ltc3869gn-2 Linear Technology Corporation, ltc3869gn-2 Datasheet - Page 16

• Manufacturer Part Number: ltc3869gn-2
• Description: Ltc3869/ltc3869-2 - Dual, 2-phase Synchronous Step-down Dc/dc Controllers
• Manufacturer: Linear Technology Corporation

LTC3869/LTC3869-2
APPLICATIONS INFORMATION
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. T
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 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 conduc-
tion 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
cycles less than 40%. For a DCR sensing application, the
actual ripple voltage will be determined by the equation:
16
R
R1|| R2 =
R1=
P
ΔV
LOSS
D
SENSE
=
R1|| R2
DCR
R1=
L(MAX)
R
D
R
=
(DCR at 20°C) • C1
(MAX)
SENSE(EQUIV)
(
V
V
IN
; R2 =
R1• C1
is the maximum inductor temperature.
IN(MAX)
− V
at T
OUT
L
L(MAX)
− V
R1 • R
1− R
R1
V
IN
OUT
V
OUT
• f
D
D
)
OSC
• V
SENSE
OUT
of 10mV for duty
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
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 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
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:
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
I
L ≥
RIPPLE
f
OSC
V
IN
=
• I
– V
V
RIPPLE
V
OUT
OUT
IN
⎛
⎜
⎝
V
•
IN
f
OUT(MAX)
OSC
V
V
OUT
– V
IN
• L
OUT
OSC
⎞
⎟
⎠
for a duty cycle less than
directly determine the
3869f