SC251EVB SEMTECH [Semtech Corporation], SC251EVB Datasheet - Page 10

• Manufacturer Part Number: SC251EVB
• Description: Step-down DC-DC Converter with Bias LDO for WCDMA Amplifi ers
• Manufacturer: SEMTECH [Semtech Corporation]

bias supply for power amplifi ers. 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 of 2.95V. When input voltages below 2.95V are
used, V
is controlled by the same enable pin as the switching
regulator.
Protection Features
The SC251 provides the following protection features:
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.
Applications Information (Cont.)
POWER MANAGEMENT
© 2006 Semtech Corp.
•
•
•
Thermal shutdown
Current limit
Under voltage lockout
REF
Applications Information (Cont.)
is equal to V
IN
- V
REF DO
. This reference supply
10
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:
The inductor should have a low DC Resistance (DCR) to
minimize the conduction losses and maximize effi ciency.
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:
Final inductor selection will depend on various design
considerations such as effi ciency, EMI, size and cost.
Table 1 lists the manufacturers of practical inductor
options.
C
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 suffi cient input voltage fi ltering
and a 22μF should be used for improved input voltage
fi ltering.
C
The internal compensation is designed to work with a
certain output fi lter corner frequency defi ned by the
equation:
This single pole fi lter 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
IN
OUT
Selection
Selection
Δ
I
L
I
f
LPK
=
C
=
=
V
2
OUT
I
OUT
π
⎛ −
⎜
⎝
L
(
MAX
1
×
L
1
)
f
×
+
OSC
C
V
V
Δ
OUT
OUT
2
I
N
I
L
⎞
⎟
⎠
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