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Bootstrap for the N-Channel MOSFET

Light Load Operation

DCM

= ´

´ d ´ T

V V

IN OUT

(14)

TPS54283 , , TPS54286

SLUS749C – JULY 2007 – REVISED OCTOBER 2007

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.

DESIGN HINT

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 IDD

SDN

will be the same as quiescent

IDD

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 .

During discontinuous comduction 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 of switching

pulses at light load conditions is a 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.

Product Folder Link(s): TPS54283 TPS54286