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| SC251 Datasheet, PDF (1/19 Pages) Semtech Corporation – Step-down DC-DC Converter with Bias LDO for WCDMA Amplifi ers | |||
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POWER MANAGEMENT
SC251
Step-down DC-DC Converter with
Bias LDO for WCDMA Ampliï¬ers
Description
The SC251 is a synchronous step-down converter designed
for use as an adaptive voltage supply for WCDMA RF
power ampliï¬ers (PAs). An analog control input is used to
adjust the output voltage dynamically between 0.5V and
3.4V using a non-linear transfer function. The non-linear
relationship maximizes total system efï¬ciency by providing
the PA with the minimum voltage it needs to maintain
linearity. For output voltages greater than 3.4V the input
is connected directly to the output via an internal PMOS
switch. An optional gate drive (GD) output to control an
external low on-resistance PMOS switch is also provided
for systems applications that require minimal voltage
drop. The SC251 also provides a 2.85V LDO reference
output that can be used to supply a PA bias input.
Low power and high power modes are provided to match
performance with dual mode PAs. In low power mode the
output voltage follows an exponential relationship with
the VDAC input until it reaches 3.4V. When the VMODE
pin changes state, VOUT follows an alternate exponential
relationship.
The SC251 is capable of supplying output current up
to 800mA. Standby current is <1μA when the device is
disabled. The internal clock runs at 1MHz so that small
surface mount inductors and capacitors can be used.
Features
 VOUT exponentially proportional to VDAC for maximum
efï¬ciency (patent pending)
 Output range and pass-through mode - 0.5V to 3.4V
 Output current - 800mA
 Shutdown current - < 1μA
 LDO PA bias supply - 2.85V, 10mA
 Internal clock - 1MHz
 Continuous short circuit protection on VOUT
 Duty cycle mode - 100%
 Internal PMOS bypass transistor
 Gate drive available for external bypass transistor
 Over 90% efï¬ciency
 Low and high power modes for optimum dual-mode
PA efï¬ciency
 Switching time (lowest to highest output) < 40μs
 Micro-lead frame package MLPD-10, 3mm x 3mm
Applications
 3G mobile phones - RF PA power supply
 WCDMA power ampliï¬er modules
 Wireless modems
Typical Application Circuit
Patent Pending
VIN
2.7 to 5V
CIN
10μF
ENABLE
VMODE
VDAC
Optional
External Pass-through
MOSFET
1
VIN
SC251
4
EN
5 VMODE
6 VDAC
9 PGND
GD 3
LX 10
VOUT 8
2
VREF
GND 7
L1
4.7μH
CREF
1μF
VOUT
0.5V to VIN
COUT
4.7μF
RF Input
Vcc
PA
BIAS
GND
RF Output
May 8, 2006
1
www.semtech.com
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