LTC3867_15 Datasheet, PDF (19/36 Pages) Linear Technology – Low IQ, Dual 2-Phase Synchronous Step-Down Controller

LTC3867
APPLICATIONS INFORMATION
10000
1000
THERMISTOR RESISTANCE
R_O = 100k
T_O = 25°C
B = 4334 FOR 25°C/100°C
100
R_ITMP
R_S = 20k
10 ^RP ^{= 43.2k}
100k NTC
1
–40 –20 0 20 40 60 80 100 120
INDUCTOR TEMPERATURE (°C)
3867 F06
Figure 6. Resistance Versus Temperature for the ITEMP Pin
Network and the 100k NTC
After determining the components for the temperature
compensation network, check the results by plotting I_MAX
versus inductor temperature using the following equations:
( ( ) ) ^IMAX
=
DCR(MAX)
V_{SENSEMAX(ADJ)} – ∆V_SENSE / 2
at 25°C • 1+ T_L(MAX) – 25°C
• 0.4
/
100
where:
1.8V – ^VITEMP
V_{SENSEMAX(ADJ)} = V_SENSE(MAX) •
2.8 _{– A}
1.3
( ) ^V_ITEMP ^{= 30µA • 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 1.
Table 1
I_LIM
V_SENSE(MAX) TYP
A
GND
30mV
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.
These are typical values for the NTC compensation network:
• NTC R_O = 100k, B-constant = 3000 to 4000
• R_S ≈ 20k
• R_P ≈ 50k
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 7 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 make sure V_ITEMP is
greater than 0.6V 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 thermistors next to the inductor as shown in
Figure 8. Take care to keep the ITEMP pin away from the
switch nodes.
25
20
CORRECTED
15
I_MAX
NOMINAL
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)
3867 F07
Figure 7. Worst-Case I_MAX Versus Inductor Temperature Curve
with and without NTC Temperature Compensation
V_OUT
R_NTC
L1
SW1
3867 F08
Figure 8. Thermistor Location. Place Thermistor Next to
Inductor for Accurate Sensing of the Inductor Temperature,
But Keep the ITEMP Pin Away from the Switch Nodes and Gate
Drive Traces
3867f
19