SC251MLTRT datasheet(9/19 Pages) SEMTECH

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SC251

POWER MANAGEMENT

Figure 4 - Load Proﬁle-High Power Mode

Maximum Load

Typical Load

Minimum Load

OUT(V)

High Power Mode

The SC251 enters high power mode when the VMODE

pin is pulled low. In this mode the V

DAC to VOUT transfer

function is set to follow the solid line curve shown in

Figure 2. The output voltage again starts at 0.5V and

increases exponentially as the power demand increases

until it reaches the maximum of 3.4V. A typical WCDMA

load proﬁle for high power mode with the minimum and

maximum current limits is shown in Figure 4. If the power

control for the PA requires the output voltage to exceed

3.4V, then the SC251 goes into pass-through mode and

OUT is equal to VIN minus the voltage dropped across the

pass-through device (V

dropout).

Figure 3 - Load Proﬁle-Low Power Mode

Maximum Load

Minimum Load

Typical

Load

OUT(V)

In high power mode the PA gain is constant, but output

impedance is lower and the subsequent input voltage

required to achieve the desired output power is less than

in low power mode. The SC251 output, therefore, switches

to the solid line curve in the V

DAC-to- VOUT in Figure 2. The

lower output voltage required improves efﬁciency over a

single mode system by lowering the voltage required for a

ﬁxed current load.

100% Duty Cycle Operation

When the input supply voltage approaches the

programmed output voltage the PMOS on-time extends

until the supply voltage gets within 400mV of the output

voltage. At this point both the internal pass device and the

PMOS switching device automatically turns on, connecting

IN to VOUT. The bypass device and PMOS switching device

remains fully on until either the V

IN voltage is increased

by 150mV or the programmed V

OUT voltage is reduced

such that V

IN – VOUT is greater than 650mV. Bypassing the

impedance of the inductor and switching PMOS device

improves efﬁciency by minimizing the voltage drop from

OUT to VIN.

Pass-Through Mode

This mode is entered when the V

DAC voltage reaches

1.28V. If the demanded output voltage is within 400mV

of the input voltage the device 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

DSon of this FET is extremely low so there is

little voltage drop across the part. If the system designer

determines that the pass-through resistance is too high

for the application, there is an optional gate-drive output

that can be used with an external switch.

The GD pin becomes active-low only when V

DAC is greater

than 1.4V. This pin can be connected to the gate of an

external low-R

DSon P-channel MOSFET whose source and

drain are connected to V

IN and VOUT, respectively. This

option allows the lowest insertion loss possible between

IN and VOUT . Note that GD should not be loaded with a DC

current. GD is monitored so that the part remains in pass-

through until GD reaches within 600mV of V

IN.

Bias Supply Output

In addition to the main output the SC251 also provides a

low current LDO reference output that can be used as a

Applications Information (Cont.)

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