12

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POWER MANAGEMENT

SC4501

Application Information

Programming Undervoltage Lockout

The SC4501 has an internal V

(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

the internal UVLO threshold. The resulting output voltage

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

the SC4501 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

V

voltages) are determined by the 1.1V threshold crossings,

V

(

)

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

SC4501

SHDN

IN

VIN

(a)

SC4501

SHDN

IN

(d)

SC4501

SHDN

IN

(b)

SC4501

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

(c) Driven from a Logic Gate with V

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