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PDF LM2706 Data sheet ( Hoja de datos )
| Número de pieza | LM2706 | |
| Descripción | Step-Down DC-DC Converter | |
| Fabricantes | National Semiconductor | |
| Logotipo |  |
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PRELIMINARY
February 2004
LM2706
Miniature, Variable, Step-Down DC-DC Converter with
Bypass for RF Power Amplifiers
General Description
The LM2706 DC-DC converter is optimized for powering RF
power amplifiers (PAs) from a single Lithium-Ion cell. It may
also be used in many other applications. It steps down an
input voltage from 2.7V to 5.5V to a variable output from
1.5V to 3.25V up to 300 mA. Output voltage is set using a
www.DataSheet4U.cVoCmON analog input for controlling power levels and efficiency
of the RF PA. An internal bypass switch allows direct con-
nection to the battery for maximum power to the RF PA.
The device offers 4 modes for mobile phones and similar RF
PA applications. Fixed-frequency PWM mode minimizes RF
interference. Forced Bypass mode turns on an internal by-
pass switch to power the PA directly from the battery. Auto-
matic bypass mode minimizes dropout by turning on the
bypass switch when the battery decays to near the output
voltage. Shutdown mode turns the device off and reduces
battery consumption to 0.1 µA (typ.).
The device also offers internal synchronous rectification for
high efficiency (95% typ at 3.25 VOUT, 200 mA, 3.9 VIN).
Current limit and thermal overload protection protects the
device and system during fault conditions.
The LM2706 is available in a 10-pin lead free micro SMD
package. This packaging uses National’s chip-scale micro
SMD technology and offers the smallest possible size. A high
switching frequency (600 kHz) allows use of tiny surface-
mount components. Only three small external surface-mount
components, an inductor and two ceramic capacitors, are
required.
n ±2% DC output voltage precision
n Internal 105 mΩ (typ) bypass switch
n 300 mA maximum load capability
n 1.4 mA typ quiescent current
n 0.1 µA typ shutdown current
n 600 kHz PWM switching frequency
n High efficiency (95% typ at 3.9 VIN, 3.25 VOUT at
200 mA) from internal synchronous rectification
Features
n Forced and Automatic Bypass modes
n Miniature 10-pin lead free micro SMD package
n Only three tiny surface-mount external components
required
n Uses small ceramic capacitors
n Low output voltage ripple (<10 mV typ)
n Internal soft start
n Current overload protection
n No external compensation
Applications
n Mobile Phones
n Hand-Held Radios
n RF PC Cards
n Battery Powered RF Devices
Key Specifications
n Operates from a single LiION cell (2.7V to 5.5V)
n Variable output voltage (1.5V to 3.25V)
Typical Application Circuit
© 2004 National Semiconductor Corporation DS200409
20040901
www.national.com
1 page  
Typical Performance Characteristics (Circuit of Figure 1, VIN = EN = 3.6V, ABD = BYP = 0V, TA =
25˚C, unless otherwise noted.)
Quiescent Supply Current vs Temperature
Quiescent Supply Current vs Supply Voltage
www.DataSheet4U.com
20040984
Shutdown Supply Current vs Temperature
20040985
Switching Frequency vs Temperature
20040986
Output Voltage vs Temperature
(VOUT = 1.5V)
20040987
Output Voltage vs Temperature
(VOUT = 3.25V)
20040988
5
20040989
www.national.com
5 Page 
Bypass Operation (Continued)
Automatic Bypass modes, the output voltage will follow the
input voltage less the voltage drop across the resistance of
the bypass PFET. These modes are more efficient than
operating in PWM mode at 100% duty cycle because the
resistance of the bypass PFET is less than the series resis-
tance of the PWM PFET and inductor. This translates into
higher voltage available on the output in bypass mode, for a
given battery voltage.
The part can be placed in bypass operation by sending BYP
pin high for Forced Bypass mode. During Forced Bypass
mode, the bypass switch remains on irrespective of the state
of ABD pin. A second way to place the part in bypass
operation is by setting ABD low for Automatic Bypass mode.
In Automatic Bypass mode, the bypass switch turns on when
the difference between the input voltage and programmed
output voltage is less than 275 mV (typ) for more than the
www.DataShebeyt4pUa.scsomdelay time 10 µs (typ). The bypass switch turns off
when the input voltage is higher than the programmed output
voltage by 440 mV (typ) for longer than the bypass delay
time. The bypass delay time is provided to prevent false
triggering into Automatic Bypass Mode modes by either
spikes or dips in VIN.
