L5993 datasheet(10/22 Pages) STMICROELECTRONICS

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Pin 5. VFB (Error Amplifier Inverting Input). The

feedback signal is applied to this pin and is com-

pared to the E/A internal reference (2.5V). The

E/A output generates the control voltage which

fixes the duty cycle.

The E/A features high gain-bandwidth product,

which allows to broaden the bandwidth of the

overall control loop, high slew-rate and current ca-

pability, which improves its large signal behavior.

Usually the compensation network, which stabi-

lizes the overall control loop, is connected be-

tween this pin and COMP (pin 6).

Pin 6. COMP (Error Amplifier Output). Usually,

this pin is used for frequency compensation and

the relevant network is connected between this

pin and VFB (pin 5). Compensation networks to-

wards ground are not possible since the L5993

E/A is a voltage mode amplifier (low output im-

pedance). See application ideas for some exam-

ple of compensation techniques.

Pin 7. SS (Soft-Start). At device start-up, a ca-

pacitor (Css) connected between this pin and

SGND (pin 12) is charged by an internal current

generator, ISSC, up to about 7V. During this

ramp, the E/A output is clamped by the voltage

across Css itself and allowed to rise linearly, start-

ing from zero, up to the steady-state value im-

posed by the control loop. The maximum time in-

terval during which the E/A is clamped, referred to

as soft-start time, is approximately:

Tss

≅

⋅ Rsense ⋅ IQpk

ISSC

⋅ Css

(6)

where Rsense is the current sense resistor (see pin

13) and IQpk is the switch peak current (flowing

through Rsense), which depends on the output

load. Usually, CSS is selected for a TSS in the or-

der of milliseconds.

As mentioned before, the soft-start intervenes

also in case of severe overload or short circuit on

the output. Referring to fig. 23, pulse-by-pulse

current limitation is somehow effective as long as

the ON-time of the power switch can be reduced

(from A to B). After the minimum ON-time is

reached (from B onwards) the current is out of

control.

To prevent this risk, a comparator trips an over-

current handling procedure, named ’hiccup’ mode

operation, when a voltage above 1.2V (point C) is

detected on current sense input (ISEN, pin 13).

Basically, the IC is turned off and then soft-started

as long as the fault condition is detected. As a re-

sult, the operating point is moved abruptly to D,

creating a foldback effect. Fig. 24 illustrates the

operation.

The oscillation frequency appearing on the soft-

VOUT

TON

D.C.M.

C.C.M.

IQpk

TON(min)

1-2 ·IQpk

IQpk(max)

OUT

SHORT

OUT(max)

D97IN495

Figure 23. Regulation characteristic and re-

lated quantities

Thic

time

SHORT

IOUT

ISEN

FAULT

0.5V

D98IN986

Figure 24. Hiccup mode operation.

L5993

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