Datasheet

LTC3865/LTC3865-1
17
3865fb
APPLICATIONS INFORMATION
always the same and varies with temperature; consult the
manufacturers’ data sheets for detailed information.
Using the inductor ripple current value from the Induc-
tor Value Calculation section, the target sense resistor
value is:
R
V
I
I
SENSE EQUIV
SENSE MAX
MAX
L
()
()
()
=
+
Δ
2
To ensure that the application will deliver full load current
over the full operating temperature range, choose the
minimum value for the Maximum Current Sense Threshold
(V
SENSE(MAX)
) in the Electrical Characteristics table (24mV,
44mV or 68mV, depending on the state of the I
LIM
pin).
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 the temperature
coeffi cient of resistance, which is approximately 0.4%/°C.
A conservative value for T
L(MAX)
is 100°C.
To scale the maximum inductor DCR to the desired sense
resistor value, use the divider ratio:
R
R
DCR at T
D
SENSE EQUIV
MAX 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:
RR
L
DCR at C C
12
20 1
||
()•
=
°
The sense resistor values are:
R
RR
R
R
RR
R
D
D
D
1
12
2
1
1
==
−
||
;
•
The maximum power loss in R1 is related to duty cycle,
and will occur in continuous mode at the maximum input
voltage:
PR
VVV
R
LOSS
IN MAX OUT OUT
1
1
=
−
()
()
•
Ensure that R1 has a power rating higher than this value.
If high effi ciency 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 effi ciency at heavy loads.
Peak effi ciency 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 to 15mV.
For a DCR sensing application, the actual ripple voltage
will be determined by the equation:
Δ=
−
V
VV
RC
V
Vf
SENSE
IN OUT OUT
IN OSC
11••
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 LTC3865/LTC3865-1
use 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
V
V
VV
fL
RIPPLE
OUT
IN
IN OUT
OSC
=
⎛
⎝
⎜
⎞
⎠
⎟
–
•