LTC1709 datasheet(21/28 Pages) LINER

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LTC1709

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!

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

and are significant only 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

3) INTVCC current is the sum of the MOSFET driver and

control currents. The MOSFET driver current results from

switching the gate capacitance of the power MOSFETs.

Each time a MOSFET gate is switched from low to high to

low again, a packet of charge dQ moves from INTVCC to

ground. The resulting dQ/dt is a current out of INTVCC that

is typically much larger than the control circuit current. In

continuous mode, IGATECHG = (QT + QB), where QT and QB

are the gate charges of the topside and bottom side

MOSFETs.

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.

4) The VIN current has two components: the first is the

DC supply current given in the Electrical Characteristics

table, which excludes MOSFET driver and control cur-

rents; the second is the current drawn from the differential

amplifier output. VIN current typically results in a small

(<0.1%) loss.

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 50W supply will typically require a

minimum of 200

µF to 300µF of output capacitance having

a maximum of 10m

Ω to 20mΩ of ESR. The LTC1709

2-phase architecture typically halves the input and output

capacitance requirement over competing solutions. Other

losses including 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 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 decided

upon because the various types and values determine the

loop 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 ITH pin waveforms that will

give a sense of the overall loop stability without breaking

the feedback loop. The initial output voltage step resulting

from the step change in output current may not be within

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