Datasheet

LTC3839
20
3839fa
APPLICATIONS INFORMATION
filter placed near the IC has been traditionally used to
reduce the effects of capacitive and inductive noise coupled
in the sense traces on the PCB. A typical filter consists of
two series 10 resistors connected to a parallel 1000pF
capacitor, resulting in a time constant of 20ns.
This same RC filter, with minor modifications, can be
used to extract the resistive component of the current
sense signal in the presence of parasitic inductance.
For example, Figure 4a illustrates the voltage waveform
across a 2m sense resistor with a 2010 footprint for a
1.2V/15A converter operating at 100% load. The waveform
is the superposition of a purely resistive component and a
purely inductive component. It was measured using two
scope probes and waveform math to obtain a differential
measurement. Based on additional measurements of the
inductor ripple current and the on-time and off-time of
the top switch, the value of the parasitic inductance was
determined to be 0.5nH using the equation:
ESL =
V
ESL(STEP)
ΔI
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
= 100, 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 10 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
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
500ns/DIV
V
SENSE
20mV/DIV
3839 F04a
V
ESL(STEP)
500ns/DIV
V
SENSE
20mV/DIV
3839 F04b