Texas Instruments LM2833Z LLP - 3MHz 3.0A Step-Down DC-DC Switching Regulator Evaluation Module LM2833ZSDEVAL LM2833ZSDEVAL Datenbogen
Produktcode
LM2833ZSDEVAL
0
0
V
IN
-V
D
T
ON
t
t
Inductor
Current
D = T
ON
/T
SW
V
SW
T
OFF
T
SW
I
L
SW
Voltage
'
i
L
I
OUT
I
LPK
SNVS505E – MAY 2008 – REVISED APRIL 2013
APPLICATION INFORMATION
THEORY OF OPERATION
The LM2833 is a constant frequency PWM buck regulator IC that delivers a 3.0A load current. The regulator is
available in preset switching frequencies of 1.5MHz or 3.0MHz. This high frequency allows the LM2833 to
operate with small surface mount capacitors and inductors, resulting in a DC/DC converter that requires a
minimum amount of board space. The LM2833 is internally compensated, therefore it is simple to use and
requires few external components. The LM2833 uses peak current-mode control to regulate the output voltage.
The following description of operation of the LM2833 will refer to the Typical Application Circuit, to the waveforms
in
available in preset switching frequencies of 1.5MHz or 3.0MHz. This high frequency allows the LM2833 to
operate with small surface mount capacitors and inductors, resulting in a DC/DC converter that requires a
minimum amount of board space. The LM2833 is internally compensated, therefore it is simple to use and
requires few external components. The LM2833 uses peak current-mode control to regulate the output voltage.
The following description of operation of the LM2833 will refer to the Typical Application Circuit, to the waveforms
in
and simplified block diagram in
. The LM2833 supplies a regulated output voltage by
switching the internal PMOS power switch at a constant frequency and variable duty cycle. A switching cycle
begins at the falling edge of the reset pulse generated by the internal oscillator. When this pulse goes low, the
output control logic turns on the internal PMOS power switch. During this on-time, the SW pin voltage (V
begins at the falling edge of the reset pulse generated by the internal oscillator. When this pulse goes low, the
output control logic turns on the internal PMOS power switch. During this on-time, the SW pin voltage (V
SW
)
swings up to approximately V
IN
, and the inductor current (I
L
) increases with a linear slope. I
L
is measured by the
current sense amplifier, which generates an output proportional to the switch current. The sense signal is
summed with the regulator’s corrective ramp and compared to the error amplifier’s output, which is proportional
to the difference between the feedback voltage and V
summed with the regulator’s corrective ramp and compared to the error amplifier’s output, which is proportional
to the difference between the feedback voltage and V
REF
. When the PWM comparator output goes high, the
internal power switch turns off until the next switching cycle begins. During the switch off-time, the inductor
current discharges through the catch diode D1, which forces the SW pin to swing below ground by the forward
voltage (V
current discharges through the catch diode D1, which forces the SW pin to swing below ground by the forward
voltage (V
D
) of the catch diode. The regulator loop adjusts the duty cycle (D) to maintain a constant output
voltage.
Figure 26. SW Pin Voltage and Inductor Current Waveforms
SOFT-START/SHUTDOWN
The LM2833 has both enable and shutdown modes that are controlled by the EN pin. Connecting a voltage
source greater than 1.8V to the EN pin enables the operation of the LM2833, while reducing this voltage below
0.4V places the part in a low quiescent current (300nA typical) shutdown mode. There is no internal pull-up on
EN pin, therefore an external signal is required to initiate switching. Do not allow this pin to float or rise to 0.3V
above V
source greater than 1.8V to the EN pin enables the operation of the LM2833, while reducing this voltage below
0.4V places the part in a low quiescent current (300nA typical) shutdown mode. There is no internal pull-up on
EN pin, therefore an external signal is required to initiate switching. Do not allow this pin to float or rise to 0.3V
above V
IN
. It should be noted that when the EN pin voltage rises above 1.8V while the input voltage is greater
than UVLO, there is 15µs delay before switching starts. During this delay the LM2833 will go through a power on
reset state after which the internal soft-start process commences. During soft-start, the error amplifier’s reference
voltage ramps from 0V to its nominal value of 0.6V in approximately 600µs. This forces the regulator output to
ramp up in a controlled fashion, which helps reduce inrush current seen at the input and minimizes output
voltage overshoot.
reset state after which the internal soft-start process commences. During soft-start, the error amplifier’s reference
voltage ramps from 0V to its nominal value of 0.6V in approximately 600µs. This forces the regulator output to
ramp up in a controlled fashion, which helps reduce inrush current seen at the input and minimizes output
voltage overshoot.
The simplest way to enable the operation of the LM2833 is to connect the EN pin to V
IN
which allows self start-up
of the LM2833 whenever the input voltage is applied. However, when an input voltage of slow rise time is used to
power the application and if both the input voltage and the output voltage are not fully established before the soft-
start time elapses, the control circuit will command maximum duty cycle operation of the internal power switch to
bring up the output voltage rapidly. When the feedback pin voltage exceeds 0.6V, the duty cycle will have to
power the application and if both the input voltage and the output voltage are not fully established before the soft-
start time elapses, the control circuit will command maximum duty cycle operation of the internal power switch to
bring up the output voltage rapidly. When the feedback pin voltage exceeds 0.6V, the duty cycle will have to
10
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