Datasheet

LTC3855
20
3855f
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.
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
Figure 5. Resistance Versus Temperature for
ITEMP Pin Network and the 100k NTC
Figure 6. Worst Case I
MAX
Versus Inductor Temperature
Curve with and without NTC Temperature Compensation
applicaTions inForMaTion
(7a) Dual Output Dual Phase DCR Sensing Application (7b) Single Output Dual Phase DCR Sensing Application
10000
1000
100
10
1
INDUCTOR TEMPERATURE (°C)
–40
RESISTANCE (kΩ)
0 40–20 20 80
3855 F05
12060 100
RITMP
R
S
= 20kΩ
R
P
= 43.2kΩ
100k NTC
THERMISTOR RESISTANCE
R
O
= 100k, T
O
= 25°C
B = 4334 for 25°C/100°C
25
20
15
10
5
0
INDUCTOR TEMPERATURE (°C)
–40
I
MAX
(A)
0 40–20 20 80
3855 F06
12060 100
R
S
= 20kΩ
R
P
= 43.2kΩ
NTC THERMISTOR:
R
O
= 100k
T
O
= 25°C
B = 4334
CORRECTED I
MAX
UNCORRECTED I
MAX
NOMINAL I
MAX
CONNECT TO
ITEMP1
NETWORK
CONNECT TO
ITEMP2
NETWORK
R
NTC2
GND
R
NTC1
GND
3855 F07a
V
OUT1
SW1
L1
V
OUT2
SW2
L2
R
NTC
3855 F07b
V
OUT
SW1
L1
SW2
L2