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LTC1929

Supplying INTVCC power through the EXTVCC switch input

from an output-derived source will scale the VIN current

required for the driver and control circuits by the ratio

(Duty Factor)/(Efficiency). For example, in a 20V to 5V

application, 10mA of INTVCC current results in approxi-

mately 3mA of VIN current. This reduces the mid-current

loss from 10% or more (if the driver was powered directly

from VIN) to only a few percent.

3) I2R losses are predicted from the DC resistances of the

fuse (if used), MOSFET, inductor, current sense resistor,

and input and output capacitor ESR. In continuous mode

the average output current flows through L and RSENSE,

but is “chopped” between the topside MOSFET and the

synchronous MOSFET. If the two MOSFETs have approxi-

mately the same RDS(ON), then the resistance of one

MOSFET can simply be summed with the resistances of L,

RSENSE and ESR to obtain I2R losses. For example, if each

RDS(ON)=10mΩ, RL=10mΩ, and RSENSE=5mΩ, then the

total resistance is 25m

Ω. This results in losses ranging

from 2% to 8% as the output current increases from 3A to

15A per output stage for a 5V output, or a 3% to 12% loss

per output stage for a 3.3V output. Efficiency varies as the

inverse square of VOUT for the same external components

and output power level. The combined effects of increas-

ingly lower output voltages and higher currents required

by high performance digital systems is not doubling but

quadrupling the importance of loss terms in the switching

regulator system!

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

and only when operating at high input voltages (typically

20V 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 input fuse

resistance losses can be minimized by making sure that

CIN has adequate charge storage and a very low ESR at the

switching frequency. A 25W supply will typically require a

minimum of 20

µF to 40µF of capacitance having a maxi-

mum of 10m

Ω to 20mΩ of ESR. The LTC1929 2-phase

architecture typically halves this input capacitance re-

quirement over competing solutions. Other losses includ-

ing Schottky conduction losses during dead-time and

inductor 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 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.

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 predominantly 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 0.2 to 5 times their suggested values) to maximize

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 decided

upon because the various types and values determine the

loop feedback factor gain and phase. An output current

pulse of 20% to 80% of full-load current having a rise time

of <2

µs will produce output voltage and ITHpinwaveforms

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