LTC3853_15 Datasheet, PDF (16/36 Pages) Linear Technology – Triple Output, Multiphase Synchronous Step-Down Controller

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_SENSE(MAX)) in the Electrical Characteristics
table (22mV, 42mV, or 65mV, depending on the state of
the I_LIM pin).
Next, determine the DCR of the inductor. Where pro-
vided, use the manufacturer’s maximum value, usually
given at 20°C. Increase this value to account for the tem-
perature coefficient of resistance, which is approximately
0.4%/°C. A conservative value for T_L(MAX) is 100°C.
However, DCR sensing eliminates a sense resistor, re-
duces conduction losses and provides higher efficiency at
heavy loads. Peak efficiency is about the same with either
method. To maintain a good signal-to-noise ratio for the
current sense signal, use a minimum ∆V_SENSE of 10mV
to 15mV. For a DCR sensing application, the actual ripple
voltage will be determined by:
∆V_SENSE
=
V_IN – V_OUT
R1• C1
•
V_OUT
V_IN • f_OSC
Slope Compensation and Inductor Peak Current
To scale the maximum inductor DCR to the desired sense
resistor value, use the divider ratio:
R_D
=
R_{SENSE(EQUIV )}
DCR_(MAX) at T_L(MAX)
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:
R1|| R2
=
(DCR
at
L
20°C)
•
C1
The sense resistor values are:
R1= ^{R1|| R2} ; R2 = ^{R1• RD}
R_D
1− R_D
The maximum power loss in R1 is related to duty cycle,
and will occur in continuous mode at the maximum input
voltage:
( ) _{PLOSS R1=}
V_IN(MAX) − V_OUT
R1
• V_OUT
Ensure that R1 has a power rating higher than this value.
If high efficiency 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.
Slope compensation provides stability in constant-
frequency architectures by preventing subharmonic
oscillations 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 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_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.
A reasonable starting point is to choose a ripple current
that is about 40% of I_OUT(MAX). Note that the largest ripple
current occurs at the highest input voltage. To guarantee
that ripple current does not exceed a specified maximum,
the inductor should be chosen according to:
L
≥
V_IN – V_OUT
f_OSC • I_RIPPLE
•
V_OUT
V_IN
3853fc
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