device enters soft-start. During soft-start, the reference volt-

age is ramped up to the nominal value of 0.8V in approxi-

mately 4ms. Duty cycle and output voltage will increase as

the reference voltage is ramped up.

UNDER VOLTAGE DETECTION

When the output voltage falls below 70% (typical) of the

normal voltage, as measured at the FB pin, the device turns

off PFET and restarts a new soft-start cycle. In short circuit,

the PFET is always on, and the converter is effectively a

resistor divider from input to output to ground. Whether the

part restarts depends on the power path resistance and the

short circuit resistance. This feature should not be consid-

ered as overcurrent protection or output short circuit protec-

tion.

PGATE

During switching, the PGATE pin swings from V

ground (on). As input voltage increases, the time it takes to

slew the gate of the PFET on and off also increases. Also, as

the PFET gate voltage approaches V

driving capability decreases. This can cause a significant

additional delay in turning the switch off when using a PFET

with a low threshold voltage. These two effects will increase

power dissipation and reduce efficiency. Therefore, a PFET

with relatively high threshold voltage and low gate capaci-

tance is recommended.

MINIMUM ON/OFF TIME

To ensure accurate comparator switching, the LM3475 im-

poses a blanking time after each comparator state change.

This blanking time is 180ns typically. Immediately after the

comparator goes high or low, it will be held in that state for

the duration of the blanking time. This helps keep the hys-

teretic comparator from improperly responding to switching

noise spikes (See Reducing Switching Noise) and ESL

spikes (See Output Capacitor Selection) at the output.

At very low or very high duty cycle operation, maximum

frequency will be limited by the blanking time. The maximum

operating frequency can be determined by the following

equations:

F

F

Where D is the duty cycle, defined as V

and toff

delay time and the PFET delay time (see Figure 1).

ENABLE PIN (EN)

The LM3475 provides a shutdown function via the EN pin to

disable the device. The device is active when the EN pin is

pulled above 1.5V (typ) and in shutdown mode when EN is

below 1.135V (typ). In shutdown mode, total quiescent cur-

rent is less than 10µA. The EN pin can be directly connected

to V

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