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LTC3417A

3417afa

Soft-Start

Soft-start reduces surge currents from VIN by gradually

increasing the peak inductor current. Power supply se-

quencing can also be accomplished by controlling the ITH

pin. The LTC3417A has an internal digital soft-start for

each regulator output, which steps up a clamp on ITH over

1024 clock cycles, as can be seen in Figures 2 and 3. As the

voltage on ITH 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 VIN enables Burst Mode operation for both

regulators, which provides the best low current efficiency

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 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 1/2∆IL). 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 SYNC/MODE pin, selecting the

forced continuous mode except at start-up. The

LTC3417A can also be synchronized to an external clock

signal by the SYNC/MODE pin. The internal oscillator

frequency 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 ITH pin compensation allows the transient response to

be optimized for a wide range of loads and output capaci-

tors. The availability of the ITH pin not only allows optimi-

zation of the 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 predomi-

nantly second order system, phase margin and/or damp-

ing 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 ITH 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 particu-

lar output capacitor type and value have been determined.

APPLICATIO S I FOR ATIO

Figure 2. Digital Soft-Start OUT1

Figure 3. Digital Soft-Start OUT2

VIN = 3.6V

VOUT = 1.8V

RL = 0.9Ω

200µs/DIV

1A/DIV

VOUT

1V/DIV

VRUN

2V/DIV

VIN = 3.6V

VOUT = 2.5V

RL = 2Ω

200µs/DIV

0.5A/DIV

VOUT

1V/DIV

VRUN

2V/DIV

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