LTC1736 datasheet(21/28 Pages) LINER

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LTC1736

APPLICATIO S I FOR ATIO

0.06

Ω. This results in losses ranging from 3% to 17%

as the output current increases from 1A to 5A for a 1.8V

output, or 4% to 20% for a 1.5V output. Efficiency

varies as the inverse square of VOUT for the same

external components and power level. I2R losses cause

the efficiency to drop at high output currents.

4. Transition losses apply only to the topside MOSFET(s),

and only become significant when operating at high

input voltages (typically 12V or greater). Transition

losses can be estimated from:

Transition Loss = (1.7)(VIN2)(IO(MAX))(CRSS)(f)

Other “hidden” losses such as copper trace and internal

battery resistances can account for an additional 5% to

10% efficiency degradation in portable systems. It is very

important to include these “system” level losses in the

design of a system. The internal battery and fuse resis-

tance losses can be minimized by making sure that CIN has

adequate charge storage and very low ESR at the switch-

ing frequency. A 25W supply will typically require a

minimum of 20

µF to 40µF of capacitance having a maxi-

mum of 0.01

Ω to 0.02Ω of ESR. Other losses including

Schottky conduction losses during dead-time and induc-

tor core losses generally account for less than 2% total

additional loss.

Checking Transient Response

The regulator loop response can be checked by looking at

the load current transient response. Switching regulators

take several cycles to respond to a step in DC (resistive)

load current. When a load step occurs, VOUT shifts by an

amount equal to

∆ILOAD (ESR), where ESR is the effective

series resistance of COUT. ∆ILOAD also begins to charge or

discharge COUT generating the feedback error signal that

forces the regulator to adapt to the current change and

return VOUT to its steady-state value. During this recovery

time VOUT can be monitored for excessive overshoot or

ringing, which would indicate a stability problem. OPTI-

LOOP compensation allows the transient response to be

optimized over a wide range of output capacitance and

ESR values. The availability of the ITH pin not only allows

optimization of 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 pre-

dominantly 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 by examining the rise time at the pin. The ITH

external components shown in the Figure 1 circuit will

provide an adequate starting point for most applications.

The ITH series RC-CC 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 the 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 10

µs will produce output voltage and ITHpinwaveforms

that will give a sense of the overall loop stability without

breaking the feedback loop. The initial output voltage step

may not be within the bandwidth of the feedback loop, so

the standard second-order overshoot/DC ratio cannot be

used determine phase margin. The gain of the loop will be

increased by increasing RC and the bandwidth of the loop

will be increased by decreasing CC. If RC is increased by the

same factor that CC 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 Application Note 76.

Improve Transient Response and Reduce Output

Capacitance with Active Voltage Positioning

Fast load transient response, limited board space and low

cost are requirements of microprocessor power supplies.

Active voltage positioning improves transient response

and reduces the output capacitance required to power a

microprocessor where a typical load step can be from 0.2A

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