lm2706 National Semiconductor Corporation, lm2706 Datasheet - Page 11

• Manufacturer Part Number: lm2706
• Description: Miniature, Variable, Step-down Dc-dc Converter With Bypass For Rf Power Amplifiers
• Manufacturer: National Semiconductor Corporation

Bypass Operation
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
bypass delay 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 V
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 (
device in Forced Bypass mode. Setting BYP low and ABD
high forces operation in PWM mode. Setting both BYP and
ABD low (
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.
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 (
LM2706 in a 0.1 µA (typ) shutdown mode. During shutdown,
Forced Bypass
Auto-Bypass / PWM
PWM
Mode
<
0.5V) or leaving them floating places the device
TABLE 1. Operating Modes
IN
.
0 or 1
0 or 1
ABD
0
(Continued)
Logic Level
<
>
0.5V) places the
1.2V) places the
BYP
1
0
0
11
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 V
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
CONTROL
pin. This feature is useful in
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