Datasheet

LTC3880/LTC3880-1
38
3880fc
For more information www.linear.com/LTC3880
APPLICATIONS INFORMATION
The Typical Application on the back page is a basic LTC3880
application circuit. The LTC3880 can be configured to use
either DCR (inductor resistance) sensing or low value
resistor sensing. The choice between the two current
sensing schemes is largely a design trade-off between
cost, power consumption and accuracy. DCR sensing
is becoming popular because it saves expensive current
sensing resistors and is more power efficient, especially
in high current applications. The LTC3880 can nominally
account for the temperature dependency of the DCR sens
-
ing element. The accuracy of the current reading and cur-
rent limit are typically limited by the accuracy of the DCR
resistor (accounted for in the IOUT_CAL_GAIN parameter
of the L
TC3880). Thus current sensing resistors provide
the most accurate current sense and limiting for the ap
-
plication. Other external component selection is driven
by the load requirement, and begins with the selection of
R
SENSE
(if R
SENSE
is used) and inductor value. Next, the
power MOSFETs are selected. Then the input and output
capacitors are selected. Finally the current limit is selected.
All of these components and ranges are required to be
determined prior to calculating the external compensation
components. The current limit range is required because
the two ranges (25mV to 50mV vs 37.5mV to 70mV) have
different EA gains set with bit 7 of the MFR_PWM_MODE_
LTC3880 command. The voltage RANGE bit also modifies
the loop gain and impacts the compensation network set
with bits 5, 6 of MFR_PWM_CONFIG_LTC3880. All other
programmable parameters do not affect the loop gain,
allowing parameters to be modified without impact to the
transient response to load.
CURRENT LIMIT PROGRAMMING
The LTC3880 has two ranges of current limit programming
and a total of eight levels within each range. Refer to the
IOUT_OC_FAULT_LIMIT section of the PMBus commands.
Within each range the error amp gain is fixed, resulting in
constant loop gain. The LTC3880 will account for the DCR
of the inductor and automatically update the current limit as
the inductor temperature changes. The temperature coef
-
ficient of the DCR is stored in the MFR_IOUT_TC register.
For the best current limit accuracy, use the 75mV setting.
The 25mV setting will allow for the use of very low DCR
inductors or sense resistors, but at the expense of cur-
rent limit accuracy. Keep in mind this operation is on a
cycle-by-cycle basis and is only a function of the peak in-
ductor current. The average inductor current is monitored
by the ADC converter and can provide a warning if too
much average output current is detected. The over
current
fault is detected when the ITH voltage hits the maximum
value. The digital processor within the LTC3880 provides
the ability to either ignore the fault, shut down and latch off
or shut down and retry indefinitely (hiccup). Refer to the
overcurrent portion of the Operation section for more detail.
I
SENSE
+
AND I
SENSE
–
PINS
The I
SENSE
+
and I
SENSE
–
pins are the inputs to the current
comparators and the A/D. The common mode input voltage
range of the current comparators is 0V to 5.5V. Both the
SENSE pins are high impedance inputs with small base
currents typically less than 1µA. When the I
SENSE
pins
ramp up from 0V to 1.4V, the small base currents flow
out of the SENSE pins. When the I
SENSE
pins are greater
than 1.4V, the base currents flow into the I
SENSE
pins. The
high impedance inputs to the current comparators allow
accurate DCR sensing. Do not to float these pins during
normal operation.
Filter components mutual to the I
SENSE
lines should be
placed close to the IC. The positive and negative traces
should be routed differentially and Kelvin connected to
the current sense element, see Figure 17. A non-Kelvin
connection elsewhere can add parasitic inductance and
capacitance to the current sense element, degrading
the information at the sense terminals and making the
programmed current limit unpredictable. In a PolyPhase
system, poor placement of the sensing element will result in
sub-optimal current sharing between power stages. If DCR
sensing is used (Figure 18a), sense resistor R1 should be
placed close to the switching node to prevent noise from
Figure 17. Optimal Sense Line Placement
C
OUT
TO SENSE FILTER,
NEXT TO THE CONTROLLER
INDUCTOR OR R
SENSE
3880 F17