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SC4503 Datasheet(PDF) 12 Page - Semtech Corporation

Part # SC4503
Description  1.3MHz Step-Up Switching Regulator with 1.4A Switch
PDF  22 Pages
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Manufacturer  SEMTECH [Semtech Corporation]
Direct Link  http://www.semtech.com
Logo SEMTECH - Semtech Corporation

SC4503 Datasheet(HTML) 12 Page - Semtech Corporation

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 2007 Semtech Corp.
www.semtech.com
POWER MANAGEMENT
SC4503
Methods of Driving the Shutdown Pin and Soft-starting the SC4503
(a) Directly Driven from a Logic Gate. R
LIM 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 < V
OH < 2V),
(e) Driven from an Open-collector NPN Transistor with Soft-start and
(f) Driven from a Logic Gate (whose V
OH > VIN) with Soft-start.
Figure 5.
Applications Information (Cont.)
The minimum
SS
SHDN SS
SHDN
voltage for switching is 1.4V. The
graph “Switch Current Limit vs. Shutdown Pin Voltage” in
the “Typical Characteristics” shows that the
SS
SHDN SS
SHDN
pin
voltage needs to be at least 2V for the SC4503 to deliver
its rated power. The effect of the
SS
SHDN SS
SHDN
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
SS
SHDN SS
SHDN
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
SS
SHDN SS
SHDN
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
R
SS. RSS limits the current drawn by the SC4503 internal
(a)
SC4503
SHDN/SS
IN
VOH > 2V
VOL < 0.4V
RLIM
SC4503
SHDN/SS
IN
VOL < 0.4V
RSS
CSS
ISHDN/SS
End of Soft-start
VSHDN/SS > 2V
(c)
(b)
SC4503
IN
SHDN/SS
CSS
RSS
End of Soft-start
VSHDN/SS > 2V
VIN
SC4503
IN
SHDN/SS
CSS
RSS
End of Soft-start
VSHDN/SS > 2V
VIN
(e)
(d)
SC4503
SHDN/SS
IN
VOL ≈ 0
CSS
ISHDN/SS
RSS
CMDSH-3
DSS
1.7V < VOH < 2V
VIN
(f)
1N4148
SC4502
IN
VIN
RSS
CSS
VOH > VIN
SHDN/SS



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