LM2833ZMY/NOPB National Semiconductor, LM2833ZMY/NOPB Datasheet - Page 10

• Manufacturer Part Number: LM2833ZMY/NOPB
• Description: IC REG BUCK 3.0MHZ 3A 10MSOP
• Manufacturer: National Semiconductor
• Type: Step-Down (Buck)r

Specifications of LM2833ZMY/NOPB

• Internal Switch(s): Yes
• Synchronous Rectifier: No
• Number Of Outputs: 1
• Voltage - Output: 0.6 ~ 4.5 V
• Current - Output: 3A
• Frequency - Switching: 3MHz
• Voltage - Input: 3 ~ 5.5 V
• Operating Temperature: -40°C ~ 125°C
• Mounting Type: Surface Mount
• Package / Case: 10-MSOP Exposed Pad, 10-HMSOP, 10-eMSOP
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Power - Output: -
• Other names: LM2833ZMYTR

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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 con-
verter 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 Figure 2
and simplified block diagram in Figure 1. The LM2833 sup-
plies a regulated output voltage by switching the internal
PMOS power switch at a constant frequency and variable du-
ty 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
rent (I
current sense amplifier, which generates an output propor-
tional 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 be-
tween the feedback voltage and V
parator 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
adjusts the duty cycle (D) to maintain a constant output volt-
age.
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
SW
FIGURE 2. SW Pin Voltage and Inductor Current
) swings up to approximately V
L
) increases with a linear slope. I
D
) of the catch diode. The regulator loop
Waveforms
REF
IN
. When the PWM com-
, and the inductor cur-
L
is measured by the
30013266
10
required to initiate switching. Do not allow this pin to float or
rise to 0.3V above V
pin voltage rises above 1.8V while the input voltage is greater
than UVLO, there is 15µs delay before switching starts. Dur-
ing 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.
The simplest way to enable the operation of the LM2833 is to
connect the EN pin to V
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 rapid-
ly. When the feedback pin voltage exceeds 0.6V, the duty
cycle will have to reduce from the maximum value according-
ly, to maintain regulation. It takes a finite amount of time for
this reduction of duty cycle and this can result in a transient
in output voltage for a short duration, as shown in Figure 3. In
applications where this output voltage overshoot is undesir-
able, one simple solution is to add a feed-forward capacitor
(C
plifier recovery. In practice, a 27nF to 100nF ceramic capac-
itor is usually a good choice to remove the overshoot
completely or limit the overshoot to an insignificant level dur-
ing startup, as shown in Figure 4. Another more effective
solution is to control EN pin voltage by a separate logic signal,
and pull the signal high only after V
this way, the chip can execute a normal, complete soft start
process, minimizing any output voltage overshoot. Under
some circumstances at cold temperature, this approach may
also be required to minimize any unwanted output voltage
transients that may occur when the input voltage rises slowly.
For a fast rising input voltage (100µs for example), there is no
need to control EN separately or add a feed-forward capacitor
since the soft-start can bring up output voltage smoothly as
shown in Figure 5.
During startup, the LM2833 gradually increases the switching
frequency from 400kHz (LM2833X) or 800kHz (LM2833Z) to
the nominal fixed value, as the feedback voltage increases
(see Frequency Foldback section for more information).
Since the internal corrective ramp signal adjusts its slope dy-
namically, and is proportional to the switching frequency dur-
ing startup, a larger output capacitance may be required to
insure a smooth output voltage rise, at low programmed out-
put voltage and high output load current.
FF
) across the top feedback resistor R1 to speed Gm Am-
IN
. It should be noted that when the EN
IN
which allows self start-up of the
IN
is fully established. In