10

KB3302

Kingbor Technology Co.,Ltd

TEL:(86)0755-26508846 FAX:(86)0755-26509052

APPLICATIONS INFORMATION

Programming Undervoltage Lockout

The KB3302 has an internal V

IN

undervoltage lockout

(UVLO) threshold of 1.4V. The transition from idle to

switching is abrupt but there is no hysteresis. If the input

voltage ramp rate is slow and the input bypass is limited,

then sudden turn on of the power transistor will cause a

dip in the line voltage. Switching will stop if V

IN

falls below

the internal UVLO threshold. The resulting output voltage

rise may be non-monotonic. The 1.1V disable threshold of

the KB3302 can be used in conjunction with a resistive

voltage divider to raise the UVLO threshold and to add an

UVLO hysteresis. Figure 7 shows the scheme. Both V

H

and

V

L

(the desired upper and the lower UVLO threshold

voltages) are determined by the 1.1V threshold crossings,

V

H

and V

L

are therefore:

()

3

HYS

H

HYS

H

L

4

3

H

R

I

V

V

V

V

V

1

.

1

R

R

1

V

−

=

−

=

⎟⎟⎠

⎞

⎜⎜⎝

⎛

+

=

(10)

Re-arranging,

HYS

HYS

3

I

V

R

=

(11)

1

1

.

1

V

R

R

H

3

4

−

=

(12)

Figure 6. Methods of Driving the Shutdown Pin

(c)

1N4148

KB3302

SHDN

IN

VIN

(a)

KB3302

SHDN

IN

(d)

KB3302

SHDN

IN

(b)

KB3302

SHDN

IN

(a) Directly Driven from a Logic Gate

(b) Driven from an Open-drain N-channel MOSFET or an Open-Collector NPN Transistor (V

OL

< 0.1V)

(c) Driven from a Logic Gate with V

OH

> V

IN

(d) Combining Shutdown with Programmed UVLO (See Section Below).