L5994 - L5994A

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the oscillator and the internal supply and differ for the pre-set output voltages.

Each converter can be turned on and off independently: RUN1 and RUN2 are control inputs which disable the

relevant section when a low logic level (below 0.8 V) is applied and enable its operation with a high logic level

(above 2.4 V). When both inputs are low the device is in stand-by condition and its current consumption is ex-

tremely reduced (less than 120mA over the entire input voltage range).

The device is able to regulate the desired output voltage in two different ways: classic PWM operation and Pulse

Skip operation (see the relevant sections).

Oscillator

The oscillator, which does not require any external timing component, controls the PWM switching frequency.

This can be either 200 or 300 kHz, depending on the logic state of the control pin OSC, or else can be synchro-

nized by an external oscillator.

If the OSC pin is grounded or connected to pin PREG5 (5V) the oscillator works at 200kHz. By connecting the

OSC pin to a 2.5 V voltage, 300 kHz operation will be selected. Moreover, if pin OSC is fed with an external

signal like the one shown in fig. 2, the oscillator will be synchronized by its falling edges.

Considering the spread of the oscillator, synchronization can be guaranteed for frequencies above 230kHz.

Even though a maximum frequency value is in practice imposed by efficiency considerations it should be noticed

that increasing frequency too much arises problems (noise, subharmonic oscillation, etc.) without significant

benefits in terms of external component size reduction and better dynamic performance.

The oscillator imposes a time interval (300 ns min.), during which the high-side MOSFET is definitely OFF, to

recharge the bootstrap capacitor (see "MOSFET's Drivers" section). This, implies a limit on the maximum duty

cycle (88.5%@fSW=300kHz, 92.6%@fsw=200kHz, worst case) which, in turn, imposes a limit on the minimum

operating input voltage.

Figure 2. Synchronization signal and operation

PWM Operation

The control loop does not employ a traditional error amplifier in favour of an error summing comparator which

sums the reference voltage, the feedback signal, the voltage drop across an external sense resistor and a slope

compensation ramp (to avoid subharmonic oscillation with duty cycles greater than 50%) with the appropriate

signs.

With reference to the schematic of fig. 3, the output latch of both controllers is set by every pulse coming from

t

t

t

5V

OSC

0V

H1GATE

H2GATE

300ns min.