LTC1149CS datasheet(12/20 Pages) LINER

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LTC1149

LTC1149-3.3/LTC1149-5

APPLICATIO S I FOR ATIO

the ground side of the output to prevent the absolute

maximum voltage ratings of the sense pins from being

exceeded. This is shown in Figure 5. When the current

sense comparator is operating at 0V common mode, the

off-time increases approximately 40%, requiring the use

of a smaller timing capacitor CT.

Efficiency Considerations

The percent 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. Percent efficiency can be

expressed as:

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

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

age of input power. (For high efficiency circuits only small

errors are incurred by expressing losses as a percentage

of output power.)

Although all dissipative elements in the circuit produce

losses, four main sources usually account for most of the

losses in LTC1149 series circuits: 1) LTC1149 DC supply

current, 2) MOSFET gate charge current, 3) I

2R losses and

4) P-channel transition losses.

1. The DC supply current is the current which flows into

VIN Pin 2 less the gate charge current. For VIN = 12V the

LTC1149 DC supply current is 0.6mA for no load, and

increases proportionally with load up to 2mA after the

LTC1149 series has entered continuous mode.

Because the DC supply current is drawn from VIN, the

resulting loss increases with input voltage. For

VIN = 24V, the DC bias losses are generally less than 3%

for load currents over 300mA. However, at very low

load currents the DC bias current accounts for nearly all

of the loss.

2. MOSFET gate charge 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 VIN to ground. The

resulting dQ/dt is a current out of VIN which is typically

much larger than the DC supply current. In continuous

mode, IGATECHG = f (QN + QP). The typical gate charge

for a 0.1

Ω N-channel power MOSFET is 25nC, and for

a P-channel about twice that value. This results in

IGATECHG = 7.5mA in 100kHz continuous operation, for

a 5% to 10% typical mid-current loss with VIN = 24V.

Note that the gate charge loss increases directly with

both input voltage and operating frequency. This is the

principal reason why the highest efficiency circuits

operate at moderate frequencies. Furthermore, it

argues against using larger MOSFETs than necessary

to control I

2R losses, since overkill can cost efficiency

as well as money!

3. I

2R losses are easily predicted from the DC resistances

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

ous 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 24V or greater). Transition losses can be esti-

mated from:

Transition Loss

≈ 5(VIN)2(IMAX)(CRSS)(f)

For example, if VIN = 48V, IMAX = 2A, CRSS = 300pF (a very

large MOSFET) and f = 100kHz, the transition loss is 0.7W.

A loss of this magnitude would not only kill efficiency but

would probably require additional heat sinking for the

MOSFET! See Design Example for further guidelines on

how to select the P-channel MOSFET.

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.

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