LTC1159CS datasheet(13/20 Pages) LINER

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LTC1159/LTC1159-3.3/LTC1159-5

APPLICATIO S I FOR ATIO

LTC1159

SENSE+

SENSE –

1000pF

100

Ω

100

Ω

RSENSE

COUT

LTC1159 • F07

Figure 7. Suppressing Burst Mode Operation

low to high to low again, a packet of charge dQ moves

from VCC to ground. The resulting dQ/dt is a current out

of VCC which is typically much larger than the control

circuit current. In continuous mode, IGATECHG ≈ f (QP +

QN), where QP and QN are the gate charges of the two

MOSFETs.

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 VCC 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. I

2R losses are easily predicted from the DC resistances

of the MOSFET, inductor, and current shunt. In con-

tinuous mode all of the output current flows through L

and RSENSE, but is “chopped” between the P-channel

and N-channel MOSFETs. If the two MOSFETs have

approximately the same RDS(ON), then the resistance of

one MOSFET can simply be summed with the resis-

tances of L and RSENSE to obtain I2R losses. For

example, if each RDS(ON) = 0.1Ω, RL = 0.15Ω, and

RSENSE = 0.05Ω, then the total resistance is 0.3Ω. This

results in losses ranging from 3% to 12% 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 P-channel MOSFET,

and only when operating at high input voltages (typi-

cally 20V or greater). Transition losses can be esti-

mated from:

Transition Loss

≈ 5(VIN)2(IMAX)(CRSS)(f)

Other losses including CINandCOUT ESRdissipativelosses,

Schottky conduction losses during dead time, and inductor

core losses, generally account for less than 2% total

additional loss.

Auxiliary Windings – Suppressing Burst Mode

Operation

The LTC1159 synchronous switch removes the normal

limitation that power must be drawn from the inductor

primary winding in order to extract power from auxiliary

windings. With synchronous switching, auxiliary out-

puts may be loaded without regard to the primary output

load, providing that the loop remains in continuous

mode operation.

Burst Mode operation can be suppressed at low output

currents with a simple external network which cancels the

0.025V minimum current comparator threshold. This tech-

nique is also useful for eliminating audible noise from

certain types of inductors in high current (IOUT > 5A)

applications when they are lightly loaded.

An external offset is put in series with the Sense – pin to

subtract from the built-in 0.025V offset. An example of this

technique is shown in Figure 7. Two 100

Ω resistors are

inserted in series with the leads from the sense resistor.

With the addition of R3, a current is generated through R1

causing an offset of:

VOFFSET = VOUT

)

) R1

R1 + R3

If VOFFSET > 0.025V, the minimum threshold will be

cancelled and Burst Mode operation is prevented from

occurring. Since VOFFSET is constant, the maximum load

current is also decreased by the same offset. Thus, to get

back to the same IMAX, the value of the sense resistor must

be reduced:

RSENSE ≈

Ω

IMAX

To prevent noise spikes from erroneously tripping the

current comparator, a 1000pF capacitor is needed across

the Sense– and Sense+ pins.

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