LTC3875_15 Datasheet, PDF (17/44 Pages) Linear Technology – Dual, 2-Phase, Synchronous Controller with Low Value DCR Sensing and Temperature Compensation

Operation
Fast Transient Disabled
LTC3875
Fast Transient Enabled
V_O
50mV/DIV
SW NODE
10V/DIV
I_O
10A/DIV
95mV
0A TO 15A
67.5mV
0A TO 15A
3875 F03
Figure 3. Worst-Case Transient Comparison Between Normal Mode Operation and
Fast Transient Mode of Operation for 12V/1.5V Application with 15A Load Step
Applications Information
The Typical Application on the first page of this data sheet
is a basic LTC3875 application circuit configured as a
dual phase single output power supply. The LTC3875
has an optional thermal balancing function that balances
the thermal stress between phases, thus increasing the
reliability of the whole system. In addition, the LTC3875 is
designed and optimized for use with a very low value DCR
inductor 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. The LTC3875 uses an
LTC proprietary technique to solve this issue with mini-
mum additional external components. In general, external
component selection is driven by the load requirement,
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 maximum
current limit of the controller. The input impedance of the
ILIM pin is 250kΩ. When ILIM is grounded, floated, or tied
to INTV_CC, the typical value for the maximum current sense
threshold will be 10mV, 20mV, or 30mV, respectively. Set-
ting ILIM to one-fourth INTV_CC and three-fourths INTV_CC
provides maximum current sense thresholds of 15mV or
25mV. The user should select the proper ILIM level based
on the inductor DCR value and targeted current limit level.
SNSD⁺, SNSA⁺ and SNS^– Pins
The SNSA⁺ and SNS^– pins are the direct inputs to the cur-
rent comparators, while the SNSD⁺ pin is the input of an
internal DC amplifier. The operating input voltage range
of 0V to 3.5V is for SNSA⁺, SNSD⁺ and SNS^– in a typical
application. All the positive sense pins that are connected
to the current comparator or the DC amplifier are high
impedance with input bias currents of less than 1µA, but
there is a resistance of about 300k from the SNS^– pin
to ground. The SNS^– pin should be connected directly
to V_OUT. The SNSD⁺ pin connects to the filter that has a
R1 • C1 time constant equals L/DCR of the inductor. The
SNSA⁺ pin is connected to the second filter, R2 • C2,
with the time constant equals (R1 • C1)/5. Care must be
taken not to float these pins. Filter components, especially
capacitors, must be placed close to the LTC3875, and the
sense lines should run close together to a Kelvin connec-
tion underneath the current sense element (Figure 4a).
Because the LTC3875 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 4b, resistors R1 and R2 are placed close to the
3875fa
For more information www.linear.com/LTC3875
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