參數(shù)資料
型號(hào)： LM3202
廠商： National Semiconductor Corporation
元件分類(lèi)： DC/DC變換器
英文描述： 650mA Miniature, Adjustable, Step-Down DC-DC Converter for RF Power Amplifiers
中文描述： 六百五十零毫安微型，可調(diào)，降壓型DC - DC轉(zhuǎn)換器的射頻功率放大器
文件頁(yè)數(shù)： 11/15頁(yè)
文件大?。?/td> 1507K
代理商： LM3202
Operation Description
(Continued)
Circuit Operation
Referring to
Figure 1
and
Figure 2
, the LM3202 operates as
follows. During the first part of each switching cycle, the
control block in the LM3202 turns on the internal PFET
(P-channel MOSFET) switch. This allows current to flow
from the input through the inductor to the output filter capaci-
tor and load. The inductor limits the current to a ramp with a
slope of around (V
IN
- V
OUT
) / L, by storing energy in a
magnetic field. During the second part of each cycle, the
controller turns the PFET switch off, blocking current flow
from the input, and then turns the NFET (N-channel MOS-
FET) synchronous rectifier on. In response, the inductor’s
magnetic field collapses, generating a voltage that forces
current from ground through the synchronous rectifier to the
output filter capacitor and load. As the stored energy is
transferred back into the circuit and depleted, the inductor
current ramps down with a slope around V
/ L. The
output filter capacitor stores charge when the inductor cur-
rent is high, and releases it when low, smoothing the voltage
across the load.
The output voltage is regulated by modulating the PFET
switch on time to control the average current sent to the load.
The effect is identical to sending a duty-cycle modulated
rectangular wave formed by the switch and synchronous
rectifier at SW to a low-pass filter formed by the inductor and
output filter capacitor. The output voltage is equal to the
average voltage at the SW pin.
While in operation, the output voltage is regulated by switch-
ing at a constant frequency and then modulating the energy
per cycle to control power to the load. Energy per cycle is set
by modulating the PFET switch on-time pulse width to con-
trol the peak inductor current. This is done by comparing the
signal from the current-sense amplifier with a slope compen-
sated error signal from the voltage-feedback error amplifier.
At the beginning of each cycle, the clock turns on the PFET
switch, causing the inductor current to ramp up. When the
current sense signal ramps past the error amplifier signal,
the PWM comparator turns off the PFET switch and turns on
the NFET synchronous rectifier, ending the first part of the
cycle. If an increase in load pulls the output down, the error
amplifier output increases, which allows the inductor current
to ramp higher before the comparator turns off the PFET.
This increases the average current sent to the output and
adjusts for the increase in the load.
Before appearing at the PWM comparator, a slope compen-
sation ramp from the oscillator is subtracted from the error
signal for stability of the current feedback loop. The minimum
on time of PFET in PWM mode is 50ns (typ.)
Shutdown Mode
Setting the EN digital pin low (
<
0.5V) places the LM3202 in
a 0.01μA(typ.) Shutdown mode. During shutdown, the PFET
switch, NFET synchronous rectifier, reference voltage
source, control and bias circuitry of the LM3202 are turned
off. Setting EN high (
>
1.2V) enables normal operation.
EN should be set low to turn off the LM3202 during power-up
and under voltage conditions when the power supply is less
than the 2.7V minimum operating voltage. The LM3202 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 additional size required for inclu-
sion of UVLO (Under Voltage Lock-Out) circuitry.
Internal Synchronous Rectification
The LM3202 uses an internal NFET as a synchronous rec-
tifier 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 and
ordinary rectifier diode.
The internal 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, elimi-
nating the need for an external diode.
Current Limiting
A current limit feature allows the LM3202 to protect itself and
external components during overload conditions. In PWM
mode, an 1200mA (max.) cycle-by-cycle current limit is nor-
mally used. If an excessive load pulls the output voltage
20141536
FIGURE 3. Typical Operating System Circuit
L
www.national.com
11
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參數(shù)描述
LM3202_06 制造商:NSC 制造商全稱(chēng):National Semiconductor 功能描述:650mA Miniature, Adjustable, Step-Down DC-DC Converter for RF Power Amplifiers
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LM3202TLX 制造商:NSC 制造商全稱(chēng):National Semiconductor 功能描述:650mA Miniature, Adjustable, Step-Down DC-DC Converter for RF Power Amplifiers
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