LM3475_14 Datasheet, PDF (8/21 Pages) Texas Instruments – Hysteretic PFET Buck Controller

LM3475
SNVS239A – OCTOBER 2004 – REVISED JANUARY 2005
V_IN
t_ON
t_OFF
-V_D
Switch Voltage
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I_out
'I_L
Inductor Current
t_d
V_OUT
ripple
V_OUT
(DC)
V_HYST
t_d
Output Voltage
Figure 12. Hysteretic Waveforms
The LM3475 operates in discontinuous conduction mode at light load current and continuous conduction mode at
heavy load current. In discontinuous conduction mode, current through the inductor starts at zero and ramps up
to the peak, then ramps down to zero. The next cycle starts when the FB voltage reaches the reference voltage.
Until then, the inductor current remains zero. Operating frequency is low, as are switching losses. In continuous
conduction mode, current always flows through the inductor and never ramps down to zero.
SOFT-START
The LM3475 includes an internal soft-start function to protect components from excessive inrush current and
output voltage overshoot. As V_IN rises above 2.7V (typical), the internal bias circuitry becomes active. When EN
goes high, the device enters soft-start. During soft-start, the reference voltage is ramped up to the nominal value
of 0.8V in approximately 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 considered as overcurrent protection or
output short circuit protection.
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 capacitance is
recommended.
MINIMUM ON/OFF TIME
To ensure accurate comparator switching, the LM3475 imposes 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 hysteretic comparator from improperly
responding to switching noise spikes (See REDUCING SWITCHING NOISE) and ESL spikes (See OUTPUT
CAPACITOR SELECTION) at the output.
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