Operating Mode Selection Controls
The LM2706 is designed for digital control of the operating
modes using BYP and ABD pins. The settings for these pins
are outlined in Table 1. Setting BYP high (>1.2V) places the
device in Forced Bypass mode. Setting BYP low and ABD
high forces operation in PWM mode. Setting both BYP and
ABD low (<0.5V) or leaving them floating places the device
in Automatic Bypass mode. The BYP and ABD pins have
5 µA (typ) pull down currents for default operation in Auto-
matic Bypass mode for automatic switchover between PWM
and Bypass operation in systems where digital mode control
is not required.
TABLE 1. Operating Modes
Mode
Forced Bypass
Auto-Bypass / PWM
PWM
Logic Level
ABD
BYP
0 or 1
1
00
0 or 1
0
Overvoltage Protection
The LM2706 has an over-voltage comparator that prevents
the output voltage from rising too high, when the device is
left in PWM mode under low-load conditions. When the
output voltage rises by 350 mV over its regulation threshold,
the OVP comparator inhibits PWM operation to skip pulses
until the output voltage returns to the regulation threshold. In
over voltage protection, output voltage and ripple increases.
At light loads when the required on time to regulate becomes
less than the minimum on time of the LM2706 (around
200ns) then the output voltage will increase till it hits the
overvoltage threshold and the part will be in overvoltage
protection mode.
Shutdown Mode
Setting the EN digital input pin low (<0.5V) places the
LM2706 in a 0.1 µA (typ) shutdown mode. During shutdown,
the PFET switch, NFET synchronous rectifier, reference,
control and bias circuitry of the LM2706 are turned off.
Setting EN high enables normal operation. While turning on,
soft start is activated. The device takes around 600 µs to
complete the soft-start interval and come into regulation
during turn-on.
EN should be set low to turn off the LM2706 during power-up
and under voltage conditions when the supply is less than
the 2.7V minimum operating voltage. The LM2706 is de-
signed for compact portable applications, such as mobile
phones. In such applications, the system controller deter-
mines power supply sequencing and requirements for small
package size outweigh the additional size required for inclu-
sion of UVLO (Under Voltage Lock-Out) circuitry.
Internal Synchronous Rectification
While in PWM mode, the LM2706 uses an internal NFET as
a synchronous rectifier to reduce rectifier forward voltage
drop and associated power loss. Synchronous rectification
provides a significant improvement in efficiency whenever
the output voltage is relatively low compared to the voltage
drop across an ordinary rectifier diode.
During moderate and heavy loads, the internal NFET syn-
chronous rectifier is turned on during the inductor current
down slope during the second part of each cycle. The syn-
chronous rectifier is turned off prior to the next cycle, or when
the inductor current ramps to zero at light loads. The NFET
is designed to conduct through it’s intrinsic body diode dur-
ing transient intervals before it turns on, eliminating the need
for an external diode.
Current Limiting
A current limit feature allows the LM2706 to protect itself and
external components during overload conditions. In PWM
mode a 750 mA max cycle-by-cycle current limit is normally
used.
In the Bypass mode the bypass FET peak current limit is
650 mA (typ). During overload conditions the LM2706 limits
output current to this value. If the output current reaches the
bypass FET peak current limit then the current folds back to
the fold back current value of around 450 mA. A reduction in
the output current below the reset value of around 300 mA
resumes normal bypass operation.
In cases where the bypass FET is turned on with a big
differential between the input and output voltages the bypass
FET may see a peak current exceeding its peak current limit
due to the charging of the output capacitor plus the load
current and will go to the foldback current value thus acting
as a constant current source. As the output capacitor gets
charged the bypass FET current will go below the reset limit
and the bypass switch will be fully on thus acting as a switch
again.
Dynamically Adjustable Output
Voltage
The LM2706 features dynamically adjustable output voltage
to eliminate the need for external feedback resistors. The
output can be set from 1.5V to 3.25V by changing the
voltage on the analog VCONTROL pin. This feature is useful in
PA applications where peak power is needed only when the
handset is far away from the base station or when data is
being transmitted. In other instances the transmitting power
can be reduced and hence the supply voltage to the PA can
11 www.national.com
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| Páginas | Total 15 Páginas | |
| PDF Descargar | [ Datasheet LM2706.PDF ] | |
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