LTC3883 Datasheet, PDF (40/112 Pages) Linear Technology – Single Phase Step-Down DC/DC Controller with Digital Power System Management

LTC3883/LTC3883-1
Applications Information
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 errors in the
temperature sensing element of 3°C to 5°C and any
additional errors associated with the proximity of the
temperature sensor element to the inductor.
C1 is usually selected to be in the range of 0.047µF to
4.7µF. This forces (R1 + R3)||R2 to be approximately 2k.
Adding optional elements R3 and C2 shown in Figure 18a
will minimize offset errors associated with the ISNS leak-
age currents. Set R3 equal to the value of R1. Set C2 to a
value of 1µF or greater to ensure adequate noise filtering.
The equivalent resistance (R1 + R3)||R2 is scaled to the
room temperature inductance and maximum DCR:
(R1+ R3) || R2
=
2
•
(DCR
L
at
20°C)
•
C1
The maximum power loss in R1 is related to the duty
cycle, and will occur in continuous mode at the maximum
input voltage:
( ) _PLOSSR1=
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.
However, DCR sensing eliminates a sense resistor, reduc-
ing conduction losses and provides higher efficiency at
heavy loads. Peak efficiency is about the same with either
method. Selecting Burst Mode operation or discontinuous
mode will improve the converter efficiency at light loads
regardless of the current sensing 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 the equation:
∆V_SENSE
=
V_IN – V_OUT
R1• C1
•
V_OUT
V_IN • f_OSC
Slope Compensation and Inductor Peak
Current
Slope compensation provides stability in constant
frequency current mode architectures by preventing
sub-harmonic oscillations at high duty cycles. This is
accomplished internally by adding a compensation ramp
to the inductor current signal at duty cycles in excess of
35%. The LTC3883 uses a patented current limit technique
that counteracts the compensating ramp. This 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 peak-to-peak ripple current:
( ) _IRIPPLE
=
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 the
lowest 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 the ripple current does not exceed a specified maxi-
mum, the inductor should be chosen according to:
( ) L ≥ VOUT VIN – VOUT
V_IN • f_OSC •I_RIPPLE
3883f
40