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SC2542 Datasheet(PDF) 11 Page - Semtech Corporation

Part # SC2542
Description  High Performance Wide Input Range Dual Synchronous Buck Controller
PDF  27 Pages
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Manufacturer  SEMTECH [Semtech Corporation]
Direct Link  http://www.semtech.com
Logo SEMTECH - Semtech Corporation

SC2542 Datasheet(HTML) 11 Page - Semtech Corporation

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© 2005 Semtech Corp.
www.semtech.com
SC2542
POWER MANAGEMENT
Applications Information (Cont.)
(b)
Figure 6. Typical waveforms of SKIP mode.
Frequency Setting
The frequency of the SC2542 is user- programmable.
The oscillator of SC2542 can be programmed with an
external resistor from the Rosc pin to ground. The step-
down controller is capable of operating up to 300KHz.
The relationship between oscillation frequency versus
oscillation resistor is shown in Figure 7.
The advantages of using constant frequency operation
include simple passive component selection and ease of
feedback compensation. Before setting the operating fre-
quency, the following trade-offs should be considered.
1) Passive component size
2) Circuitry efficiency
3) EMI condition
4) Minimum switch on time
5) Maximum duty ratio
For a given output power, the sizes of the passive com-
ponents are inversely proportional to the switching
frequency, whereas MOSFETs/Diodes switching losses
are proportional to the operating frequency. Other issues
such as heat dissipation, packaging, and cost also need
to be considered. The frequency bands for signal trans-
mission should be avoided because of EM interference.
50
100
150
200
250
300
350
Rosc(KOHM)
Figure 7. Switching frequency versus Rosc.
Power on Sequence
After applying the input voltage and with STBY and EN
inputs high, Vcc and 5VSTBY will rise. When Vcc rises
above the UVLO threshold both switcher outputs will also
ramp up. By pulling the SS pins low, the two switcher
outputs can be disabled, with Vcc and 5VSTBY maintain-
ing regulation. Vcc will turn off when STBY is pulled low,
with 5VSTBY maintaining regulation. And finally 5VSTBY
will turn off when EN is pulled low, and the device will be
in shutdown mode.
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Soft Start
During start-up, the reference voltage of the error ampli-
fier equals 30% of the voltage on Css (soft-start capaci-
tor) which is connected between the SS pin and ground.
When the controller is enabled (by pulling EN pin or STBY
TG
BG
Vo
Inductor
Current
TG
BG
Vo
Inductor
Current



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