LTC3855 LINER [Linear Technology], LTC3855 Datasheet - Page 22

• Manufacturer Part Number: LTC3855
• Description: Dual, Fast, Accurate Step-Down DC/DC Controller
• Manufacturer: LINER [Linear Technology]

LTC3838
APPLICATIONS INFORMATION
For example, Figure 4a illustrates the voltage waveform
across a 2mΩ 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:
where V
and shown in Figure 4a, and t
on-time and off-time respectively. If the RC time constant
22
ESL =
20mV/DIV
20mV/DIV
Figure 4b. Voltage Waveform Measured After the
Sense Resistor Filter. C
V
ESL(STEP)
V
SENSE
SENSE
V
Figure 4a. Voltage Waveform Measured
Directly Across the Sense Resistor
ESL(STEP)
ΔI
L
is the voltage step caused by the ESL
•
t
t
ON
ON
500ns/DIV
500ns/DIV
+ t
F
• t
= 1000pF , R
OFF
ON
OFF
and t
OFF
F
3838 F04a
3838 F04b
= 100Ω
are top MOSFET
V
ESL(STEP)
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
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
Note that the SENSE1
for sensing the output voltage for the adjustment of top
gate on time, t
internal 500k resistor from each SENSE
therefore there is an impedance mismatch with their cor-
responding SENSE
R
R
offset V
small offset may seem harmless for current limit, but
could be significant for current reversal detection (I
causing excess negative inductor current at discontinuous
mode. Also, at V
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
it is recommended to use an external 500k resistor from
each SENSE
resistor at its corresponding SENSE
The previous discussion generally applies to high density/
high current applications where I
inductor values are used. For applications where I
F
F
causes an offset in sense voltage. For example, with
= 100Ω, at V
SENSE(MAX)
SENSE(OFFSET)
+
pin to SGND, to balance the internal 500k
ON
, check the sense resistor manufacturer’s
. For this purpose, there is an additional
OUT
SENSE(MAX)
+
pins. The voltage drop across the
= V
–
= V
and SENSE2
SENSE
SENSE
= 30mV, a mere 1mV offset
–
–
RSENSE
= 5V, the sense-voltage
OUT(MAX)
• R
F
–
–
/500k = 1mV. Such
pins are also used
pin.
.
F
–
> 10A and low
value is used,
pin to SGND,
OUT(MAX)
REV
3838fa
),