L5994 - L5994A

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Although the Schottky might appear to be redundant, it is not in a system where a very high efficiency is re-

quired. In fact, its lower threshold prevents the lossy body-diode of the synchronous rectifier MOSFET from turn-

ing on during the above mentioned dead-time. Both conduction and reverse recovery losses are cut down and

efficiency can improve of 1-2% in some cases. Besides a small diode is sufficient since it conducts for a very

short time.

See the "Power Management" section to see how both synchronous rectifiers are used to ensure zero voltage

output in stand-by conditions or in case of overvoltage.

Pulse-skipping operation

To achieve high efficiency at light load current as well, under this condition the regulators change their operation

(unless this feature is disabled): they abandon PWM and enter the so-called pulse-skipping mode, in which a

single switching cycle takes place every many oscillator periods.

exceed the pulse skipping threshold (13mV typ.) while the high-side MOSFET is conducting. When the reset

extra energy that maintains the output voltage above its nominal value for a while. The oscillator pulses now set

the output latch only when the feedback signal indicates that the output voltage has fallen below its nominal

value. In this way, most of oscillator pulses are skipped and the resulting switching frequency is much lower, as

expressed by the following relationship:

where K = 3.2×10

account for power dissipation at low output power, are considerably reduced.

The section 1 can work with the input voltage very close to the output one (i.e. the output voltage is 5V), where

the current waveform may be so flat to prevent pulse-skipping from being activated. To avoid this, the pulse-

in the above formula the constant K becomes 12.8×10

When in pulse-skipping, the output voltage is some ten mV higher than in PWM mode, just because of its mode

of operation. If this "load regulation" effect is undesirable for any reason, the pulse skipping feature can be dis-

abled (see "Power Management" section) to the detriment of efficiency at light load.

MOSFET's Drivers

To get the gate-drive voltage for the high-side N-channel MOSFET a bootstrap technique is employed. A ca-

pacitor is alternately charged through a diode from the 5V PREG5 line when the high-side MOSFET is OFF and

then connected to its gate-source leads by the internal floating driver to turn the MOSFET on. The PREG5 line

is used to drive the synchronous rectifier as well, and therefore the use of low-threshold MOSFET's (the so-

called "logic-level" devices) is highly recommended.

The drivers are of "dynamic" type, which means they do not give origin to current consumption when they are

in static conditions (ON or OFF), but only during transitions. This feature is aimed at minimizing the power con-

sumption of the device even during stand-by when both low-side MOSFET's are ON.

Adaptative anti shoot-through protection is implemented to prevent cross-conduction: the low side mosfet turn

on is disbled until the HSRC pin is above 2V and, in the same way, the high side mosfet turn on is disabled until

the RGATE pin is above 0.3V. During the time in which both mosfets are in off state, the recirculation of the

current is insured by the schottky diode. The resulting dead time depends on the mosfets used and on the cur-

rent flowing in the inductor; in this way many kinds of mosfets may be used and cross conduction is avoided.

f

PS

K

R

SEN SE

2

L

---------------------- I

OU T

V

OUT

1

V

OUT

V

IN

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