SC2544 View Datasheet(PDF) - Semtech Corporation
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SC2544 Datasheet PDF : 24 Pages
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SC2544
POWER MANAGEMENT
Applications Information
Overview
The SC2544 is a constant frequency 2-phase voltage
mode step-down PWM switching controller driving all
N-channel MOSFETs. The two channels of the controller
operate at 180 degree out of phase from each other.
Since input currents are interleaved in a two-phase
converter, input ripple current is lower and smaller
input capacitance can be used for filtering. Also, with
lower inductor current and smaller inductor ripple
current per phase, overall I2 R losses are reduced.
Frequency Setting
The frequency of the SC2544 is user-
programmable.The oscillator of SC2544 can be pro-
grammed with an external resistor from the Rosc
pin to the ground. The step-down controller is ca-
pable of operating up to 300KHz. The relationship
between oscillation frequency versus oscillation resis-
tor is shown in Figure 1.
The advantages of using constant frequency
operation are simple passive component selection and
ease of feedback compensation. Before setting the
operating frequency, 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
components 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
issues are also to be considered. The frequency bands
for signal transmission should be avoided because of
EM interference.
350
300
Soft Start
During start-up, the reference voltage of the error
amplifier equals 30% of the voltage on Css (soft-
start capacitor) which is connected between the SS
pin and ground. When the controller is enabled (by
pulling EN pin high), one internal 84uA current
source, ISS, (soft start current) will charge the soft-
start capacitor gradually. The PWM output starts
pulsing when the soft start voltage reaches 1V.
This soft start scheme will ensure the duty cycle to
increase slowly, therefore limiting the charging
current into the output capacitor and also ensuring
the inductor does not saturate. The soft start
capacitor will eventually be charged up to 2.5V.
The soft-start sequence is initiated when EN pin is
high and Vcc >4.5V or during recovery from a fault
condition ( OCP, OVP, or UVLO).
The period of start up can be programed by the
soft start capacitor:
Tss
=
Css × 2.5V
84µA
Shutdown
When the EN pin is pulled low, an internal 15uA
current source discharges the soft-start capacitor
and DRVH/DRVL signals stop pulsing. The output
voltage ramps down at a rate determined by the
load condition.
The SC2544 can also be shutdown by pulling down
directly on the SS pin. The designer needs to
consider the slope of the SS pin voltage and choose
a suitable pull down resistor to prevent the output
from undershooting.
250
200
150
100
50
Rosc (KOHM)
Shutdown can also be triggered when an OCP condi-
tion occurs. When an OCP condition is detected,
DRVH and DRVL will stop pulsing and enter a “tri-
state shutdown” with the output voltage ramping
down at a rate determined by the load condition.
The internal 15uA current source will begin dis-
charging the soft-start capacitor and when the
soft-start voltage reaches 0.65V, DRVL will go high.
Figure 1. Switching frequency versus Rosc.
2005 Semtech Corp.
8
www.semtech.com
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