11

 2005 Semtech Corp.

www.semtech.com

POWER MANAGEMENT

SC4502/SC4502H

gate whose V

SC4502/SC4502H. The diode clamps the maximum

shutdown pin voltage to one diode voltage above the

input power supply.

Programming Undervoltage Lockout

The SC4502/SC4502H has an internal V

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 SC4502/SC4502H can be used

in conjunction with a resistive voltage divider to raise the

UVLO threshold and to add an UVLO hysteresis. Figure 6

shows the scheme. Both V

and the lower UVLO threshold 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

1.1V

R

R

1

V

−

=

−

=

⋅









+

=

(10)

Re-arranging,

HYS

HYS

3

I

V

R

=

(11)

1

1.1V

V

R

R

H

3

4

−

=

(12)

Figure 5. Methods of Driving the Shutdown Pin

(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).

(c)

1N4148

SHDN

IN

VIN

(a)

SC4502

SC4502H

SHDN

IN

(d)

SHDN

IN

(b)

SHDN

IN

SC4502

SC4502H

SC4502

SC4502H

SC4502

SC4502H

Applications Information