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SC251EVB Datasheet

  • SC251EVB

  • Step-down DC-DC Converter with Bias LDO for WCDMA Amplifi er...

  • 19頁

  • SEMTECH   SEMTECH

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POWER MANAGEMENT
Description
The SC251 is a synchronous step-down converter designed
for use as an adaptive voltage supply for WCDMA RF
power ampli鏗乪rs (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 ef鏗乧iency 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, V
OUT
follows an alternate exponential
relationship.
The SC251 is capable of supplying output current up
to 800mA. Standby current is <1渭A when the device is
disabled. The internal clock runs at 1MHz so that small
surface mount inductors and capacitors can be used.
Step-down DC-DC Converter with
Bias LDO for WCDMA Ampli鏗乪rs
Features
V
OUT
exponentially proportional to V
DAC
for maximum
ef鏗乧iency (patent pending)
Output range and pass-through mode - 0.5V to 3.4V
Output current - 800mA
Shutdown current - < 1渭A
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% ef鏗乧iency
Low and high power modes for optimum dual-mode
PA ef鏗乧iency
Switching time (lowest to highest output) < 40渭s
Micro-lead frame package MLPD-10, 3mm x 3mm
SC251
Applications
3G mobile phones - RF PA power supply
WCDMA power ampli鏗乪r modules
Wireless modems
Typical Application Circuit
Patent Pending
Optional
External Pass-through
MOSFET
V
IN
2.7 to 5V
1
C
IN
10渭F
GD
VIN
LX
VOUT
EN
3
10
8
L1
4.7渭H
V
OUT
0.5V to V
IN
C
OUT
4.7渭F
RF Input
BIAS
C
REF
1渭F
Vcc
SC251
4
5
VMODE
6
VDAC
9
PGND
ENABLE
VMODE
VDAC
PA
GND
RF Output
VREF
GND
2
7
May 8, 2006
1
www.semtech.com

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