage of the
output voltage if the RCONFIG pins are used to determined
output voltage:
n _{VOUT_OV_FAULT_LIMIT..................................... +10%}
n _{VOUT_OV_WARN_LIMIT................................... +7.5%}
n _{VOUT_MAX....................................................... +7.5%}
n _{VOUT_MARGIN_HIGH..........................................+5%}
n _{POWER_GOOD_ON..............................................–7%}
n _{POWER_GOOD_OFF.............................................–8%}
n _{VOUT_MARGIN_LOW...........................................–5%}
n _{VOUT_UV_WARN_LIMIT...................................–6.5%}
n _{VOUT_UV_FAULT_LIMIT.......................................–7%}
The FREQ_CFG pin settings are described in Table 14. This
pin selects the switching frequency and phase relationship
between the PWM channel and SYNC pin. To synchronize
to an external clock, the part must be put into external clock
3883f