LM3203TLX National Semiconductor Corporation, LM3203TLX Datasheet - Page 12

• Manufacturer Part Number: LM3203TLX
• Description: Step-down Dc-dc Converter With Bypass Mode For Cdma / Wcdma Rf Power Amplifiers
• Manufacturer: National Semiconductor Corporation

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Shutdown Mode
Setting the EN digital pin low (
a 0.1µA (typ.) Shutdown mode. During shutdown, the PFET
switch, NFET synchronous rectifier, reference voltage
source, control and bias circuitry of the LM3203 are turned
off. Setting EN high (
EN should be set low to turn off the LM3203 during power-up
and under voltage conditions when the power supply is less
than the 2.7V minimum operating voltage. The LM3203 is
designed 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 benefit of including UVLO (Under
Voltage Lock-Out) circuitry.
Dynamically Adjustable Output
Voltage
The LM3203 features dynamically adjustable output voltage.
The output can be set by changing the voltage on the analog
V
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, promot-
ing longer battery life. See Setting the Output Voltage in the
Application Information section for further details.
Over Voltage Protection
The LM3203 has an over voltage comparator that prevents
the output voltage from rising too high, when the device is
left in PWM mode under light-load conditions, during output
voltage steps, or during startup. When the FB pin voltage
rises by 100mV over the V
inhibits PWM operation to skip pulses until the feedback
voltage returns to the V
protection mode, both the PWM PFET and the NFET syn-
chronous rectifier are off. When the part comes out of the
over-voltage protection mode, the NFET synchronous recti-
fier remains off for approximately 3.5µs to avoid inductor
current going negative.
Internal Synchronous Rectification
While in PWM mode, the LM3203 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.
With medium and heavy loads, the internal NFET synchro-
nous 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. There is no zero
cross detect, which means that the NFET can conduct cur-
rent in both directions and inductor current is always con-
tinuous. The advantage of this method is that the part re-
mains in PWM mode at light loads or no load conditions. The
NFET has a current limit. 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
A current limit feature allows the LM3203 to protect itself and
external components during overload conditions. In PWM
CON
pin. This feature is useful in PA applications where
>
1.2V) enables normal operation.
CON
CON
voltage. During the over voltage
<
voltage, the OVP comparator
0.4V) places the LM3203 in
12
mode, a 940mA (max.) cycle-by-cycle current limit is nor-
mally used. If an excessive load pulls the output voltage
down to below approximately 0.375V, indicating a possible
short to ground, then the device switches to a timed current
limit mode. In timed current limit mode, the internal PFET
switch is turned off after the current comparator trips, and the
beginning of the next cycle is inhibited for 3.5µs to force the
instantaneous inductor current to ramp down to a safe value.
After the 3.5µs interval, the internal PFET is turned on again.
This cycle is repeated until the load is reduced and the
output voltage exceeds approximately 0.375V. Therefore,
the device may not startup if an excessive load is connected
to the output when the device is enabled. The synchronous
rectifier is off in the timed current limit mode. Timed current
limit prevents the loss of current control seen in some prod-
ucts when the output voltage is pulled low in serious over-
load conditions. This timed current limit function is disabled if
the BYPOUT pin is connected to V
A current limit is also provided for the NFET. This is approxi-
mately −500mA. Both the NFET and the PFET are turned off
in negative current limit until the PFET is turned on again at
the beginning of the next cycle. The negative current limit
inhibits buildup of excessive negative inductor current.
In the Bypass mode, the bypass current limit is 1000mA
(typ.). The output voltage drops when the bypass current
limit kicks in.
Thermal Overload Protection
The LM3203 has a thermal overload protection function to
protect the device 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 in PWM mode, and the Bypass PFET is
turned off in Bypass mode. When the temperature drops
below 130˚C, normal operation resumes. Prolonged opera-
tion in thermal overload conditions may damage the device.
Application Information
SETTING THE OUTPUT VOLTAGE
The LM3203 features a pin-controlled variable feedback volt-
age. It can be programmed for an output voltage from 0.8V
to 3.6V by setting the voltage on the V
formula below ( Refer Figure 1). In this scheme the output
voltage is directly proportional to V
When V
age will follow the above equation. For example, the V
proportionally 3 times of V
ers are used as in Figure 1 .
If V
because of insufficient slope compensation.
If the target voltage is less than 0.8V, the output voltage may
not be regulated due to the required on-time being less than
the minimum on-time (50ns). The output voltage can go
lower than 0.8V providing a limited V
to datasheet curve (Low V
details. This curve is for a typical part and there could be part
to part variation for output voltages less than 0.8V over the
limited V
approx. 0.15V, the LM3203 output is off, but the internal bias
circuits are still active.
OUT
CON
IN
is over 3.6V, sub-harmonic oscillation may occur
range. In addition, if the V
V
is between 0.267V and 1.20V, the output volt-
OUT
= (( R1 + R2) / R2 ) x V
CON
CON
Voltage vs Output Voltage) for
when external resistor divid-
CON
DD
CON
IN
.
.
range is used. Refer
voltage is less than
CON
CON
pin, as in the
OUT
is