Texas Instruments LM3242 6MHz, 750mA Miniature, Adjustable, Step-Down DC-DC EVM LM3242TM-EV/NOPB LM3242TM-EV/NOPB Datenbogen
Produktcode
LM3242TM-EV/NOPB
SNOSB48D – OCTOBER 2011 – REVISED MARCH 2013
Table 1. Description of Modes
Mode
EN
BPEN
VCON
I
OUT
Shutdown
0
X
X
X
Sleep
1
0
< 80 mV
X
Pulse Width Modulation (PWM)
1
0
> 100 mA
> 130 mV, < (V
IN
−
0.2V)/2.5
Economy (ECO)
1
0
< 50 mA
Bypass (BP)
1
0
> (V
IN
−
0.2V)/2.5
X
1
1
X
X
Internal Synchronous Rectification
While in PWM mode, the LM3242 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.
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.
With medium and heavy loads, the NFET synchronous rectifier is turned on during the inductor current down
slope in the second part of each cycle. The synchronous rectifier is turned off prior to the next cycle. The NFET
is designed to conduct through its intrinsic body diode during transient intervals before it turns on, eliminating the
need for an external diode.
slope in the second part of each cycle. The synchronous rectifier is turned off prior to the next cycle. The NFET
is designed to conduct through its intrinsic body diode during transient intervals before it turns on, eliminating the
need for an external diode.
Current Limiting
The current limit feature allows the LM3242 to protect itself and external components during overload conditions.
In PWM mode, the cycle-by-cycle current limit is a 1450 mA (typ.). If an excessive load pulls the output voltage
down to less than 0.3V (typ.), the NFET synchronous rectifier is disabled and the current limit is reduced to 530
mA (typ.). Moreover, when the output voltage becomes less than 0.15V (typ.), the switching frequency will
decrease to 3MHz, thereby preventing excess current and thermal stress.
In PWM mode, the cycle-by-cycle current limit is a 1450 mA (typ.). If an excessive load pulls the output voltage
down to less than 0.3V (typ.), the NFET synchronous rectifier is disabled and the current limit is reduced to 530
mA (typ.). Moreover, when the output voltage becomes less than 0.15V (typ.), the switching frequency will
decrease to 3MHz, thereby preventing excess current and thermal stress.
Dynamically Adjustable Output Voltage
The LM3242 features dynamically adjustable output voltage to eliminate the need for external feedback resistors.
The output can be set from 0.4V to 3.6V by changing the voltage on the analog VCON 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. Hence the supply voltage to
the PA can be reduced, promoting longer battery life. See
The output can be set from 0.4V to 3.6V by changing the voltage on the analog VCON 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. Hence the supply voltage to
the PA can be reduced, promoting longer battery life. See
in the
section for further details. The LM3242 moves into Pulse Skipping mode when duty cycle is over
approximately 92% or less than approximately 15% and the output voltage ripple increases slightly.
Thermal Overload Protection
The LM3242 has a thermal overload protection function that operates to protect itself from short-term misuse and
overload conditions. When the junction temperature exceeds around 150°C, the device inhibits operation. Both
the PFET and the NFET are turned off. When the temperature drops below 125°C, normal operation resumes.
Prolonged operation in thermal overload conditions may damage the device and is considered bad practice.
overload conditions. When the junction temperature exceeds around 150°C, the device inhibits operation. Both
the PFET and the NFET are turned off. When the temperature drops below 125°C, normal operation resumes.
Prolonged operation in thermal overload conditions may damage the device and is considered bad practice.
Soft Start
The LM3242 has a soft-start circuit that limits in-rush current during startup. During startup the switch current limit
is increased in steps. Soft start is activated if EN goes from low to high after VIN reaches 2.7V.
is increased in steps. Soft start is activated if EN goes from low to high after VIN reaches 2.7V.
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