sc250wltrt Semtech Corporation, sc250wltrt Datasheet - Page 8

• Manufacturer Part Number: sc250wltrt
• Description: Sc250 Step-down Dc-dc Converter With Bias Ldo For Rf Power Amplifiers
• Manufacturer: Semtech Corporation

SC250 Detailed Description
The SC250 adaptive power controller is a step-down,
fi xed frequency pulse-width modulated DC-DC converter
designed for use with RF Power Amplifi ers (PAs) in
CDMA and WCDMA handsets and modules. The SC250
output is used to supply DC power to the PA rather than
connecting the DC input pin directly to the battery supply.
A substantial system power effi ciency improvement can
be achieved by allowing the system controller to adaptively
adjust the DC power to the PA, reducing the total power
consumption of the device when in low-power mode. To
improve effi ciency at all RF output gain settings, the PA
supply voltage is adjusted in a linear fashion, minimizing
PA supply headroom and losses.
A consequence of using the SC250 to power the PA, rather
than using a linear regulator or direct connection to the
battery, is that less current is needed. Reduced current
consumption results in more talk-time for the handset.
Operation Modes
The SC250 output voltage is dependent on the V
control voltage, defi ned by the following relationship:
In a typical PA system application, the system controller
determines what output power level is needed from the
PA and adjusts the VDAC voltage to match the required PA
headroom for optimized effi ciency.
Pass-Through Mode
When the VDAC voltage reaches 1.36V, the SC250
enters pass-through mode. If the demanded output
voltage is within 430mV of the input voltage, the SC250
automatically enters pass-through as this exceeds the
maximum controlled duty cycle of the power converter.
In pass-through mode, the device enables an internal P-
Channel MOSFET that bypasses the converter, connecting
the output directly to the input. The R
extremely low, so there is little voltage drop across the part.
Pass-through allows the lowest insertion loss possible
between V
maintaining maximum effi ciency under these conditions.
© 2006 Semtech Corp.
POWER MANAGEMENT
Applications Information
IN
and V
V
OUT
OUT
= 3 × V
under high-power conditions, thereby
DAC
DSON
of this FET is
DAC
analog
8
Bias Supply Output
In addition to the main output, the SC250 also provides a
low current LDO output that can be used as a bias supply
for power amplifi ers. This output provides a regulated
2.85V with output current capability up to 20mA. The
2.85V output is guaranteed for input supply voltages in
excess of 2.95V.
Protection Features
The SC250 provides the following protection features:
Thermal Shutdown
The device has a thermal shutdown feature to protect the
device if the junction temperature exceeds 150°C. In
thermal shutdown, the PWM drive is disabled, effectively
tri-stating the LX output. The device will not be enabled
again until the temperature reduces by 10°C.
Short-Circuit Protection
The PMOS and NMOS power devices of the buck switcher
stage are protected by current limit functions. In the case
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 also protected by a current
limit function. When the part is fi rst enabled in pass-
through, the output capacitor charges up with a large
surge current. This surge current is internally limited for
protection purposes, but the limit is set high enough to
meet fast start-up times. In order to protect against a
short-circuit condition and to allow the transient response
time, an internal timer allows the part to operate under
current limit conditions for a maximum of 64 cycles of the
internal clock (1MHz typical). If the short-circuit conditions
persists, the pass-through PMOS will turn off for 1ms,
after which the fi rst timer is restarted. This allows the part
to manage thermal dissipation while giving it the ability to
recover when the fault condition is removed.
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•
•
Thermal shutdown
Current limit
Under-voltage lockout
PRELIMINARY
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SC250