TSM007

Functional Description

6/12

Soft start

A capacitor from the SS pin to GND provides the

soft start function. The capacitor starts to charge

when VIN reaches the UVLO threshold and Vref is

good.The soft start block enables the IC to start

with a progessive PWM duty cycle. The soft start

period is set by the external capacitor Css. The

soft start comparator drives the output driver low

when the SS pin voltage is greater than the CT pin

voltage minus one VF voltage..

Tss = Cssx(Vthct-VF)/Iiss

Let’s assume Tss = 50ms

==> Css = TssxIiss/(Vthct-VF)

==> Css = 210nF

Brown out

A voltage proportionnal to the Bulk capacitor

voltage is applied to the BRNOUT pin. When this

pin signal is less than 2.5V, the IC is disabled: SS

and CT pins go low, Vref is disabled and the IC

goes to standby mode. All bias currents are

switched off. The 2.5V voltage at the negative

input of comparator Comp_Brown out is still

active. When Brown out voltage goes higher than

2.5V, Vref is enabled again. A startup period is

started, and then a soft start occurs..

Let’s assume BRNOUT acts for Vfiltered = 200V

R2 = 15K

Ω

R1 = R2(Vfiltered - UVLO) / UVLO

When BRNOUT is bellow 2.5V

IR2 = Vfiltered(R1+R2)

When BRNOUT is above 2.5V

IR2 = Ibr + Vfiltered(R1+R2)

BRNOUT = R2x(Ibr + Vfiltered(R1+R2))

R1 = 1185K

Ω

R2 = 167µA

When BRNOUT is above 2.5V

IR2 = 237µA

BRNOUT = 3.55V

Vref

Idss

4mA

Iiss

10µA

Css

SS

Comp_SS

VF

CT

CT

Comp_osc

1.4V_3.0V

Out

Out

R

S

Q

!Q

Cs_latch

SS

VCT-VF

Tss = Soft start period

Out

2.4V

0.8V

Iiss=10µA

Iiss

Vref

1mA

400µA

CT

CT

Comp_osc

1.4V_3.0V

Out

Out

Vcc

UVLO

Vref

Vref_Status

R

S

Q

!Q

Startup Latch

Comp_brown out

Ibr = 50µA

Vin

2.50V

Brown out

R1

R2

filtered

Ibr

Brown out

2.5V

Ibr=50µA