LINE REGULATION

The LM2696 regulates to the lowest point of the output

voltage (V

may be represented by a waveform that is some average

voltage with ripple. The LM2696 will regulate to the trough of

the ripple.

The output voltage is given by the following relationship:

as discussed in the Feedback Resistor section of this docu-

ment.

TRANSIENT RESPONSE

Constant on-time architectures have inherently excellent

transient line and load response. This is because the control

loop is extremely fast. Any change in the line or load condi-

tions will result in a nearly instantaneous response in the

PWM off time.

If one considers the switcher response to be nearly cycle-

by-cycle, and amount of energy contained in a single PWM

pulse, there will be very little change in the output for a given

change in the line or load.

EFFICIENCY

The constant on-time architecture features high efficiency

even at light loads. The ability to achieve high efficiency at

light loads is due to the fact that the off-time will become

necessarily long at light loads. Having extended the off-time,

there is little mechanism for loss during this interval.

The efficiency is easily estimated using the following rela-

tionships:

Power loss due to FET:

P

Where:

P

P

P

Typical values are:

R

Ω

V

Q

t

t

Power loss due to catch diode:

P

Power loss due to DCR and ESR:

P

DCR

P

ESR_OUTPUT

P

ESR_INPUT

Power loss due to Controller:

P

I

The efficiency may be calculated as shown below:

Total power loss = P

P

Power Out = I

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FIGURE 7. Parasitic Tank Circuit at the Switchpin

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FIGURE 8. Average Output Voltage and Regulation Point

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