TPS54383, TPS54386
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SLUS774B–AUGUST 2007–REVISED OCTOBER 2007
Bootstrap for the N-Channel MOSFET
A bootstrap circuit provides a voltage source higher than the input voltage and of sufficient energy to fully
enhance the switching MOSFET each switching cycle. The PWM duty cycle is limited to a maximum of 90%,
allowing an external bootstrap capacitor to charge through an internal synchronous switch (between BP and
BOOTx) during every cycle. When the PWM switch is commanded to turn ON, the energy used to drive the
MOSFET gate is derived from the voltage on this capacitor.
To allow the bootstrap capacitor to charge each switching cycle, an internal pulldown MOSFET (from SW to
GND) is turned ON for approximately 140 ns at the beginning of each switching cycle. In this way, if, during light
load operation, there is insufficient energy for the SW node to drive to ground naturally, this MOSFET forces the
SW node toward ground and allow the bootstrap capacitor to charge.
Because this is a charge transfer circuit, care must be taken in selecting the value of the bootstrap capacitor. It
must be sized such that the energy stored in the capacitor on a per cycle basis is greater than the gate charge
requirement of the MOSFET being used.
DESIGN HINT
For the bootstrap capacitor, use a ceramic capacitor with a value between 22 nF and
82 nF.
NOTE:
For 5-V input applications, connect PVDDx to BP directly. This connection bypasses
the internal control circuit regulator and provides maximum voltage to the gate drive
circuitry. In this configuration, shutdown mode IDDSDN will be the same as quiescent
IDDQ.
Light Load Operation
There is no special circuitry for pulse skipping at light loads. The normal characteristic of a nonsynchronous
converter is to operate in the discontinuous conduction mode (DCM) at an average load current less than
one-half of the inductor peak-to-peak ripple current. Note that the amplitude of the ripple current is a function of
input voltage, output voltage, inductor value, and operating frequency, as shown in Equation 14.
V
IN - VOUT
1
IDCM
=
´
´ d ´ TS
2
L
(14)
Further, during discontinuous mode operation the commanded pulse width may become narrower than the
capability of the converter to resolve. To maintain the output voltage within regulation, skipping switching pulses
at light load conditions is a natural by-product of that mode. This condition may occur if the output capacitor is
charged to a value greater than the output regulation voltage, and there is insufficient load to discharge the
capacitor. A by-product of pulse skipping is an increase in the peak-to-peak output ripple voltage.
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