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SC251 Datasheet PDF : 19 Pages
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SC251
POWER MANAGEMENT
Applications Information (Cont.)
bias supply for power amplifiers. This output provides a
regulated 2.85V with output current capability up to 10mA.
The 2.85V output is guaranteed for input supply voltages
in excess oAfp2p.9li5cVa.tWiohnesn iInnpfuotrvmolatatgieosnb(eCloown2t..9)5V are
used, VREF is equal to VIN - VREF DO. This reference supply
is controlled by the same enable pin as the switching
regulator.
Protection Features
The SC251 provides the following protection features:
• Thermal shutdown
• Current limit
• Under voltage lockout
Thermal Shutdown
The device has a thermal shutdown feature to protect
the device if the junction temperature exceeds 160°C. In
thermal shutdown the on-chip power devices are disabled,
effectively tri-stating the LX output. Switching will resume
when the temperature drops by 15°C.
Short Circuit Protection
The PMOS and NMOS power devices of the buck switcher
stage are protected by current limit functions. In the event
of a short to ground on the output, the LX pin will switch
with minimum duty cycle. The duty cycle is short enough to
allow the inductor to discharge during each cycle, thereby
preventing the inductor current from “staircasing”.
The pass-through PMOS is protected by a current limit
function. When the part is enabled in pass-through, the
output capacitor charges up with a large surge current.
In order to support this surge current and to protect
against short circuits, an internal timer is used. A short
circuit condition must exist for more than 128 clock
cycles before the pass-through device is disabled. After
an additional 2048 clock cycles, the pass-through device
will turn back on. This cycle will continue until the short
circuit is removed. This method allows the part to manage
thermal dissipation and recover when the fault condition
is removed.
Under Voltage Lockout
The part will turn itself off if the input supply voltage falls
below 2.4V typical. The device is allowed to turn on again
when the input supply voltage increases above the lockout
voltage. Hysteresis is included to prevent chattering.
Inductor Selection
The SC251 is designed for use with a 4.7μH inductor. The
magnitude of the inductor current ripple is dependent on
the inductor value and can be determined by the following
equation:
ΔIL
=
VOUT⎜⎝⎛1 −
VOUT
VI N
⎟⎠⎞
L × fOSC
The inductor should have a low DC Resistance (DCR) to
minimize the conduction 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
following equation:
ΔIL
I = I + LPK
OUT (MAX )
2
Final inductor selection will depend on various design
considerations such as efficiency, EMI, size and cost.
Table 1 lists the manufacturers of practical inductor
options.
CIN Selection
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
OUT
The internal compensation is designed to work with a
certain output filter corner frequency defined by the
equation:
fC =
1
2π L × COUT
This single pole filter is designed to operate with an output
capacitor value of 4.7μF.
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 reduces the
© 2006 Semtech Corp.
10
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