L5991(1999) Ver la hoja de datos (PDF) - STMicroelectronics

Número de pieza

componentes Descripción

Fabricante

L5991
(Rev.:1999)

PRIMARY CONTROLLER WITH STANDBY

STMicroelectronics 

L5991 - L5991A

during ESD tests), it can be connected to VREF

through a 4.7kΩ resistor.

Figure 23. Duty cycle control.

VREF 4

R1

3µA

DC 3

23K

RA

R2

28K

ST-BY

16

RB RCT 2

+ TO PWM LOGIC

-

CT

D97IN727A

Pin 4. VREF (Reference Voltage). The device is

provided with an accurate voltage reference

(5V±1.5%) able to deliver some mA to an external

circuit.

A small film capacitor (0.1 µF typ.), connected

between this pin and SGND, is recommended to

ensure the stability of the generator and to prevent

noise from affecting thereference.

Before device turn-on, this pin has a sink current ca-

pability of 0.5mA.

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 L5991

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

pedance). See application ideas for some exam-

ple of compensation techniques.

It is worth mentioning that the calculation of the

part values of the compensation network must

take the standby frequency operation into ac-

count. In particular, this means that the open-loop

crossover frequency must not exceed fSB/4 ÷

fSB/5.

The voltage on pin 6 is monitored in order to re-

10/23

duce the oscillator frequency when the converter

is lightly loaded (standby).

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

≅

3

⋅

Rsense ⋅

ISSC

IQpk

⋅

Css

(7)

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. 24, pulse-by-pulse

current limitation is somehow effective as long as

Figure 24. Regulation characteristic and re-

lated quantities.

VOUT

D.C.M.

A

C.C.M.

TON

D97IN495

B

D

ISHORT IOUT(max)

IQpk

1-2 ·IQpk

IQpk(max)

C

TON(min)

IOUT

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. 25 illustrates the

operation.

The oscillation frequency appearing on the soft-

start capacitor in case of permanent fault, referred

to as ’hiccup” period, is approximately given by:

Thic

≅

4.5

⋅

1

ISSC

+

1

ISSD





⋅

Css

(8)

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