L5993 datasheet(11/22 Pages) STMICROELECTRONICS

11 / 22 page

start capacitor in case of permanent fault, referred

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

Thic

≅ 4.5 ⋅ 



ISSC

ISSD







⋅ Css (7)

Since the system tries restarting each hiccup cy-

cle, there is not any latchoff risk.

”Hiccup” keeps the system in control in case of

short circuits but does not eliminate power com-

ponents overstress during pulse-by-pulse limita-

tion (from A to C). Other external protection cir-

cuits are needed if a better control of overloads is

required.

Pin 8. VCC (Controller Supply). This pin supplies

the signal part of the IC. The device is enabled as

VCC voltage exceeds the start threshold and

works as long as the voltage is above the UVLO

threshold. Otherwise the device is shut down and

the

current

consumption

extremely

low

(<150

µA). This is particularly useful for reducing

the consumption of the start-up circuit (in the sim-

plest case, just one resistor), which is one of the

most significant contributions to power losses

when a converter is lightly loaded.

An internal Zener limits the voltage on VCC to

25V. The IC current consumption increases con-

siderably if this limit is exceeded.

A small film capacitor between this pin and SGND

(pin 12), placed as close as possible to the IC, is

recommended to filter high frequency noise.

Pin 9. VC (Supply of the Power Stage). It supplies

the driver of the external switch and therefore ab-

sorbs a pulsed current. Thus it is recommended to

place a buffer capacitor (towards PGND, pin 11,

as close as possible to the IC) able to sustain

these current pulses and in order to avoid them

inducing disturbances.

This pin can be connected to the buffer capacitor

directly or through a resistor, as shown in fig. 25,

to control separately the turn-on and turn-off

speed of the external switch, typically a Power-

MOS. At turn-on the gate resistance is Rg +Rg’, at

turn-off is Rg only.

Pin 10. OUT (Driver Output). This pin is the out-

put of the driver stage of the external power

switch. Usually, this will be a PowerMOS, al-

though the driver is powerful enough to drive

BJT’s (1.6A source, 2A sink, peak).

The driver is made up of a totem pole with a high-

side NPN Darlington and a low-side VDMOS, thus

there is no need of an external diode clamp to

prevent voltage from going below ground. An in-

ternal clamp limits the voltage delivered to the

gate at 13V. Thus it is possible to supply the

driver (Pin 9) with higher voltages without any risk

of damage for the gate oxide of the external MOS.

The clamp does not cause any additional in-

crease of power dissipation inside the chip since

the current peak of the gate charge occurs when

the gate voltage is few volts and the clamp is not

active. Besides, no current flows when the gate

voltage is 13V, steady state.

Under UVLO conditions an internal circuit (shown

in fig.26) holds the pin low in order to ensure that

the external MOS cannot be turned on acciden-

tally. The peculiarity of this circuit is its ability to

mantain the same sink capability (typically, 20mA

@ 1V) from VCC = 0V up to the start-up threshold.

When the threshold is exceeded and the L5993

starts operating, VREFOK is pulled high (refer to fig.

26) and the circuit is disabled.

It is then possible to omit the ”bleeder” resistor

(connected between the gate and the source of

the MOS) ordinarily used to prevent undesired

switching-on of the external MOS because of

some leakage current.

Pin 11. PGND (Power Ground). The current loop

during the discharge of the gate of the external

MOS is closed through this pin. This loop should

be as short as possible to reduce EMI and run

separately from signal currents return.

OUT

DRIVE &

CONTROL

13V

VCC

Rg’

PGND

Rg(ON)=Rg+Rg’

Rg(OFF)=Rg

D97IN767

L5993

Figure 25. Turn-on and turn-off speeds adjust-

ment

SGND

OUT

VREFOK

D97IN538

Figure 26. Pull-Down of the output in UVLO

L5993

11/22

L5993 Datasheet(PDF) 11 Page - STMicroelectronics

L5993 Datasheet(HTML) 11 Page - STMicroelectronics

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22

Datasheet Download

Link URL