Datasheet

LTC3417A-2
13
3417a2fa
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 LTC3417A-2 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 SYNC/MODE pin is a multipurpose pin which provides
mode selection and frequency synchronization. Connect-
ing this pin to V
IN
enables Burst Mode operation for both
regulators, which provides the best low current effi ciency
at the cost of a higher output voltage ripple. When SYNC/
MODE is connected to ground, pulse skipping operation
is selected for both regulators, which provides the low-
est output voltage and current ripple at the cost of low
current effi ciency. Applying a voltage that is more than
1V from either supply results in forced continuous mode
for both regulators, which creates a fi xed 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 fi lter out. In many cases, the output
voltage can be simply connected to the SYNC/MODE pin,
selecting the forced continuous mode except at start-up.
The LTC3417A-2 can be synchronized to an external clock
signal by the SYNC/MODE pin. The internal oscillator fre-
quency should be set to 20% lower than the external clock
frequency to ensure adequate slope compensation, since
slope compensation is derived from the internal oscillator.
During synchronization, the mode is set to pulse skipping
and the top switch turn-on is synchronized to the rising
edge of the external clock.
When using an external clock, with the PHASE pin low, the
switching of the two channels occur at the edges of the
external clock. A 50% duty cycle will therefore produce
180° out-of-phase operation.
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 fi lited closed-loop response
test point. The DC step, rise time, and settling at this test
point truly refl ects the closed-loop response. Assuming a
predominantly second order system, phase margin and/or
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.
APPLICATIONS INFORMATION
V
IN
= 3.6V
V
OUT
= 1.8V
R
L
= 0.9Ω
200µs/DIV
I
L
1A/DIV
V
OUT
1V/DIV
V
RUN
2V/DIV
3417A-2 F02
Figure 2. Digital Soft-Start OUT1
V
IN
= 3.6V
V
OUT
= 2.5V
R
L
= 2Ω
200µs/DIV
I
L
0.5A/DIV
V
OUT
1V/DIV
V
RUN
2V/DIV
3417A-2 F03
Figure 3. Digital Soft-Start OUT2