L5991D datasheet(9/19 Pages) STMICROELECTRONICS

9 / 19 page

The oscillation frequency can be established with

the aid of the diagrams of fig. 13, where RT will be

intended as the parallel of RA and RB in normal

operation and RT =RA in standby, or considering

the following approximate relationships:

fosc

≅

⋅ (0.693 ⋅ (RA // RB)+ KT

(1),

which gives the normal operating frequency, and:

fSB

≅

⋅ (0.693 ⋅ RA + KT

)

(2),

which gives the standby frequency, that is the one

the converter will operate at when lightly loaded.

In the above expressions, RA // RB means:

RA//RB

⋅ RB

+ RB

while KT is defined as:

= 



90 V15

= VREF

160 V15 = GND/OPEN

(3),

and is related to the duration of the falling-edge of

the sawtooth:

Td ≈ 30 ⋅ 10

−9 + K

⋅ CT (4).

Td is also the duration of the sync pulses deliv-

ered at pin 1 and defines the upper extreme of the

duty cycle range, Dx (see pin 15 for DX definition

and calculation) since the output is held low dur-

ing the falling edge.

In case V15 is connected to VREF, however, the

switching frequency will be a half the values taken

from fig. 13 or resulting from (1) and (2).

To prevent the oscillator frequency from switching

back and forth from fosc to fSB, the ratio fosc /fSB

must not exceed 5.5.

If during normal operation the IC is to be synchro-

nized to an external oscillator, RA,RB and CT

should be selected for a fosc lower than the master

frequency in any condition (typically, 10-20% ),

depending also on the tolerance of the parts.

Pin 3. DC (Duty Cycle Control). By biasing this

pin with a voltage between 1 and 3 V it is possible

to set the maximum duty cycle between 0 and the

upper extreme Dx (see pin 15).

If Dmax is the desired maximum duty cycle, the

voltage V3 to be applied to pin 3 is:

V3 =5 -2

(2-Dmax)

(5)

Dmax is determined by internal comparison be-

tween V3 and the oscillator ramp (see fig. 23),

thus in case the device is synchronized to an ex-

ternal frequency fext (and therefore the oscillator

amplitude is reduced), (5) changes into:

= 5 − 4 ⋅ exp 



−

Dmax

⋅ CT ⋅ fext







(6)

A voltage below 1V will inhibit the driver output

stage. This could be used for a not-latched device

disable, for example in case of overvoltage pro-

tection (see application ideas).

If no limitation on the maximum duty cycle is re-

quired (i.e. DMAX =DX), the pin has to be left float-

ing. An internal pull-up (see fig. 23) holds the volt-

age above 3V. Should the pin pick up noise (e.g.

CLAMP

Ω

600

µA

D97IN729A

VREF

RCT

SYNC

CLK

ST-BY

STANDBY

Figure 22. Oscillator and synchronization internal schematic.

L5991 - L5991A

9/23

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