LTC3875_15 Datasheet, PDF (21/44 Pages) Linear Technology – Dual, 2-Phase, Synchronous Controller with Low Value DCR Sensing and Temperature Compensation

LTC3875
Applications Information
V_SENSEMAX(
ADJ)
=
V_SENSE(MAX
)
•
2.2
– V_ITEMP
1.5
V_ITEMP = 30µA • (R_S + R_P||R_NTC)
I_DC(MAX) = Maximum average inductor current.
TC is the inductor temperature.
The resulting current limit should be greater than or equal
to I_MAX for inductor temperatures between 25°C and 100°C.
Typical values for the NTC compensation network are:
• NTC R_O = 100k, B-constant = 3000 to 4000
• R_S ≈ 3.92k
• R_P ≈ 24.3k
Generating the I_MAX versus inductor temperature curve plot
first using the above values as a starting point, and then
adjusting the R_S and R_P values as necessary, is another
approach. Figure 6 shows a curve of I_MAX versus inductor
temperature. For PolyPhase^® applications, tie the TCOMP/
ITEMP pins together and calculate for an TCOMP/ITEMP
pin current of 30µA • #phases.
For the most accurate temperature detection, place the
thermistors next to the inductors as shown in Figure 7.
Take care to keep the TCOMP/ITEMP pins away from the
switch nodes.
Slope Compensation and Inductor Peak Current
70
60
50
NOMINAL
I_MAX
CORRECTED I_MAX
R_S = 3.92k
40
R_P = 24.3k
NTC THERMISTOR:
R_O = 100k
T_O = 25°C
B = 4334
30
–40 –20 0
UNCORRECTED
I_MAX
20 40 60 80 100 120
INDUCTOR TEMPERATURE (°C)
3875 F06
Figure 6. Worst-Case I_MAX vs Inductor Temperature Curve with
and without NTC Temperature Compensation
CONNECT TO
ITEMP1
NETWORK
R_NTC1
GND
V_OUT1
L1
V_OUT2
L2
CONNECT TO
ITEMP2
NETWORK
R_NTC2
GND
SW1
SW2
3875 F07a
(7a) Dual Output Dual Phase DCR Sensing Application
V_OUT
L1
R_NTC
L2
SW1
SW2
3875 F07b
(7b) Single Output Dual Phase DCR Sensing Application
Figure 7. Thermistor Locations. Place Thermistor Next to
Inductor(s) for Accurate Sensing of the Inductor Temperature,
but Keep the ITEMP Pins away from the Switch Nodes and
Gate Traces
Slope compensation provides stability in constant fre-
quency architectures by preventing sub-harmonic oscil-
lations at high duty cycles. It is accomplished internally
by adding a compensating ramp to the inductor current
signal at duty cycles in excess of 40%. Normally, this re-
sults in a reduction of maximum inductor peak current for
duty cycles > 40%. However, the LTC3875 uses a scheme
that counteracts this compensating ramp, which allows
the maximum inductor peak current to remain unaffected
throughout all duty cycles.
Inductor Value Calculation
Given the desired input and output voltages, the inductor
value and operating frequency, f_OSC, directly determine
the inductor’s peak-to-peak ripple current:
I_RIPPLE
=
V_OUT
V_IN


V_IN – V_OUT
f_OSC •L


Lower ripple current reduces core losses in the inductor,
ESR losses in the output capacitors, and output voltage
ripple. Thus, highest efficiency operation is obtained at
low frequency with a small ripple current. Achieving this,
however, requires a large inductor.
For more information www.linear.com/LTC3875
3875fa
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