LTC3786 LINER [Linear Technology], LTC3786 Datasheet - Page 23

• Manufacturer Part Number: LTC3786
• Description: Multi-Phase Current Mode Step-Up DC/DC Controller
• Manufacturer: LINER [Linear Technology]

The SENSE
The SENSE
to the CMOS current comparators for each channel.
Nominally, there is no DC current into or out of these
pins. There are ESD protection diodes connected from
these pins to SGND, although even at hot temperature the
leakage current into the SENSE
be less than 1μA.
Since the LTC3862-2 contains leading edge blanking, an
external RC filter is not required for proper operation.
However, if an external filter is used, the filter components
should be placed close to the SENSE
the IC, as shown in Figure 15. The positive and negative
sense node traces should then run parallel to each other
to a Kelvin connection underneath the sense resistor, as
shown in Figure 16. Sensing current elsewhere on the
board can add parasitic inductance and capacitance to
the current sense element, degrading the information at
the sense pins and making the programmed current limit
unpredictable. Avoid the temptation to connect the SENSE
line to the ground plane using a PCB via; this could result
in unpredictable behavior.
The sense resistor should be connected to the source
of the power MOSFET and the ground node using short,
wide PCB traces, as shown in Figure 16. Ideally, the bot-
tom terminal of the sense resistors will be immediately
operaTion
Figure 15. Proper Current Sense Filter Component Placement
+
+
LTC3862-2
and SENSE
and SENSE
SENSE
SENSE
INTV
PGND
GATE
V
CC
IN
+
–
–
–
pins are high impedance inputs
Pins
FILTER COMPONENTS
PLACED NEAR
SENSE PINS
+
and SENSE
R
SENSE
+
V
and SENSE
IN
38622 F15
V
OUT
–
pins should
–
pins on
–
adjacent to the negative terminal of the output capacitor,
since this path is a part of the high di/dt loop formed by
the switch, boost diode, output capacitor and sense resis-
tor. Placement of the inductors is less critical, since the
current in the inductors is a triangle waveform.
Checking the Load Transient Response
The regulator loop response can be checked by looking at
the load current transient response. Switching regulators
take several cycles to respond to a step in DC (resistive)
load current. When a load step occurs, V
amount equal to ∆I
series resistance of C
discharge C
forces the regulator to adapt to the current change and
return V
time V
ringing, which would indicate a stability problem.
The availability of the ITH pin not only allows optimization
of control loop behavior but also provides a DC-coupled
and AC-filtered closed-loop response test point. The DC
step, rise time and settling at this test point truly reflects the
closed-loop response. Assuming a predominantly second
order system, phase margin and/or damping factor can be
estimated using the percentage of overshoot seen at this
pin. The bandwidth can also be estimated by examining
the rise time at the pin.
Figure 16. Connecting the SENSE
Sense Resistor Using a Kelvin Connection
OUT
OUT
can be monitored for excessive overshoot or
OUT
to its steady-state value. During this recovery
TO CONTROLLER
, generating the feedback error signal that
FILTER NEXT
TO SENSE
LOAD
OUT
(ESR), where ESR is the effective
. ∆I
LOAD
GND
MOSFET SOURCE
+
R
and SENSE
SENSE
38622 F16
also begins to charge or
LTC3862-2
OUT
–
Traces to the
shifts by an
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