STV9118 datasheet(35/46 Pages) STMICROELECTRONICS

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STV9118

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During the operation, a sawtooth is to be found on

pin BISense, generated externally by the applica-

tion. According to BOutPh I2C bus bit, the R-S flip-

flop is set either at H-drive signal edge (rising or

falling, depending on BOHEdge I2C bus bit), or a

certain delay (tBTrigDel /TH) after middle of H-fly-

back. The output is set On at the end of a short

pulse generated by the monostable trigger.

Timing of reset of the R-S flip-flop affects duty cy-

cle of the output square signal and so the energy

transferred from DC/DC converter input to its out-

put. A reset edge is provided by comparator C2 if

the voltage on pin BISense exceeds the internal

threshold VThrBIsCurr. This represents current limi-

tation if a voltage proportional to the current

through the power component or deflection stage

is available on pin BISense. This threshold is af-

fected by the voltage on pin HPosF, which rises at

soft start and descends at soft stop. This ensures

self-contained soft control of duty cycle of the out-

put signal on pin BOut. Refer to Figure 10. Another

condition for the reset of the R-S flip-flop, OR-ed

with the one described before, is that the voltage

on pin BISense exceeds the voltage VC1, which

depends on the voltage applied on input BISense

of the error amplifier O1. The two voltages are

compared, and the reset signal generated by the

comparator C1. The error amplifier amplifies (with

a factor defined by external components) the dif-

ference between the input voltage proportional to

DC/DC convertor output voltage and internal refer-

ence VBReg. The internal reference and so the out-

put voltage is I2C bus adjustable by means of

BREF I2C bus control.

DC/DC controller Off-to-On edge timing

Figure 14. DC/DC converter controller block diagram

BOutPh

(Sad07/

D7)

BOHEdge

(Sad17/

D3)

Timing of Off-to-On transition

on BOut output

don’t care Middle of H-flyback plus tBTrigDel

Falling edge of H-drive signal

Rising edge of H-drive signal

BRegIn

BComp

BIsense

BOut

Soft start

H-drive edge

H-flyback

~500ns

Monostable

HBOutEn

VCC

Feedback

XRayAlarm

(I2C)

BOutPh

(I2C)

HPosF

VThrBIsCurr

(+delay)

BOHEdge

(I2C)

VBReg

VC1

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