SC2545 データシートの表示（PDF） - Semtech Corporation
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SC2545 Datasheet PDF : 24 Pages
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SC2545
POWER MANAGEMENT
Applications Information
Overview
The SC2545 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 SC2545 is user- programmable.
The oscillator of SC2545 can be programmed with an
external resistor from the Rosc pin to the ground. The
step-down controller is capable of operating up to
300kHz. The relationship between oscillation frequency
versus oscillation resistor 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 and ”Peel off ”Tracking
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
By tieing the two soft start pins together, the two
output rails track each other at start up till the rail with
lower set level reaching regulation. Please refer the
waveform on page 20.
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 SC2545 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
Ro s c ( KOHM)
Figure 1. Switching frequency versus Rosc.
Shutdown can also be triggered when an OCP condition
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 deter-
mined by the load condition. The internal 15uA current
source will begin discharging the soft-start capacitor
and when the soft-start voltage reaches 0.65V, DRVL
will go high.
© 2005 Semtech Corp.
8
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