Datasheet
LTC3880/LTC3880-1
21
3880fc
For more information www.linear.com/LTC3880
OPERATION
relationships are completely independent of each other
providing the numerous application options for the user.
If the LTC3880 is configured to drive the SYNC pin using
the programmed FREQUENCY_SWITCH command value,
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 LTC3880 TG outputs. Multiple LTC3880s can be
synchronized in order to realize PolyPhase arrays.
OUTPUT VOLTAGE SENSING
The channel 0 differential amplifier allows remote, dif-
ferential sensing of the load voltage with V
SENSE0n
pins.
The channel 1 sense pin (V
SENSE1
) is referenced to SGND.
The telemetry ADC is fully differential and makes measure-
ments of channels 0 and 1 output voltages at the V
SENSE0n
and V
SENSE1
/SGND pins, respectively. Due to head room
limitations of the internal amplifier for V
SENSE0
, the maxi-
mum allowed differential sense voltage is 4.096V.
C
URRENT 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 func
-
tion 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 register. The external temperature is sensed
Figure 3. Load Sharing Connections for 3-Phase Operation
LTC3880 + POWER STAGE
I
TH0
1/2 LTC3880 + POWER STAGE
I
TH0
V
DD33
NOTE: SOME CONNECTORS
AND COMPONENTS OMITTED
FOR CLARITY
I
SENSE0
+
I
SENSE0
–
V
SENSE0
+
V
SENSE0
–
I
TH1
10k
GPIO0
RUN0
RUN1
ALERT
GPIO1
SYNC
SHARE_CLK
V
DD33
RUN0
ALERT
GPIO1
SYNC
SHARE_CLK
I
SENSE0
+
I
SENSE0
–
I
SENSE1
+
I
SENSE1
–
V
SENSE1
PGNDSGND
1µF
PGND
3880 F03
SGND
V
SENSE0
+
V
SENSE0
–
1µF
LOAD
10k10k4.99k10k
RUN
SHARE_CLK
ALERT
GPIO
SYNC