Datasheet

LTC3862
22
3862fc
For more information www.linear.com/LTC3862
pins. There are ESD protection diodes connected from
these pins to SGND, although even at hot temperature the
leakage current into the SENSE
+
and SENSE
–
pins should
be less than 1μA.
Since the LTC3862 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
+
and SENSE
–
pins on
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 bottom
terminal of the sense resistors will be immediately 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 resistor. Place
-
ment of the inductors is less critical, since the current in
the inductors is a triangle waveform.
MOSFET SOURCE
TO SENSE
FILTER NEXT
TO CONTROLLER
R
SENSE
GND
3862 F16
Figure 16. Connecting the SENSE
+
and SENSE
–
Traces to the
Sense Resistor Using a Kelvin Connection
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
OUT
shifts by an
amount equal to ∆I
LOAD
(ESR), where ESR is the effective
series resistance of C
OUT
. ∆I
LOAD
also begins to charge or
discharge C
OUT
, generating the feedback error signal that
forces the regulator to adapt to the current change and
return V
OUT
to its steady-state value. During this recovery
time V
OUT
can be monitored for excessive overshoot or
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.
The ITH series R
C
• C
C
filter sets the dominant pole-zero
loop compensation. The transfer function for boost and
flyback converters contains a right half plane zero that
normally requires the loop crossover frequency to be
reduced significantly in order to maintain good phase
operaTion
SENSE
–
SENSE
+
LTC3862
GATE
V
OUT
R
SENSE
3862 F15
FILTER COMPONENTS
PLACED NEAR
SENSE PINS
V
IN
PGND
INTV
CC
V
IN
Figure 15. Proper Current Sense Filter Component Placement