L5994 - L5994A

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thus both should be considered:

with it as well.

Further constraints on the minimum output capacitance can arise from specifications regarding the maximum

undershoot,

whichever is greater, and where Dmax is the maximum duty cycle and the quantities are relevant either to the

+3.3V or +5.1V section. High performance capacitors should be employed to reduce the capacitance needed

for a given ESR, to avoid paralleling several parts with a considerable waste of space. Although excellent elec-

trolytic capacitors are available, OS-CON or tantalums may be preferred especially if very compact design is

required, or in case of surface mount assemblies. Multilayer ceramic capacitors with extremely low ESR are now

available, but they have a large spread of the capacitance value, so they should be paralleled with another more

stable, high-ESR capacitor.

Miscellaneous Components

The feedback loop has virtually unlimited bandwidth, thus a filter is necessary to make the system insensitive

to the switching frequency ripple and, in general, to prevent noise from disturbing the correct operation of the

error summing comparator. Anyway, the cut-off frequency of this filter can be very high, so that line and load

transient response is extremely fast. This filter is a simple R-C type where resistance and capacitance can be

chosen for a typical 3dB cut-off frequency of 60kHz.

As to the bootstrap diodes, even though small signal diodes might be effectively used, it is preferable to employ

low-power Schottky rectifiers, since that increase slightly the gate drive voltage, in favour of efficiency. The boot-

strap capacitor can be a 100nF film capacitor.

The soft-start capacitors determine the time during which the current limitation circuit moves gradually the set-

point from zero up to 50mV in order to limit the current inflow at start-up. This ramp lasts approximately 1 ms

per nF of soft-start capacitance (10 to 100 nF typical values), but the actual time necessary to the output voltage

to reach the steady-state value depends on the load current and the output filter capacitance.

There are some critical points of the IC that may require by-pass capacitors to prevent noise from disturbing the

circuit. These points are the reference voltage VREF, the IC supply pin VIN, the PREG5 line and the alternative

supply pin V5SW. Use film capacitors suitable for AC decoupling.

External PNP bipolar transistor

As the output of the auxiliary winding on the 5.1V section is dimensioned for 13V, considering an output voltage

of 12V the power loss across the external PNP transistor is of:

The collector-emitter breakdown voltage must be greater than the one delivered by the transformer on the 5.1V

section and this is true also for the collector-base junction. A small signal trnsistor is enough for the considered

application.

V

RPP X

R

()

0.011

ESR

X

R

SENS EX

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

⋅

=

V

RPP X

R

()

3.1 10

6

–

L

X

C

OUTX

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

1

R

SENS EX

2

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

1

V

IN M IN

V

OUT

–

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

1

V

OUT

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

–





⋅⋅

⋅

⋅

=

C

OU T

LI

OUT

2

∆

⋅

V

OUT

-

∆

V

IN M IN

D

MAX

V

OUT

–

⋅

()

⋅

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

>

C

OU T

LI

OUT

2

∆

⋅

V

OUT

+

V

OU T

⋅

∆

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

>