LTC3883 Datasheet, PDF (16/112 Pages) Linear Technology – Single Phase Step-Down DC/DC Controller with Digital Power System Management

LTC3883/LTC3883-1
Operation
with the resistor configuration pins detailed in Tables 12
and 13 or by the V_OUT command (either from NVM or
by PMBus command). Refer to the PMBus command
section of the data sheet or the PMBus specification for
more details. The output voltage can be modified by the
user at any time with a PMBus VOUT_COMMAND. This
command will typically have a latency less than 10ms.
The user is encouraged to reference the PMBus Power
System Management Protocol Specification to understand
how to program the LTC3883. This specification can be
found at http://www.pmbus.org/specs.html.
Continuing the basic operation description, the current
mode controller will turn off the top gate when the peak
current is reached. If the load current increases, V_SENSE
will slightly droop with respect to the DAC reference.
This causes the I_TH voltage to increase until the average
inductor current matches the new load current. After the
top MOSFET has turned off, the bottom MOSFET is turned
on. In continuous conduction mode, the bottom MOSFET
stays on until the end of the switching cycle.
EEPROM
The LTC3883 contains internal EEPROM (nonvolatile
memory) to store configuration settings and fault log
information. EEPROM endurance retention and mass write
operation time are specified in the Electrical Characteristics
and Absolute Maximum Ratings sections. Write opera-
tions above T_J = 85°C or below 0°C are possible although
the Electrical Characteristics are not guaranteed and the
EEPROM will be degraded. Read operations performed at
temperatures between 85°C and 125°C will not degrade
the EEPROM. Writing to the EEPROM above 85°C will
result in a degradation of retention characteristics. The
fault logging function, which is useful in debugging system
problems that may occur at high temperatures, only writes
to fault log EEPROM locations. If occasional writes to these
registers occur above 85°C, the slight degradation in the
data retention characteristics of the fault log will not take
away from the usefulness of the function.
It is recommended that the EEPROM not be written
when the die temperature is greater than 85°C. If the die
temperature exceeds 130°C, the LTC3883 will disable all
EEPROM write operations. All EEPROM write operations
16
will be re-enabled when the die temperature drops below
125°C. (The controller will also disable when the die tem-
perature exceeds the internal overtemperature fault limit.)
The degradation in EEPROM retention for temperatures
>125°C can be approximated by calculating the dimen-
sionless acceleration factor using the following equation:
AF
=
e




Ea
k



•_
T_USE
1
+
273
–
1
T_STRESS
+
273





where:
AF = acceleration factor
Ea = activation energy = 1.4eV
K = 8.617 • 10^–5 eV/°K
T_USE = 125°C specified junction temperature
T_STRESS = actual junction temperature in °C
Example: Calculate the effect on retention when operating
at a junction temperature of 135°C for 10 hours.
T_STRESS = 130°C
T_USE = 125°C
AF= e^{[(1.4/8.617 • 10–5) • (1/398 – 1/403)]} = 1.66
The equivalent operating time at 125°C = 16.6 hours.
Thus the overall retention of the EEPROM was degraded
by 16.6 hours as a result of operating at a junction tem-
perature of 130°C for 10 hours. The effect of the overstress
is negligible when compared to the overall EEPROM
retention rating of 87,600 hours at a maximum junction
temperature of 125°C.
Power Up and Initialization
The LTC3883 is designed to provide standalone supply
sequencing and controlled turn-on and turn-off opera-
tion. It operates from a single input supply (4.5V to 24V)
while three on-chip linear regulators generate internal
2.5V, 3.3V and 5V. If V_IN is below 6V, the INTV_CC and V_IN
pins must be tied together. The controller configuration
is initialized by an internal threshold based UVLO where
V_IN must be approximately 4V and the 5V, 3.3V and 2.5V
linear regulators must be within approximately 20% of
the regulated values. The LTC3883-1 does not have an
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