LTC3866 Datasheet, PDF (14/36 Pages) Linear Technology – Current Mode Synchronous Controller for Sub Milliohm DCR Sensing

LTC3866
Operation
Another way to detect an undervoltage condition is to
monitor the V_IN supply. Because the RUN pin has a preci-
sion turn-on reference of 1.22V, one can use a resistor
divider to V_IN to turn on the IC when V_IN is high enough.
An extra 4.5µA of current flows out of the RUN pin once
the RUN pin voltage passes 1.22V. The RUN comparator
itself has about 80mV of hysteresis. One can program
additional hysteresis for the RUN comparator by adjust-
ing the values of the resistive divider. For accurate V_IN
undervoltage detection, V_IN needs to be higher than 4.75V.
Applications Information
The Typical Application on the first page of this data sheet
is a basic LTC3866 application circuit. The LTC3866 is
designed and optimized for use with a very low DCR
value by utilizing a novel approach to reduce the noise
sensitivity of the sensing signal by a factor of 14dB. DCR
sensing is becoming popular because it saves expensive
current sensing resistors and is more power efficient,
especially in high current applications. However, as the
DCR value drops below 1mΩ, the signal-to-noise ratio
is low and current sensing is difficult. LTC3866 uses an
LTC proprietary technique to solve this issue. In general,
external component selection is driven by the load require-
ment, and begins with the DCR and inductor value. Next,
power MOSFETs are selected. Finally, input and output
capacitors are selected.
Current Limit Programming
The ILIM pin is a 5-level logic input which sets the maxi-
mum current limit of the controller. When ILIM is either
grounded, floated or tied to INTV_CC, the typical value for
the maximum current sense threshold will be 10mV,
20mV or 30mV, respectively. Setting ILIM to one-fourth
INTV_CC and three-fourths INTV_CC for maximum current
sense thresholds of 15mV and 25mV.
Which setting should be used? For the best current limit
accuracy, use the highest setting that is applicable to the
output requirements.
SNSD⁺, SNSA⁺ and SNS^– Pins
The SNSA⁺ and SNS^– pins are the inputs to the current
comparators, while the SNSD⁺ pin is the input of an internal
amplifier. The operating input voltage range of 0V to 3.5V
is for SNSA⁺, SNS^– and SNSD⁺ when the internal differen-
tial amplifier is used to remotely sense the output. All the
positive sense pins that are connected to the current com-
parator or the amplifier are high impedance with input bias
currents of less than 1µA, but there is also a resistance of
about 300k from the SNS^– pin to ground. The SNS^– should
be connected directly to V_OUT. The SNSD⁺ pin connects
to the filter that has a R1C1 time constant matched to
L/DCR of the inductor. The SNSA⁺ pin is connected to the
second filter with the time constant one-fifth that of R1C1.
Care must be taken not to float these pins during normal
operation. Filter components, especially capacitors, must
be placed close to the LTC3866, and the sense lines should
run close together to a Kelvin connection underneath the
current sense element (Figure 2). Because the LTC3866
is designed to be used with a very low DCR value to
sense inductor current, without proper care, the parasitic
resistance, capacitance and inductance will degrade the
current sense signal integrity, making the programmed
current limit unpredictable. As shown in Figure 3, resistors
R1 and R2 are placed close to the output inductor and
capacitors C1 and C2 are close to the IC pins to prevent
noise coupling to the sense signal.
TO SENSE FILTER,
NEXT TO THE CONTROLLER
INDUCTOR
C_OUT
3866 F02
Figure 2. Sense Lines Placement with Inductor DCR
3866fa
14