LTC3883 Datasheet, PDF (20/112 Pages) Linear Technology – Single Phase Step-Down DC/DC Controller with Digital Power System Management

LTC3883/LTC3883-1
Operation
the SYNC pin will pull low at the desired clock rate with
500ns low pulse. Care must be taken in the application to
assure the capacitance on SYNC is minimized to assure
the pull-up resistor versus the capacitor load has a low
enough time constant for the application. In addition,
a phase-locked loop (PLL) is available to synchronize
the internal oscillator to an external clock source that is
connected to the SYNC pin. All phase relationships are
between the falling edge of SYNC and the rising edge
of the LTC3883 TG output. Multiple LTC3883s can be
synchronized in order to realize PolyPhase arrays.
Output Voltage Sensing
The differential amplifier allows remote, differential sens-
ing of the load voltage with V_SENSEn pins. The telemetry
ADC is fully differential and makes measurements of the
output voltage at the V_SENSEn pins.
Output Current Sensing
For DCR current sense applications, a resistor in series
with a capacitor is placed across the inductor. In this
configuration, the resistor is tied to the FET side of the
inductor while the capacitor is tied to the load side of the
inductor as shown in Figure 3. If the RC values are cho-
sen such that the RC time constant matches the inductor
time constant (L/DCR, where DCR is the inductor series
resistance), the resultant voltage (V_DCR) appearing across
the capacitor will equal the voltage across the inductor
series resistance and thus represent the current flowing
through the inductor. The RC calculations are based on
the room temperature DCR of the inductor.
The RC time constant should remain constant, as a function
of temperature. This assures the transient response of
the circuit is the same regardless of the temperature. The
DCR of the inductor has a large temperature coefficient,
approximately 3900ppm/°C. The temperature coefficient
of the inductor must be written to the MFR_IOUT_CAL_
GAIN_TC command. The external temperature is sensed
near the inductor and is used to modify the internal current
limit circuit to maintain an essentially constant current
limit with temperature. In this application, the I_SENSE+
pin is connected to the FET side of the capacitor while
the I_SENSE– pin is placed on the load side of the capacitor.
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The current sensed from the input is then given by the
expression V_DCR/DCR. V_DCR is digitized by the LTC3883’s
telemetry ADC with an input range of ±128mV, a noise
floor of 7µV_RMS, and a peak-peak noise of approximately
46.5µV. The LTC3883 computes the inductor current using
the DCR value stored in the IOUT_CAL_GAIN command
and the temperature coefficient stored in command
MFR_IOUT_CAL_GAIN_TC. The resulting current value
is returned by the READ_IOUT command.
Auto Calibration
Using a patent pending auto-calibration routine, the
LTC3883 can measure the actual DC resistance for DCR
current sense applications. The measured value is used
in READ_IOUT measurements and eliminates the need
for the user to know the actual resistance of the inductor.
Reference the subsection titled Inductor DCR Calibration
in the Applications Information section for further detail.
Accurate DCR Temperature Compensation
The LTC3883 uses a patent pending algorithm to dy-
namically model the temperature rise from the external
temperature sensor to the inductor core. Refer to the
Accurate DCR Temperature Compensation subsection in
the Applications Information section for complete details.
Input Current Sensing
To sense the total input current consumed by the LTC3883
and the power stage, a resistor is placed between the
supply voltage and the drain of the top N-channel MOSFET.
The V_{IN_SNS} and I_{IN_SNS} pins are connected to the sense
resistor through 100Ω filter resistors. Both pins need to be
decoupled to GND. A filter capacitor needs to be connected
across the V_{IN_SNS} and I_{IN_SNS} pins. Refer to Figure 25,
Low Noise Input Current Sense Circuit for further details.
The filtered voltage is amplified by the internal high side
current sense amplifier and digitized by the LTC3883’s
telemetry ADC. The input current sense amplifier has
three gain settings of 2x, 4x, and 8x set by the bits 5:4 of
the MFR_PWM_MODE command. The maximum input
sense voltage for the three gain settings is 50mV, 20mV,
and 8mV respectively. The LTC3883 computes the input
current using the R value stored in the IIN_CAL_GAIN
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