LTC3851-1_15 Datasheet, PDF (23/28 Pages) Linear Technology – Synchronous Step-Down Switching Regulator Controller

LTC3851-1
APPLICATIONS INFORMATION
HIGH CURRENT PATH
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SENSE⁺ SENSE^–
CURRENT SENSE
RESISTOR
(R_SENSE)
Figure 10. Kelvin Sensing R_SENSE
4. Does the (+) terminal of C_IN connect to the drain of
the topside MOSFET(s) as closely as possible? This
capacitor provides the AC current to the MOSFET(s).
5. Is the INTV_CC decoupling capacitor connected closely
between INTV_CC and GND? This capacitor carries the
MOSFET driver peak currents. An additional 1μF ceramic
capacitor placed immediately next to the INTV_CC and
GND pins can help improve noise performance.
6. Keep the switching node (SW), top gate node (TG) and
boost node (BOOST) away from sensitive small-signal
nodes, especially from the voltage and current sensing
feedback pins. All of these nodes have very large and
fast moving signals and therefore should be kept on
the “output side” (Pin 9 to Pin 16) of the LTC3851-1
and occupy minimum PC trace area.
PC Board Layout Debugging
It is helpful to use a DC-50MHz current probe to monitor
the current in the inductor while testing the circuit. Monitor
the output switching node (SW pin) to synchronize the
oscilloscope to the internal oscillator and probe the actual
output voltage as well. Check for proper performance over
the operating voltage and current range expected in the
application. The frequency of operation should be main-
tained over the input voltage range down to dropout and
until the output load drops below the low current opera-
tion threshold—typically 10% of the maximum designed
current level in Burst Mode operation.
The duty cycle percentage should be maintained from cycle
to cycle in a well designed, low noise PCB implementation.
Variation in the duty cycle at a subharmonic rate can suggest
noise pick-up at the current or voltage sensing inputs or
inadequate loop compensation. Overcompensation of the
loop can be used to tame a poor PC layout if regulator
bandwidth optimization is not required.
Reduce V_IN from its nominal level to verify operation
of the regulator in dropout. Check the operation of the
undervoltage lockout circuit by further lowering V_IN while
monitoring the outputs to verify operation.
Investigate whether any problems exist only at higher out-
put currents or only at higher input voltages. If problems
coincide with high input voltages and low output currents,
look for capacitive coupling between the BOOST, SW, TG
and possibly BG connections and the sensitive voltage
and current pins. The capacitor placed across the current
sensing pins needs to be placed immediately adjacent to
the pins of the IC. This capacitor helps to minimize the
effects of differential noise injection due to high frequency
capacitive coupling. If problems are encountered with
high current output loading at lower input voltages, look
for inductive coupling between C_IN, the Schottky and the
top MOSFET to the sensitive current and voltage sensing
traces. In addition, investigate common ground path
voltage pickup between these components and the GND
pin of the IC.
Design Example
As a design example, assume V_IN = 12V (nominal), V_IN =
22V (maximum), V_OUT = 1.8V, I_MAX = 5A, and f = 250kHz
(refer to Figure 12).
The inductance value is chosen first based on a 30%
ripple current assumption. The highest value of ripple
current occurs at the maximum input voltage. Connect a
160k resistor between the FREQ/PLLFLTR and GND pins,
generating 250kHz operation. The minimum inductance
for 30% ripple current is:
ΔI_L
=
1
(^f)(^L)
V_OUT
⎛
⎜1−
⎝
V_OUT
V_IN
⎞
⎟
⎠
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