LTC3855_15 Datasheet, PDF (20/44 Pages) Linear Technology – Dual, Multiphase Synchronous DC/DC Controller with Differential Remote Sense

LTC3855
Applications Information
10000
1000
THERMISTOR RESISTANCE
R_O = 100k, T_O = 25°C
B = 4334 for 25°C/100°C
100
RITMP
R_S = 20kΩ
10 RP = 43.2kΩ
100k NTC
1
–40 –20 0 20 40 60 80 100 120
INDUCTOR TEMPERATURE (°C)
3855 F05
Figure 5. Resistance Versus Temperature for
ITEMP Pin Network and the 100k NTC
25
20
15
NOMINAL I_MAX
CORRECTED I_MAX
10
UNCORRECTED I_MAX
R_S = 20kΩ
R_P = 43.2kΩ
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)
3855 F06
Figure 6. Worst Case I_MAX Versus Inductor Temperature
Curve with and without NTC Temperature Compensation
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 thermistors next to the inductors as shown in
Figure 7. Take care to keep the ITEMP pins away from the
switch nodes.
Slope Compensation and Inductor Peak Current
Slope compensation provides stability in constant-
frequency architectures by preventing subharmonic 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 results in
a reduction of maximum inductor peak current for duty
cycles > 40%. However, the LTC3855 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.
CONNECT TO
ITEMP1
NETWORK
R_NTC1
GND
V_OUT1
L1
V_OUT2
L2
CONNECT TO
ITEMP2
NETWORK
R_NTC2
GND
V_OUT
L1
R_NTC
L2
SW1
SW2
3855 F07a
SW1
SW2
3855 F07b
(7a) Dual Output Dual Phase DCR Sensing Application
(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 Drive Traces
3855f
20