LM2833ZMY/NOPB National Semiconductor, LM2833ZMY/NOPB Datasheet - Page 13

• 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

THERMAL SHUTDOWN
Thermal shutdown limits total power dissipation by turning off
the internal power switch when the IC junction temperature
typically exceeds 165°C. After thermal shutdown occurs, the
power switch does not turn on again until the junction tem-
perature drops below approximately 150°C.
Design Guide
INDUCTOR SELECTION
The Duty Cycle (D) can be approximated quickly using the
ratio of output voltage (V
The catch diode (D1) forward voltage drop and the voltage
drop across the internal PMOS must be included to calculate
a more accurate duty cycle. Calculate D by using the following
formula:
V
Where I
(V
the diode. The lower the V
cy of the converter.
The inductor value determines the output ripple current (Δi
as defined in Figure 2). Lower inductor values decrease the
size of the inductor, but increase the output ripple current. An
increase in the inductor value will decrease the output ripple
current. In general, the ratio of ripple current to the output
current is optimized when it is set between 0.2 and 0.4 for
output currents above 2A. This ratio r is defined as:
One must ensure that the minimum current limit (3.4A) is not
exceeded, so the peak current in the inductor must be calcu-
lated. The peak current (I
When the designed maximum output current is reduced, the
ratio r can be increased. At a current of 0.1A, r can be made
as high as 0.9. The ripple ratio can be increased at lighter
loads because the net ripple is actually quite low, and if r re-
mains constant the inductor value can be made quite large.
An equation empirically developed for the maximum ripple
ratio at any current below 2A is:
Note that this is just a guideline, and it needs to be combined
with two important factors for proper selection of inductance
values at any operating condition. The first consideration is at
output voltage above 2.5V, one needs to ensure that the in-
ductance given by the above guideline should not be less than
1µH for the LM2833X or 0.5µH for the LM2833Z. Since the
LM2833 has a fixed internal corrective ramp signal, a very low
inductance value at high output voltage will generate a very
SW
D
) can range from 0.3V to 0.7V depending on the quality of
can be approximated by:
OUT
is output load current. The diode forward drop
r = 0.387 x I
V
SW
I
LPK
= I
OUT
LPK
= I
D
, the higher the operating efficien-
OUT
) to input voltage (V
) in the inductor is calculated by:
OUT
x R
OUT
+ Δi
DS(ON)
-0.3667
L
/2
IN
):
L
,
13
steep down slope of inductor current, which will result in an
insufficient slope compensation, and cause instability known
as sub-harmonic oscillation. Another consideration is at low
load current, one needs to ensure that the inductance value
given by the guideline should not exceed 10µH for the
LM2833X and 4.7µH for the LM2833Z, since too much induc-
tance effectively flattens the down slope of the inductor cur-
rent, and may significantly limit the system bandwidth and
phase margin resulting in instability.
The LM2833 operates at frequencies allowing the use of ce-
ramic output capacitors without compromising transient re-
sponse. Ceramic capacitors allow higher inductor ripple
without significantly increasing output ripple. See the output
capacitor section for more details on calculating output volt-
age ripple.
Now that the ripple current is determined, the inductance is
calculated by:
where f
ductor, make sure that it is capable of supporting the peak
output current without saturating. Inductor saturation will re-
sult in a sudden reduction in inductance and prevent the
regulator from operating properly. Because of the operating
frequency of the LM2833, ferrite based inductors are pre-
ferred to minimize core losses. This presents little restriction
since the variety and availability of ferrite-based inductors is
large. Lastly, inductors with lower series resistance (DCR) will
provide better operating efficiency. For recommended induc-
tor selection, refer to Design Examples.
INPUT CAPACITOR
An input capacitor is necessary to ensure that V
drop excessively during switching transients. The primary
specifications of the input capacitor are capacitance, voltage
rating, RMS current rating, and ESL (Equivalent Series In-
ductance). The input voltage rating is specifically stated by
the capacitor manufacturer. Make sure to check any recom-
mended deratings and also verify if there is any significant
change in capacitance at the operating input voltage and the
operating temperature. The input capacitor maximum RMS
input current rating (I
Neglecting inductor ripple simplifies the above equation to:
It can be shown from the above equation that maximum RMS
capacitor current occurs when D = 0.5. Always calculate the
RMS at the point where the duty cycle D is closest to 0.5. The
ESL of an input capacitor is usually determined by the effec-
tive cross sectional area of the current path. As a rule of
thumb, a large leaded capacitor will have high ESL and a 1206
ceramic chip capacitor will have very low ESL. At the operat-
ing frequencies of the LM2833, leaded capacitors may have
an ESL so large that the resulting impedance (2
higher than that required to provide stable operation. It is
strongly recommended to use ceramic capacitors due to their
low ESR and low ESL. A 22µF multilayer ceramic capacitor
SW
is the switching frequency. When selecting an in-
RMS-IN
) must be greater than:
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π
IN
fL) will be
does not