SC4501 Semtech Corporation, SC4501 Datasheet - Page 12

• Manufacturer Part Number: SC4501
• Description: 2MHz Step-Up Switching Regulator
• Manufacturer: Semtech Corporation

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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,
Application Information
POWER MANAGEMENT
L
2005 Semtech Corp.
(the desired upper and the lower UVLO threshold
Figure 6. 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).
V IN
1N4148
IN
(a)
(c)
undervoltage lockout
IN
SHDN
IN
SHDN
SC4501
SC4501
IN
falls below
OH
H
> V
and
IN
12
V
Re-arranging,
H
and V
R
R
V
V
H
L
3
4
V
V
1
I
L
1
H
HYS
V
HYS
are therefore:
1 .
H
R
3
V
R
R
HYS
3
4
1
1
1 .
V
H
V
(b)
(d)
I
HYS
IN
SHDN
IN
SHDN
R
3
SC4501
SC4501
OL
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< 0.1V)
SC4501
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