LTC3417 datasheet(13/20 Pages) LINER

13 / 20 page

LTC3417

3417fb

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 LTC3417 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 MODE pin provides mode selection. Connecting this

pin to VIN enables Burst Mode operation for both regula-

tors, 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 1/2

∆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 MODE pin, selecting the forced

continuous mode except at start-up.

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.

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 1

µs to 10µs

will produce output voltage and ITH pin waveforms that will

give a sense of overall loop stability without breaking the

feedback loop.

Switching regulators take several cycles to respond to a

step in load current. When a load step occurs, VOUT

immediately shifts by an amount equal to

∆ILOAD•ESRCOUT,

where ESRCOUT is the effective series resistance of COUT.

∆ILOAD also begins to charge or discharge COUT generat-

ing a feedback error signal used by the regulator to return

VOUT to its steady-state value. During this recovery time,

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

200

µs/DIV

1A/DIV

VOUT

1V/DIV

VRUN

2V/DIV

VIN = 3.6V

VOUT = 2.5V

RL = 2Ω

200

µs/DIV

0.5A/DIV

VOUT

1V/DIV

VRUN

2V/DIV

LTC3417 Datasheet(PDF) 13 Page - Linear Technology

LTC3417 Datasheet(HTML) 13 Page - Linear Technology

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

Datasheet Download

Link URL