LM3475 Datasheet, PDF (8/14 Pages) National Semiconductor (TI) – Hysteretic PFET Buck Controller

Operation Description (Continued)
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_IN (off) to
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_IN, the PGATE current
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_MAX = D / ton_min
F_MAX = (1-D) / toff_min
Where D is the duty cycle, defined as V_OUT/V_IN, and ton_min
and toff_min is the sum of the blanking time, the propagation
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_IN for always-on operation.
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