Datasheet

LTC3613
17
3613fa
APPLICATIONS INFORMATION
The inductor DCR is sensed by connecting an RC filter
across the inductor. This filter typically consists of one
or two resistors (R1 and R2) and one capacitor (C1) as
shown in Figure 4. If the external R1||R2 • C1 time constant
is chosen to be exactly equal to the L/DCR time constant,
the voltage drop across the external capacitor is equal
to the voltage drop across the inductor DCR multiplied
by R2/(R1 + R2). Therefore, R2 may be used to scale
the voltage across the sense terminals when the DCR is
greater than the target sense resistance. With the ability
to program current limit through the V
RNG
pin, R2 may
be optional. C1 is usually selected to be in the range of
0.01F to 0.47F. This forces R1|| R2 to around 2k to 4k,
reducing error that might have been caused by the sense
pins’ input bias currents.
The first step in designing DCR current sensing is to
determine the DCR of the inductor. Where provided, use
the manufacturer’s maximum value, usually given at 25°C.
Increase this value to account for the temperature coef-
ficient of resistance, which is approximately 0.4%/°C. A
conservative value for inductor temperature T
L
is 100°C.
The DCR of the inductor can also be measured using a good
RLC meter, but the DCR tolerance is not always the same
and varies with temperature; consult the manufacturers’
datasheets for detailed information.
From the DCR value, V
SENSE(MAX)
is calculated as:
V
SENSE(MAX)
=DCR
MAX
at 25°C• 1+0.4% T
L(MAX)
–25°C
()
⎡
⎣
⎤
⎦
• I
OUT(MAX)
–ΔI
L
/2
⎡
⎣
⎤
⎦
If V
SENSE(MAX)
is within the maximum sense voltage of
the LTC3613 as programmed by the V
RNG
pin (30mV to
100mV), then the RC filter only needs R1. If V
SENSE(MAX)
is
higher, then R2 may be used to scale down the maximum
sense voltage so that it falls within range.
The maximum power loss in R1 is related to duty cycle,
and will occur in continuous mode at the maximum input
voltage:
P
LOSS
R1
()
=
V
IN(MAX)
–V
OUT
()
• V
OUT
R1
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
R
SENSE
sensing. 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 ΔV
SENSE
of 10mV. For a
DCR sensing application, the actual ripple voltage will be
determined by:
ΔV
SENSE
=
V
IN
–V
OUT
R1•C1
•
V
OUT
V
IN
• f
Operating Multiple Units in Parallel
The LTC3613’s current mode control architecture makes
it straightforward to parallel multiple units for higher
output current. Figure 13 shows an example circuit of two
LTC3613s placed in parallel to provide 30A at 1.2V from
a 6V to 24V input. The signals at MODE/PLLIN are 180°
out of phase, to reduce stress on the input and output
capacitors.
Since the ITH pin voltage determines the cycle-by-cycle
valley inductor current, sharing is achieved by connecting
the ITH pins together. Because the ITH pin is sensitive to
noise, a small 22pF to 47pF decoupling capacitor should