VOVP (HIGH)

VOVP (LOW)

VFB

Hysteretic Mode:

PWM Mode:

- Light load current

- D < DMIN

- Transient response

overshoot too high

- Normal

operation

t

Feedback Voltage

VOVP(HYS)

LM3477

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SNVS141K – OCTOBER 2000 – REVISED MARCH 2013

OVP will cause the drive pin to go low, forcing the power MOSFET off. With the MOSFET off, the output voltage

will drop. The LM3477/A will begin switching again when the feedback voltage reaches VFB + (VOVP - VOVP(HYS)).

See Electrical Characteristics for limits on VOVP(HYS).

OVP can be triggered by any event that causes the output voltage to rise out of regulation. There are several

common circumstances in which this can happen, and it is beneficial for a designer to be aware of these for

debugging purposes, since the mode of operation changes from the normal Pulse Width Modulation (PWM)

mode to the hysteretic mode. In the hysteretic mode the output voltage is regulated between a high and low

value that results in a higher ripple magnitude and lower ripple frequency than in the PWM mode, see Figure 20.

See different Ripple Components in PWM and Hysteretic Modes.

Figure 20. The Feedback Voltage is related to the Output Voltage

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:

VOUT = VIN*D. The LM3477/A has a minimum duty cycle of 16.5% (typical), due to the blank-out timing, TMIN. If

the input voltage increases such that the duty cycle wants to be less than DMIN, the duty cycle will hold at DMIN

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

hysteretic mode. In Figure 19, the limits shown are with respect to the peak voltage across the sense resistor

RSN, (VSNpk); they can be referred to the peak inductor current by dividing through by RSN. VSNpk is bound to the

shaded regions. In normal circumstances VSNpk is required to be in the shaded region, and the LM3477/A will

operate in the PWM mode. If operating conditions are chosen such that VSNpk would not normally fall in the

shaded regions, then the mode of operation is changed so that VSNpk will be in the shaded region, and the part

will operate in the hysteretic mode. What actually happens is that the LM3477/A will not allow VSNpk to be outside

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

The output voltage transient response overshoot can also trigger OVP. As discussed in Output Capacitor

Selection, 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 DMIN or

DMAX), the LM3477/A will return to PWM mode after a few cycles of hysteretic mode operation.

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 21. 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.

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