ltc3853euj Linear Technology Corporation, ltc3853euj Datasheet - Page 16

• Manufacturer Part Number: ltc3853euj
• Description: Triple Output, Multiphase Synchronous Step-down Controller
• Manufacturer: Linear Technology Corporation

LTC3853
APPLICATIONS INFORMATION
To ensure that the application will deliver full load cur-
rent over the full operating temperature range, choose
the minimum value for the Maximum Current Sense
Threshold (V
table (22mV, 42mV, or 65mV, depending on the state of
the I
Next, determine the DCR of the inductor. Where provided,
use the manufacturer’s maximum value, usually given
at 20°C. Increase this value to account for the tempera-
ture coeffi cient of resistance, which is approximately
0.4%/°C. A conservative value for T
To scale the maximum inductor DCR to the desired sense
resistor value, use the divider ratio:
C1 is usually selected to be in the range of 0.047μF to
0.47μF . This forces R1||R2 to around 2kΩ, reducing error
that might have been caused by the SENSE pins’ ±1μA
current.
The equivalent resistance R1||R2 is scaled to the room
temperature inductance and maximum DCR:
The sense resistor values are:
The maximum power loss in R1 is related to duty cycle,
and will occur in continuous mode at the maximum input
voltage:
Ensure that R1 has a power rating higher than this value.
If high effi ciency is necessary at light loads, consider this
power loss when deciding whether to use DCR sensing or
sense resistors. Light load power loss can be modestly
higher with a DCR network than with a sense resistor,
due to the extra switching losses incurred through R1.
16
R
R
R
P
LOSS
LIM
D
1
1
||
=
=
R
pin).
R
DCR
2
1
R
R
=
||
1
D
R
SENSE(MAX)
R
=
(
(
SENSE EQUIV
DCR at
MAX
2
(
V
;
IN MAX
)
R
(
(
at T
2
L
=
20
) in the Electrical Characteristics
L MAX
)
(
R
1
−
°
)
1
C
R
−
V
•
) •
1
R
OUT
R
)
D
D
C
1
)
•
L(MAX)
V
OUT
is 100°C.
However, DCR sensing eliminates a sense resistor, reduces
conduction losses and provides higher effi ciency at heavy
loads. Peak effi ciency is about the same with either method.
To maintain a good signal-to-noise ratio for the current
sense signal, use a minimum ΔV
For a DCR sensing application, the actual ripple voltage
will be determined by:
Slope Compensation and Inductor Peak Current
Slope compensation provides stability in constant-
frequency architectures by preventing subharmonic
oscillations at high duty cycles. It is accomplished inter-
nally 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 cur-
rent for duty cycles >40%. However, the LTC3853 uses
a patented 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
the inductor’s peak-to-peak ripple current:
Lower ripple current reduces core losses in the inductor,
ESR losses in the output capacitors, and output voltage
ripple. Thus, highest effi ciency operation is obtained at
low frequency with a small ripple current. Achieving this,
however, requires a large inductor.
A reasonable starting point is to choose a ripple current
that is about 40% of I
current occurs at the highest input voltage. To guarantee
that ripple current does not exceed a specifi ed maximum,
the inductor should be chosen according to:
I
L
Δ
RIPPLE
V
≥
SENSE
f
OSC
V
IN
=
=
•
–
V
I
RIPPLE
V
V
V
OUT
IN
OUT
IN
R C
1 1
–
⎛
⎝ ⎜
•
V
OUT(MAX)
V
OUT
•
IN
f
OSC
V
V
–
OUT
•
IN
V
V
•
OUT
IN
L
. Note that the largest ripple
V
OUT
•
SENSE
⎞
⎠ ⎟
OSC
f
OSC
, directly determine
of 10mV to 15mV.
3853f