LTC3869-2_15 Datasheet, PDF (17/42 Pages) Linear Technology – Dual, 2-Phase Synchronous Step-Down DC/DC Controllers

LTC3869/LTC3869-2
Applications Information
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 er-
ror that might have been caused by the SENSE pins’ ±1µA
current. T_L(MAX) is the maximum inductor temperature.
The equivalent resistance R1|| R2 is scaled to the room
temperature inductance and maximum DCR:
R1|| R2 =
L
(DCR at 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.
However, DCR sensing eliminates a sense resistor, reduces
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 for duty
cycles less than 40%. 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 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 LTC3869 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.
A reasonable starting point is to choose a ripple current
that is about 40% of I_OUT(MAX) for a duty cycle less than
40%. 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 ≥ ^{VIN – VOUT} • ^VOUT
f_OSC • I_RIPPLE V_IN
For duty cycles greater than 40%, the 10mV current
sense ripple voltage requirement is relaxed because the
slope compensation signal aids the signal-to-noise ratio
and because a lower limit is placed on the inductor value
to avoid subharmonic oscillations. To ensure stability for
For more information www.linear.com/LTC3869
38692fa
17