SC4502H 查看數據表（PDF） - Semtech Corporation
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SC4502H Datasheet PDF : 19 Pages
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POWER MANAGEMENT
Applications Information
IN
SC4502
SC4502H
SHDN
(a)
VIN
1N4148
IN
SC4502
SC4502H
SHDN
SC4502/SC4502H
IN
SC4502
SC4502H
SHDN
(b)
IN
SC4502
SC4502H
SHDN
(c)
(d)
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 (VOL < 0.1V)
(c) Driven from a Logic Gate with V > V
OH IN
(d) Combining Shutdown with Programmed UVLO (See Section Below).
gate whose V is higher than the supply voltage of the
OH
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 VIN 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 VIN
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 VH and VL (the desired upper
and the lower UVLO threshold voltages) are determined
by the 1.1V threshold crossings, VH and VL are therefore:
VH
=
1+
R3
R4
⋅
(1.1V )
VL = VH − VHYS = VH − IHYSR3
(10)
Re-arranging,
R3
=
VHYS
IHYS
(11)
R4
=
R3
VH
1.1V
−1
(12)
2005 Semtech Corp.
11
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