SC251 데이터 시트보기 (PDF) - Semtech Corporation
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SC251 Datasheet PDF : 19 Pages
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SC251
POWER MANAGEMENT
Applications Information
SC251 Detailed Description
The SC251 is a step-down, fixed frequency pulse-width
modulated DC-DC converter designed for use with RF power
amplifiers (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 efficiency
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 efficiency at all RF output gain settings, the PA
supply voltage is adjusted exponentially, minimizing PA
supply headroom and losses. The benefit of having an
exponential VOUT vs. VDAC relationship is clearly seen when
plotted on the same graph as linear relationships, see
following figure. The SC251 VOUT vs. VDAC transfer function is
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 efficiency 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.
Operation Modes
The SC251 output voltage is dependent on the VDAC analog
control voltage and the VMODE digital control input. In each
mode V follows a different V transfer function that is
OUT
DAC
designed to produce maximum power amplifier efficiency.
When VMODE is high the device is in low power mode, and
when VMODE is low the device switches to high power
mode. The relationships between VOUT and VDAC in both
modes are optimized to achieve the best efficiency from
a dual-mode PA design These relationships are shown
in the following figure. The system controller determines
the output power level needed from the PA and adjusts
the VDAC voltage accordingly. The SC251 monitors the VDAC
voltage and adjusts the output voltage supply to the PA to
optimize efficiency and maintain PA linearity.
VIN - Vdropout
LOW
POWER
MODE
HIGH
POWER
MODE
Exponential function
advantage over linear
function
Margin to maintain
PA linear operation
VDAC(V)
Figure 2 - VDAC to VOUT Transfer Functions
Signal
amplitude (V)
VDAC(V)
Figure 1 - Advantage of exponential Transfer Function
Low Power Mode
The SC251 enters low power mode when the V pin is
MODE
pulled high. In this mode the VDAC to VOUT transfer function
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 VOUT is equal to VIN minus the voltage
dropped across the pass-through device (see pass-through
mode for more details).
© 2006 Semtech Corp.
A typical WCDMA load profile for low power mode, with
the minimum and maximum current limits, is shown in
Figure 3.
8
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