SC251EVB SEMTECH [Semtech Corporation], SC251EVB Datasheet - Page 8

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

SC251 Detailed Description
The SC251 is a step-down, fi xed frequency pulse-width
modulated DC-DC converter designed for use with RF power
amplifi ers (PAs) in WCDMA handsets and modules.
The 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 voltage to the PA,
reducing the total power consumption of the device. To
maximize effi ciency at all RF output gain settings, the PA
supply voltage is adjusted exponentially, minimizing PA
supply headroom and losses. The benefi t of having an
exponential V
plotted on the same graph as linear relationships, see
following fi gure. The SC251 V
optimized to provide the lowest supply voltage to maintain
the PA’s linearity. This provides the best possible balance
between Adjacent Channel Leakage Ratio (ACLR) margin
and effi ciency requirements.
By using a switching regulator, less current is needed
than when the PA is connected directly to the battery or
an LDO. Reduced current consumption results in more
talk-time for the handset.
Applications Information
POWER MANAGEMENT
© 2006 Semtech Corp.
Figure 1 - Advantage of exponential Transfer Function
Exponential function
advantage over linear
function
OUT
vs. V
DAC
relationship is clearly seen when
V
OUT
DAC
vs. V
(V)
DAC
Margin to maintain
PA linear operation
transfer function is
Signal
amplitude (V)
8
Operation Modes
The SC251 output voltage is dependent on the V
control voltage and the V
mode V
designed to produce maximum power amplifi er effi ciency.
When V
when V
mode. The relationships between V
modes are optimized to achieve the best effi ciency from
a dual-mode PA design These relationships are shown
in the following fi gure. The system controller determines
the output power level needed from the PA and adjusts
the V
voltage and adjusts the output voltage supply to the PA to
optimize effi ciency and maintain PA linearity.
Low Power Mode
The SC251 enters low power mode when the V
pulled high. In this mode the V
is set to follow the dotted line curve shown in Figure 2.
The output voltage starts at 0.5V for low power settings
and increases exponentially until it reaches the maximum
of 3.4V. If the power control for the PA requires the output
voltage to exceed 3.4V, then the SC251 goes into pass-
through mode and V
dropped across the pass-through device (see pass-through
mode for more details).
A typical WCDMA load profi le for low power mode, with
the minimum and maximum current limits, is shown in
Figure 3.
DAC
OUT
MODE
MODE
voltage accordingly. The SC251 monitors the V
follows a different V
Figure 2 - V
is high the device is in low power mode, and
is low the device switches to high power
OUT
DAC
LOW
POWER
MODE
is equal to V
MODE
to V
V
DAC
digital control input. In each
OUT
DAC
(V)
DAC
Transfer Functions
transfer function that is
to V
IN
OUT
OUT
minus the voltage
and V
transfer function
V
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HIGH
POWER
MODE
IN
- V
SC251
dropout
DAC
DAC
MODE
in both
analog
pin is
DAC