Datasheet

LTC3876
23
3876f
the target sense resistance. With the ability to program
current limit through the V
RNG
pin, R2 may be optional. C1
is usually selected in the range of 0.01µF to 0.47µF. This
forces R1||R2 to around 2k to 4k, reducing error that might
have been caused by the SENSE pins’ input bias currents.
Resistor R1 should be placed close to the switching node,
to prevent noise from coupling into sensitive small-signal
nodes. Capacitor C1 should be placed close to the IC pins.
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’
data sheets for detailed information.
From the DCR value, V
SENSE(MAX)
is easily calculated as:
V
SENSE(MAX)
=DCR
MAX(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 (30mV
to 100mV) of the LTC3876 as programmed by the V
RNG
pin, 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, start with a ∆V
SENSE
of 10mV. For a DCR
sensing application, the actual ripple voltage will be de-
termined by:
ΔV
SENSE
=
V
IN
–V
OUT
R1• C1
•
V
OUT
V
IN
•f
Power MOSFET Selection
Two external N-channel power MOSFETs must be selected
for each channel of the LTC3876 controller: one for the
top (main) switch and one for the bottom (synchronous)
switch. The gate drive levels are set by the DRV
CC
voltage.
This voltage is typically 5.3V. Pay close attention to the
BV
DSS
specification for the MOSFETs as well; most of the
logic-level MOSFETs are limited to 30V or less.
Selection criteria for the power MOSFETs include the on-
resistance, R
DS(ON)
, Miller capacitance, C
MILLER
, input
voltage and maximum output current. Miller capacitance,
C
MILLER
, can be approximated from the gate charge curve
usually provided on the MOSFET manufacturers’ data
sheet. C
MILLER
is equal to the increase in gate charge along
the horizontal axis while the curve is approximately flat,
divided by the specified V
DS
test voltage.
When the IC is operating in continuous mode, the duty
cycles for the top and bottom MOSFETs are given by:
D
TOP
=
V
OUT
V
IN
D
BOT
=1–
V
OUT
V
IN
APPLICATIONS INFORMATION