SC908 View Datasheet(PDF) - Semtech Corporation
Part Name
Description
MFG CO.
'SC908' PDF : 30 Pages View PDF
SC908
Applications Information (continued)
Switcher Protection Features
The protection features are:
• Current limit
• Over-voltage protection
• Soft-start
Current Limit
The PMOS power device in the buck switcher stage is pro-
tected by a current limit function. If a short to ground on
the output occurs, the part enters frequency foldback
mode, which causes the switching frequency to divide by
a factor determined by the output voltage. This prevents
the inductor current from stair-casing.
Over-Voltage Protection
In the event of over-voltage on the output in PWM mode,
the PWM drive is disabled. When disabled, the SLX output
becomes high impedance (both high-side and low-side
switches are turned off ). The switcher will not resume
switching until the output voltage has fallen to 2% below
the programmed regulation voltage.
Soft-Start
The soft-start mode is enabled after every shutdown cycle
to limit in-rush current. This controls the maximum current
during start-up. The PMOS current limit is stepped up
using three soft-start levels to the full value by a timer
driven from the internal oscillator. During soft-start, the
switching frequency is stepped by 1/8, 1/4, and 1/2 of the
internal oscillator frequency up to the full value, under
control of three output voltage thresholds. When the
output voltage rises to 98% of the regulation voltage, soft-
start mode is disabled.
The inductor should have a low DCR to minimize the con-
duction losses and maximize efficiency. As a minimum
requirement, the DC current rating of the inductor should
be equal to the maximum load current plus half of the
inductor current ripple as shown by the equation
ILS (Peak )
IOUT(MAX)
'ILS
2
Final inductor selection will depend on various design
considerations such as efficiency, EMI, PSAVE entry, size
and cost.
C Selection
BAT
C functions as both the charger output capacitor and as
BAT
the switching regulator input capacitor. The source input
current to a buck converter is non-continuous. To prevent
large input voltage ripple a low ESR ceramic capacitor is
required. A minimum value of 10μF should be used for
sufficient input voltage filtering and a 22μF should be
used for improved input voltage filtering.
C Selection
SVOUT
The internal compensation is designed to operate with a
minimum output capacitor value of 10μF. Larger output
capacitor values will improve transient performance.
Output voltage ripple is a combination of the voltage
ripple from the inductor current charging and discharging
the output capacitor and the voltage created from the
inductor current ripple through the output capacitor ESR.
Selecting an output capacitor with a low ESR will reduce
the output voltage ripple component, as can be seen in
the equation
Switcher External Components
The SC908 is designed for use with the inductor L = 4.7μH,
S
although other values can be used. The magnitude of the
inductor current ripple is dependent on the inductor value
and can be determined by the equation
'ILS
VSVOUT
LS u fosc
¨¨©§1
VSVOUT
VVOUT
¸¸¹·
This equation demonstrates the relationship between
input voltage, output voltage, and inductor ripple
current.
'V 'I u ESR SVOUT(ESR)
LS (ripple)
CSVOUT
Capacitors with X7R or X5R ceramic dielectric are recom-
mended for their low ESR and superior temperature and
voltage characteristics. Y5V capacitors should not be used
as their temperature coefficients make them unsuitable
for this application.
When selecting an output capacitor, it is essential that
C capacitance be evaluated at the V programmed
SVOUT
SVOUT
voltage. The specified capacitance of 0402, and even
0603, package size devices is often severely derated at just
27
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