VIN (V)

VFB (V)

7.2V

OVP

(1.315V)

1.265V

t

t

xxxxx

LM3477

www.ti.com

SNVS141J – MAY 2004 – REVISED JULY 2004

If the load current becomes too low, the LM3477/A will increase the duty cycle, causing the voltage to rise and

trigger the OVP. The reasons for this involve the way the LM3477/A regulates the output voltage, using a control

waveform at the pulse width modulator. This control waveform has upper and lower bounds.

Another way OVP can be tripped is if the input voltage rises higher than the LM3477/A is able to regulate in

pulse width modulation (PWM) mode. The output voltage is related to the input voltage by the duty cycle as:

and the output voltage will increase with the input voltage until it trips OVP.

It is useful to plot the operational boundaries in order to illustrate the point at which the device switches into

of the shaded regions, so the duty cycle is adjusted.

The output voltage transient response overshoot can also trigger OVP. As discussed in the OUTPUT

CAPACITOR section, if the capacitance is too low or ESR too high, the output voltage overshoot will rise high

enough to trigger OVP. However, as long as there is room for the duty cycle to adjust (the converter is not near

There is one last way that OVP can be triggered. If the unregulated input voltage crosses 7.2V, the output

voltage will react as shown in Figure 5. The internal bias of the LM3477/A switches supplies at 7.2V. When this

happens, a sudden small change in bias voltage is seen by all the internal blocks of the LM3477/A. The control

voltage, VC, shifts because of the bias change, the PWM comparator tries to keep regulation. To the PWM

comparator, the scenario is identical to step change in the load current, so the response at the output voltage is

the same as would be observed in a step load change. Hence, the output voltage overshoot here can also trigger

OVP. The LM3477/A will regulate in hysteretic mode for several cycles, or may not recover and simply stay in

hysteretic mode until the load current drops. Note that the output voltage is still regulated in hysteric mode.

Predicting whether or not the LM3477/A will come out of hysteretic mode in this scenario is a difficult task,

however it is largely a function of the output current and the output capacitance. Triggering hysteretic mode in

this way is only possible at higher load currents. The method to avoid this is to increase the output capacitance.

Figure 5. The Feedback Voltage Experiences an Oscillation if the Input Voltage Crosses the 7.2V Internal

Bias Threshold

DEFAULT/ADJUSTABLE SLOPE COMPENSATION

The LM3477/A uses a current mode control scheme. There are many advantages in a current mode architecture

including inherent cycle-by-cycle current limiting and simple compensation of the control loop. However, there are

consequences to using current mode control that one must be aware of while selecting circuit components. One

of these consequences is the inherent possibility of subharmonic oscillations in the inductor current. This is a

form of instability and should be avoided.

Copyright © 2004, Texas Instruments Incorporated

Submit Documentation Feedback

11

Product Folder Links: LM3477