SC4503EVB View Datasheet(PDF) - Semtech Corporation
Part Name
Description
MFG CO.
'SC4503EVB' PDF : 22 Pages View PDF
SC4503
POWER MANAGEMENT
Applications Information (Cont.)
The minimum SHDN SS voltage for switching is 1.4V. The
graph “Switch Current Limit vs. Shutdown Pin Voltage” in
the “Typical Characteristics” shows that the SHDN SS pin
voltage needs to be at least 2V for the SC4503 to deliver
its rated power. The effect of the SHDN SS voltage on the
SC4503 is analog between 1.4V and 2V. Within this range
the switch current limit is determined not by ILIM but in-
stead by the PWM signal path (see Figure 2). Moreover it
varies with duty cycle and the shutdown pin voltage.
Pulling the SHDN SS pin below 0.4V shuts down the SC4503,
drawing less than 1µA from the input power supply. For
voltages above 2V and below 0.4V, the SHDN SS pin can be
regarded as a digital on/off input. Figure 5 shows several
ways of interfacing the control logic to the shutdown pin. In
Figure 5(a) soft-start is not used and the logic gate drives
the shutdown pin through a small ( ≈ 1kΩ ) optional resistor
RSS. RSS limits the current drawn by the SC4503 internal
VOH > 2V
VOL < 0.4V
RLIM
IN
SC4503
SHDN/SS
(a)
VIN
End of Soft-start
VSHDN/SS > 2V
RSS
CSS
IN
SC4503
SHDN/SS
(b)
End of Soft-start
VSHDN/SS > 2V
VOL < 0.4V
RSS
ISHDN/SS
IN
SC4503
SHDN/SS
CSS
(c)
VIN
1.7V < VOH < 2V
VOL ≈ 0
DSS
CMDSH-3
IN
RSS
SC4503
SHDN/SS
ISHDN/SS
CSS
(d)
VIN
End of Soft-start
VSHDN/SS > 2V
RSS
IN
SC4503
SHDN/SS
CSS
VIN
VOH > VIN 1N4148
RSS
IN
SC4502
SHDN/SS
CSS
(e)
(f)
Figure 5. Methods of Driving the Shutdown Pin and Soft-starting the SC4503
(a) Directly Driven from a Logic Gate. RLIM Limits the Gate Output Current during Fault,
(b) Soft-start Only,
(c) Driven from a Logic Gate with Soft-start,
(d) Driven from a Logic Gate with Soft-start (1.7V < VOH < 2V),
(e) Driven from an Open-collector NPN Transistor with Soft-start and
(f) Driven from a Logic Gate (whose VOH > VIN) with Soft-start.
2007 Semtech Corp.
12
www.semtech.com
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