LTC1439CG datasheet(21/32 Pages) LINER

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LTC1438/LTC1439

APPLICATIONS INFORMATION

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

MOSFET, inductor and current sense R. In continuous

mode the average output current flows through L and

RSENSE, but is “chopped” between the topside main

MOSFET and the synchronous MOSFET. If the two

MOSFETs have approximately the same RDS(ON), then

the resistance of one MOSFET can simply be summed

with the resistances of L and RSENSE to obtain I2R

losses. For example, if each RDS(ON) = 0.05Ω, RL =

0.15

Ω and RSENSE = 0.05Ω, then the total resistance is

0.25

Ω. This results in losses ranging from 3% to 10%

as the output current increases from 0.5A to 2A. I2R

losses cause the efficiency to roll off at high output

currents.

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

≈ 2.5(VIN)1.85(IMAX)(CRSS)(f)

Other losses including CIN and COUT ESR dissipative

losses, 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 which

forces the regulator loop to adapt to the current change

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

recovery time VOUT can be monitored for overshoot or

ringing which would indicate a stability problem. The ITH

external components shown in Figure 1 will prove ad-

equate compensation for most applications.

A second, more severe transient is caused by switching in

loads with large (> 1

µF) supply bypass capacitors. The

discharged bypass capacitors are effectively put in parallel

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

deliver enough current to prevent this problem if the load

Efficiency Considerations

The efficiency of a switching regulator is equal to the

output power divided by the input power times 100%. It is

often useful to analyze individual losses to determine what

is limiting the efficiency and which change would produce

the most improvement. Efficiency can be expressed as:

Efficiency = 100% – (L1 + L2 + L3 + ...)

where L1, L2, etc. are the individual losses as a percentage

of input power.

Although all dissipative elements in the circuit produce

losses, four main sources usually account for most of the

losses in LTC1438/LTC1439 circuits. LTC1438/LTC1439

VINcurrent,INTVCCcurrent,I

2RlossesandtopsideMOSFET

transition losses.

1. The VIN current is the DC supply current given in the

Electrical Characteristics which excludes MOSFET driver

and control currents. VIN current typically results in a

small (<< 1%) loss which increases with VIN.

2. 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 which is typically much larger than the

control circuit current. In continuous mode, IGATECHG =

f(QT + QB), where QT and QB are the gate charges of the

topside and bottom side MOSFETs. It is for this reason

that the large topside and synchronous MOSFETs are

turned off during low current operation in favor of the

small topside MOSFET and external Schottky diode,

allowing efficient, constant-frequency operation at low

output currents.

By powering EXTVCC from an output-derived source,

the additional VIN current resulting from the driver and

control currents will be scaled by a factor of Duty Cycle/

Efficiency. For example, in a 20V to 5V application,

10mA of INTVCC current results in approximately 3mA

of VIN current. This reduces the midcurrent loss from

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

VIN) to only a few percent.

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