LTC3875_15 Datasheet, PDF (19/44 Pages) Linear Technology – Dual, 2-Phase, Synchronous Controller with Low Value DCR Sensing and Temperature Compensation

LTC3875
Applications Information
R1 • C1 is the filter time constant of the SNSD⁺ pin.
R2 • C2 is the filter time constant of the SNSA⁺ pin.
For example, for a 12V_IN, 1.2V/30A step-down buck con-
verter running at 400kHz frequency, a 0.15µH, 0.4mΩ
inductor is chosen. This inductor provides 15A peak-to-
peak ripple current, which is 50% of the 30A full load
current. At full load, the inductor peak current is 30A +
15A/2 = 37.5A.
IL(PK) • DCR = 37.5A • 0.4mΩ = 15mV.
In this case, choose the 20mV ILIM setting which is the
closest but higher than 15mV to provide margin for cur-
rent limit.
Select the two R/C sensing network:
Filter on SNSD⁺ pin: R1 • C1 = L/DCR,
Filter on SNSA⁺ pin: R2 • C2 = (L/DCR)/5.
In this case, the ripple sense signal across SNSA⁺ and
SNS^– pins is ∆IL_P-P • DCR • 5 = 15A • 0.4mΩ • 5 = 30mV.
This signal should be more than 15mV for good signal-to-
noise ratio. In this case, it is certainly sufficient.
The peak inductor current at current limit is:
ILIM(PK) = 20mV/DCR = 20mV/0.4mΩ = 50A.
The average inductor current, which is also the output
current, at current limit is :
ILIM(AVG) = ILIM(PK) – ∆IL_P-P/2 = 50A – 15A/2 = 42.5A.
To ensure that the load current will be delivered over the full
operating temperature range, the temperature coefficient of
DCR resistance, approximately 0.4%/°C, should be taken
into account. The LTC3875 features a DCR temperature
compensation circuit that uses an NTC temperature sensing
resistor for this purpose. See the Inductor DCR Sensing
Temperature Compensation section for details.
Typically, C1 and C2 are selected in the range of 0.047µF
to 0.47µF. If C1 and C2 are chosen to be 100nF, and an
inductor of 150nH with 0.4mΩ DCR is selected, R1 and R2
will be 4.64k and 931Ω respectively. The bias current at
SNSD⁺ and SNSA⁺ is about 30nA and 500nA respectively,
and it causes some small error to the sense signal.
There will be some power loss in R1 and R2 that relates to
the duty cycle, and will be the most in continuous mode
at the maximum input voltage:
( ) _{( ) PLOSS}
R
=
V_IN(MAX) – V_OUT
R
• V_OUT
Ensure that R1 and R2 have a power rating higher than this
value. However, DCR sensing eliminates the conduction
loss of a sense resistor; it will provide a better efficiency
at heavy loads. To maintain a good signal-to-noise ratio
for the current sense signal, using ∆V_SENSE of 15mV be-
tween SNSA⁺ and SNS^– pins or an equivalent 3mV ripple
on the current sense signal. The actual ripple voltage
across SNSA⁺ and SNS^– pins will be determined by the
following equation:
∆V_SENSE
=
V_OUT
V_IN
• ^{VIN – VOUT}
R2 •C2 • f_OSC
Inductor DCR Sensing Temperature Compensation
with NTC Thermistor
For DCR sensing applications, the temperature coefficient
of the inductor winding resistance should be taken into
account when the accuracy of the current limit is criti-
cal over a wide range of temperature. The main element
used in inductors is copper; that has a positive tempco
of approximately 4000ppm/°C. The LTC3875 provides
a feature to correct for this variation through the use of
the TCOMP/ITEMP pin. There is a 30µA precision current
source flowing out of the TCOMP/ITEMP pin. A thermistor
with a NTC (negative temperature coefficient) resistance
can be used in a network, R_ITEMP (Figure 4b) connected to
maintain the current limit threshold constant over a wide
operating temperature. The TCOMP/ITEMP voltage range
that activates the correction is from 0.7V or less. If this
pin is floating, its voltage will be at INTV_CC potential, about
5.5V. When the TCOMP/ITEMP voltage is higher than 0.7V,
the temperature compensation is inactive. Floating the
ENTMPB pin enables the temperature compensation
function.
The following guidelines will help to choose components
for temperature correction. The initial compensation is for
25°C ambient temperature:
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