SC251 datasheet(8/19 Pages) SEMTECH

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SC251

POWER MANAGEMENT

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 V

OUT vs. VDAC relationship is clearly seen when

plotted on the same graph as linear relationships, see

following ﬁgure. The SC251 V

OUT 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 V

DAC analog

control voltage and the V

MODE digital control input. In each

mode V

OUT follows a different VDAC transfer function that is

designed to produce maximum power ampliﬁer efﬁciency.

When V

MODE is high the device is in low power mode, and

when V

MODE is low the device switches to high power

mode. The relationships between V

OUT 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 V

DAC voltage accordingly. The SC251 monitors the VDAC

voltage and adjusts the output voltage supply to the PA to

optimize efﬁciency and maintain PA linearity.

Figure 2 - V

DAC to VOUT Transfer Functions

Figure 1 - Advantage of exponential Transfer Function

DAC(V)

Exponential function

advantage over linear

function

Signal

amplitude (V)

Margin to maintain

PA linear operation

Low Power Mode

The SC251 enters low power mode when the V

MODE pin is

pulled high. In this mode the V

DAC 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 V

OUT is equal to VIN minus the voltage

dropped across the pass-through device (see pass-through

mode for more details).

A typical WCDMA load proﬁle for low power mode, with

the minimum and maximum current limits, is shown in

Figure 3.

DAC(V)

IN - Vdropout

LOW

POWER

MODE

HIGH

POWER

MODE

Applications Information

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