LTC3890-2
18
38902f
APPLICATIONS INFORMATION
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 drop across
the inductor DCR multiplied by R2/(R1 + R2). R2 scales the
voltage across the sense terminals for applications where
the DCR is greater than the target sense resistor value.
To properly dimension the external filter components, the
DCR of the inductor must be known. It can be measured
using a good RLC meter, but the DCR tolerance is not
always the same and varies with temperature; consult
the manufacturers’ data sheets for detailed information.
Using the inductor ripple current value from the Inductor
Value Calculation section, the target sense resistor value is:
R
SENSE(EQUIV)
=
V
SENSE(MAX)
I
MAX
+
ΔI
L
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 (30mV,
50mV or 75mV, depending on the state of the I
LIM
pin).
Next, determine the DCR of the inductor. When provided,
use the manufacturer’s maximum value, usually given at
20°C. Increase this value to account for the temperature
coefficient of copper 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 (R
D
), use the divider ratio:
R
D
=
R
SENSE(EQUIV)
DCR
MAX
atT
L(MAX)
C1 is usually selected to be in the range of 0.1µF to 0.47µF.
This forces R1|| R2 to around 2k, reducing error that might
have been caused by the SENSE
+
pin’s ±1µA current.
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
R
D
; R2 =
R1• 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:
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
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.
Inductor Value Calculation
The operating frequency and inductor selection are inter-
related in that higher operating frequencies allow the use
of smaller inductor and capacitor values. So why would
anyone ever choose to operate at lower frequencies with
larger components? The answer is efficiency. A higher
frequency generally results in lower efficiency because of
MOSFET switching and gate charge losses. In addition to
this basic trade-off, the effect of inductor value on ripple
current and low current operation must also be considered.
