Datasheet

LTC3838-1
22
38381f
For more information www.linear.com/3838-1
APPLICATIONS INFORMATION
500ns/DIV
V
SENSE
20mV/DIV
38381 F04b
Figure 4a. Voltage Waveform Measured
Directly Across the Sense Resistor
Figure 4b. Voltage Waveform Measured After the
Sense Resistor Filter. C
F
= 1000pF, R
F
= 100Ω
the top switch, the value of the parasitic inductance was
determined to be 0.5nH using the equation:
ESL =
V
ESL(STEP)
∆I
L
•
t
ON
• t
OFF
t
ON
+ t
OFF
where V
ESL(STEP)
is the voltage step caused by the ESL
and shown in Figure 4a, and t
ON
and t
OFF
are top MOSFET
on-time and off-time respectively. If the RC time constant
is chosen to be close to the parasitic inductance divided by
the sense resistor (L/R), the resulting waveform looks re-
sistive again, as shown in Figure 4b. For applications using
low V
SENSE(MAX)
, check the sense resistor manufacturer’s
data sheet for information about parasitic inductance. In
the absence of data, measure the voltage drop directly
across the sense resistor to extract the magnitude of the
ESL step and use the equation above to determine the ESL.
However, do not over filter. Keep the RC time constant less
than or equal to the inductor time constant
to maintain a
high enough ripple voltage on V
RSENSE
.
Note that the SENSE1
–
and SENSE2
–
pins are also used
for sensing the output voltage for the adjustment of top
gate on time, t
ON
. For this purpose, there is an additional
internal 500k resistor from each SENSE
–
pin to SGND,
therefore there is an impedance mismatch with their cor-
responding SENSE
+
pins. The voltage drop across the
R
F
causes an offset in sense voltage. For example, with
R
F
= 100Ω, at V
OUT
= V
SENSE
–
= 5V, the sense-voltage
offset V
SENSE(OFFSET)
= V
SENSE
–
• R
F
/500k = 1mV. Such
small offset may seem harmless for current limit, but
could be significant for current reversal detection (I
REV
),
causing excess negative inductor current at discontinuous
mode. Also, at V
SENSE(MAX)
= 30mV, a mere 1mV offset
will cause a significant shift of zero-current ITH voltage
by (2.4V – 0.8V) • 1mV/30mV = 53mV. Too much shift
may not allow the output voltage to return to its regulated
value after the output is shorted due to ITH foldback.
Therefore, when a larger filter resistor R
F
value is used,
it is recommended to use an external 500k resistor from
each SENSE
+
pin to SGND, to balance the internal 500k
resistor at its corresponding SENSE
–
pin.
The previous discussion generally applies to high density/
high current applications where I
OUT(MAX)
> 10A and low
inductor values are used. For applications where I
OUT(MAX)
< 10A, set R
F
to 10Ω and C
F
to 1000pF. This will provide
a good starting point.
The filter components need to be placed close to the
IC.
The positive and negative sense traces need to be routed
as a differential pair and Kelvin (4-wire) connected to the
sense resistor.
DCR Inductor Current Sensing
For applications requiring higher efficiency at high load
currents, the LTC3838-1 is capable of sensing the volt-
age drop across the inductor DCR, as shown in Figure 5.
The DCR of the inductor represents the small amount of
DC winding resistance
, which can be less than 1mΩ for
today’s low value, high current inductors.
In a high current application requiring such an inductor,
conduction loss through a sense resistor would cost several
points of efficiency compared to DCR sensing.
The inductor DCR is sensed by connecting an RC filter
across the inductor. This filter typically consists of one or
500ns/DIV
V
SENSE
20mV/DIV
38381 F04a
V
ESL(STEP)