LTC3856_15 Datasheet, PDF (20/40 Pages) Linear Technology – 2-Phase Synchronous Step-Down DC/DC Controller with Diffamp

LTC3856
Applications Information
After determining the components for the temperature
compensation network, check the results by plotting
I_MAX versus inductor temperature using the following
equations:
I_MAX =
V_{SENSEMAX(ADJ)}
−
∆V_SENSE
2
DCR_MAX
AT
25°C •
_{1+ (TL(MAX)}
−
25°C)
•
0.4
100


where
V_{SENSEMAX(ADJ)} = V_SENSE(MAX) •
1.8V − V_ITMP
1.3
−
A
V_ITMP = 10µA • (R_S + R_P||R_NTC)
Use typical values for V_SENSE(MAX). Subtracting constant A
will provide a minimum value for V_SENSE(MAX). These values
are summarized in Table 2.
Table 2. Values for V_SENSE(MAX)
I_LIM
GND
V_SENSE(MAX) Typ
30mV
A
5mV
FLOAT
50mV
5mV
INTV_CC
75mV
7mV
The resulting current limit should be greater than or
equal to I_MAX for inductor temperatures between 25°C
and 100°C.
25
20
CORRECTED
I_MAX
NOMINAL
15
I_MAX
10
R_S = 20k
UNCORRECTED
I_MAX
R_P = 43.2k
5
NTC THERMISTOR:
R_O = 100k
T_O = 25°C
B = 4334
0
–40 –20 0 20 40 60 80 100 120
INDUCTOR TEMPERATURE (°C)
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Figure 6. Worst Case I_MAX vs Inductor Temperature Curve
with and without NTC Temperature Compensation
Typical values for the NTC compensation network are:
• NTC R_O = 100k, B-constant = 3000 to 4000
• R_S ≈ 20k
• R_P ≈ 50k
Another approach for generating the I_MAX versus inductor
temperature curve plot is to first use the aforementioned
values as a starting point and then adjusting the R_S and
R_P values as necessary. Figure 6 shows a typical curve
of I_MAX versus inductor temperature.
The same thermistor network can be used to correct for
temperatures less than 25°C. But, ensure that V_ITEMP is
greater than 0.2V for duty cycles of 25% or more, oth-
erwise temperature correction may not occur at elevated
ambients. For the most accurate temperature detection,
place the thermistor next to the inductors, as shown in
Figure 7. Take care to keep the ITEMP pins away from the
switch nodes.
RNTC
V_OUT
L1
SW1
L2
SW2
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Figure 7. Thermistor Location. Place Thermistor Next to
Inductor(s) for Accurate Sensing of the Inductor Temperature,
But Keep the ITEMP Pin Away from the Switch Nodes and
Gate Drive Traces.
Slope Compensation and Inductor Peak Current
Slope compensation provides stability in constant-frequen-
cy, current mode architectures by preventing subharmonic
oscillation 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
results in a reduction of maximum inductor peak current
for duty cycles greater than 40%. However, the LTC3856
uses a scheme that counteracts this compensating ramp,
which allows the maximum inductor peak current to remain
unaffected throughout all duty cycles.
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