SC251EVB SEMTECH [Semtech Corporation], SC251EVB Datasheet - Page 9

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

High Power Mode
The SC251 enters high power mode when the VMODE
pin is pulled low. In this mode the V
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 profi le 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
V
pass-through device (V
Applications Information (Cont.)
POWER MANAGEMENT
© 2006 Semtech Corp.
OUT
is equal to V
Figure 4 - Load Profi le-High Power Mode
Figure 3 - Load Profi le-Low Power Mode
Maximum Load
IN
Maximum Load
minus the voltage dropped across the
dropout
V
V
OUT
).
OUT
(V)
(V)
Minimum Load
Minimum Load
DAC
Typical Load
to V
Typical
Load
OUT
transfer
9
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
lower output voltage required improves effi ciency over a
single mode system by lowering the voltage required for a
fi xed 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
V
remains fully on until either the V
by 150mV or the programmed V
such that V
impedance of the inductor and switching PMOS device
improves effi ciency by minimizing the voltage drop from
V
Pass-Through Mode
This mode is entered when the V
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
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
than 1.4V. This pin can be connected to the gate of an
external low-R
drain are connected to V
option allows the lowest insertion loss possible between
V
current. GD is monitored so that the part remains in pass-
through until GD reaches within 600mV of V
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
OUT
IN
IN
and V
to V
to V
OUT
IN
OUT
. The bypass device and PMOS switching device
.
IN
. Note that GD should not be loaded with a DC
– V
DSon
DSon
OUT
P-channel MOSFET whose source and
is greater than 650mV. Bypassing the
of this FET is extremely low so there is
IN
and V
DAC
-to- V
OUT
IN
OUT
voltage is increased
DAC
OUT
, respectively. This
voltage is reduced
voltage reaches
in Figure 2. The
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IN
SC251
DAC
.
is greater