LTC3417_15 Datasheet, PDF (13/20 Pages) Linear Technology – Dual Synchronous 1.4A/800mA 4MHz Step-Down DC/DC Regulator

APPLICATIONS INFORMATION
LTC3417
V_RUN
2V/DIV
V_OUT
1V/DIV
I_L
1A/DIV
V_IN = 3.6V
V_OUT = 1.8V
R_L = 0.9Ω
200μs/DIV
Figure 2. Digital Soft-Start Out1
V_RUN
2V/DIV
V_OUT
1V/DIV
I_L
0.5A/DIV
V_IN = 3.6V
V_OUT = 2.5V
R_L = 2Ω
200μs/DIV
Figure 3. Digital Soft-Start Out2
Soft-Start
Soft-start reduces surge currents from V_IN by gradu-
ally increasing the peak inductor current. Power supply
sequencing can also be accomplished by controlling the
I_TH pin. The LTC3417 has an internal digital soft-start for
each regulator output, which steps up a clamp on I_TH over
1024 clock cycles, as can be seen in Figures 2 and 3. As
the voltage on I_TH ramps through its operating range, the
internal peak current limit is also ramped at a proportional
linear rate.
Mode Selection
The MODE pin provides mode selection. Connecting this pin
to V_IN enables Burst Mode operation for both regulators,
which provides the best low current efficiency at the cost
of a higher output voltage ripple. When MODE is connected
to ground, pulse skipping operation is selected for both
regulators, which provides the lowest output voltage and
current ripple at the cost of low current efficiency. Applying
a voltage that is more than 1V from either supply results
in forced continuous mode for both regulators, which
creates a fixed output ripple and allows the sinking of
some current (about 1/2ΔI_L). Since the switching noise is
constant in this mode, it is also the easiest to filter out. In
many cases, the output voltage can be simply connected
to the MODE pin, selecting the forced continuous mode
except at start-up.
Checking Transient Response
The I_TH pin compensation allows the transient response
to be optimized for a wide range of loads and output
capacitors. The availability of the I_TH pin not only allows
optimization of the control loop behavior, but also pro-
vides 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 using the percentage of overshoot seen at this
pin or by examining the rise time at this pin.
The I_TH external components shown in the Figure 4 circuit
will provide an adequate starting point for most applica-
tions. The series RC filter sets the dominant pole-zero
loop compensation. The values can be modified slightly
(from 0.5 to 2 times their suggested values) to optimize
transient response once the final PC layout is done and
the particular output capacitor type and value have been
determined. The output capacitors need to be selected
because of various types and values determine the loop
feedback factor gain and phase. An output current pulse
of 20% to 100% of full load current having a rise time
of 1μs to 10μs will produce output voltage and I_TH pin
waveforms that will give a sense of overall loop stability
without breaking the feedback loop.
3417fd
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