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L5994 - L5994A

the oscillator. That turns off the low-side MOSFET (synchronous rectifier) and, when the low-side gate voltage

falls below 0.3V to prevent cross-conduction, turns on the high-side one, thus allowing energy to be drawn from

the input source and stored in the inductor.

The error summing, by comparing the above mentioned signals, determines the moment in which the output

latch is to be reset. The high-side MOSFET is then turned off and the synchronous rectifier is turned on when

the voltage on the high-side MOSFET source falls below 2V to prevent cross-conduction, thus making the in-

ductor current recirculate. The high side mosfet is in any case turned off on the clock signal falling edge: this is

the reason why the duty cicle is limited in its maximum value.

The reached state is maintained until the next oscillator pulse.

The open-loop transfer function of such a kind of control system, under the assumption of an ideal slope com-

pensation, is:

where A is the gain of the error summing comparator, which is 2 by design.

The system is inherently very fast since it tends to correct output voltage deviations nearly on a cycle-by-cycle

basis. Actually, in case of line or load changes, few switching cycles can be sufficient for the transient to expire.

The operation above illustrated is modified during particular or anomalous conditions. Leaving out other circum-

stances (described in "Protections" section) for the moment, consider when the load current is low enough or

during the first switching cycles at start-up: the inductor current may become discontinuous, so it is zero during

the last part of each cycle. In such a case, a "zero current comparator" detects the event and turns off the syn-

chronous rectifier, avoiding inductor current reversal and reproducing the natural turn-off of a diode when re-

verse biased. This allows to increase the efficiency in ligth load. Both MOSFET's stay in off state until the next

oscillator pulse.

Figure 3. Control loop.

Synchronous Rectification

Very high efficiency is achieved at high load current with the synchronous rectification technique, which is par-

ticularly advantageous because of the low output voltage. The low-side MOSFET, that is the synchronous rec-

tifier, is selected with a very low on-resistance, so that the paralleled Schottky diode is not turned on, except for

the small time in which neither MOSFET is conducting. The effect is a considerable reduction of power loss dur-

ing the recirculation period.

Fs

()

A

R

o

2R

s ens e

⋅

--------------------------

1

s ESR C

O

⋅⋅

+

1s R

o

C

o

⋅⋅

+

() 1s R

F

C

F

⋅⋅

+

()

⋅

---------------------------------------------------------------------------------------

⋅⋅

=

`CLOCK

S

R

Q

NQ

L

Rsense

ERROR

SUMMING

+

-

+

+

-

-

SLOPE

COMPENSATION

VREF

Co

ESR

Ro

Vin

HSTRAP

REG5

Rf

Cf