LTC3600 datasheet(14/28 Pages) LINER

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LTC3600

3600fb

Once the value for L is known, the type of inductor must be

selected. Ferrite designs have very low core losses and are

preferred at high switching frequencies, so design goals

can concentrate on copper loss and preventing satura-

tion. Ferrite core material saturates “hard”, which means

that inductance collapses abruptly when the peak design

current is exceeded. This results in an abrupt increase in

inductor ripple current and consequent output voltage

ripple. Do not allow the core to saturate!

Different core materials and shapes will change the size/

current and price/current relationship of an inductor. Toroid

or shielded pot cores in ferrite or permalloy materials are

small and do not radiate much energy, but generally cost

more than powdered iron core inductors with similar

characteristics. The choice of which style inductor to use

mainly depends on the price versus size requirements

and any radiated field/EMI requirements. New designs for

surface mount inductors are available from Toko, Vishay,

NEC/Tokin, Cooper, TDK, and Würth Elektronik. Refer to

Table 1 for more details.

Checking Transient Response

The OPTI-LOOP compensation allows the transient re-

sponse to be optimized for a wide range of loads and

output capacitors. The availability of the ITH pin not only

allows optimization 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

predominantly second order system, phase margin and/

or damping factor can be estimated using the percentage

of overshoot seen at this pin.

The ITH external components shown in the Figure 1 circuit

will provide an adequate starting point for most applica-

tions. The series R-C 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 their 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 ITH

pin waveforms that will give a sense of the 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 ΔILOAD • ESR,

where ESR is the effective series resistance of COUT .

ΔILOAD 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,

VOUT can be monitored for overshoot or ringing that would

indicate a stability problem.

The initial output voltage step may not be within the band-

width of the feedback loop, so the standard second order

overshoot/DC ratio cannot be used to determine phase

margin. The gain of the loop increases with the RITH and

the bandwidth of the loop increases with decreasing CITH. If

RITH is increased by the same factor that CITH is decreased,

the zero frequency will be kept the same, thereby keeping

the phase the same in the most critical frequency range

of the feedback loop.

The output voltage settling behavior is related to the stability

of the closed-loop system and will demonstrate the actual

overall supply performance. For a detailed explanation of

optimizing the compensation components, including a

review of control loop theory, refer to Linear Technology

Application Note 76.

In some applications, a more severe transient can be caused

by switching in loads with large (>10μF) load capacitors.

The discharged load capacitors are effectively put in paral-

lel with COUT , causing a rapid drop in VOUT . No regulator

can deliver enough current to prevent this problem, if the

switch connecting the load has low resistance and is driven

quickly. The solution is to limit the turn-on speed of the

load switch driver. A Hot Swap™ controller is designed

specifically for this purpose and usually incorporates cur-

rent limit, short-circuit protection, and soft-start.

APPLICATIONS INFORMATION

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