LM2833XMYEVAL National Semiconductor, LM2833XMYEVAL Datasheet - Page 14

• Manufacturer Part Number: LM2833XMYEVAL
• Description: BOARD EVAL LM2833 1.5MHZ 10MSOP
• Manufacturer: National Semiconductor

Specifications of LM2833XMYEVAL

• Main Purpose: DC/DC, Step Down
• Outputs And Type: 1, Non-Isolated
• Voltage - Output: 1.2V
• Current - Output: 3A
• Voltage - Input: 3 ~ 5.5V
• Regulator Topology: Buck
• Frequency - Switching: 1.5MHz
• Board Type: Fully Populated
• Utilized Ic / Part: LM2833
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Power - Output: -

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(MLCC) is a good choice for most applications. In cases
where large capacitance is required, use surface mount ca-
pacitors such as Tantalum capacitors and place at least a
4.7µF ceramic capacitor close to the V
recommended to use X7R or X5R dielectrics. Consult capac-
itor manufacturer datasheet to see how rated capacitance
varies over operating conditions.
OUTPUT CAPACITOR
The output capacitor is selected based upon the desired out-
put ripple and transient response. The initial current of a load
transient is provided mainly by the output capacitor. The out-
put ripple of the converter is:
When using MLCCs, the ESR is typically so low that the ca-
pacitive ripple may dominate. When this occurs, the output
ripple will be approximately sinusoidal and 90° phase shifted
from the switching action. Given the availability and quality of
MLCCs and the expected output voltage of designs using the
LM2833, there is really no need to review any other capacitor
technologies. Another benefit of ceramic capacitors is their
ability to bypass high frequency noise. A certain amount of
switching edge noise will couple through parasitic capaci-
tances in the inductor to the output. A ceramic capacitor will
bypass this noise while a tantalum will not. Since the output
capacitor is one of the two external components that control
the stability of the regulator control loop, most applications will
require a minimum of 22µF output capacitance. In the case of
low output voltage, a larger output capacitance is required to
ensure sufficient phase margin. Capacitance can often, but
not always, be increased significantly with little detriment to
the regulator stability. Like the input capacitor, recommended
multilayer ceramic capacitors are X7R or X5R types. Again,
verify actual capacitance at the desired operating voltage and
temperature. Check the RMS current rating of the capacitor.
The maximum RMS current rating of the capacitor is:
One may select a 1206 size MLCC for output capacitor, since
its current rating is typically above 1A, more than enough for
the requirement.
CATCH DIODE
The catch diode conducts during the switch off-time. A Schot-
tky diode is recommended for its fast switching time and low
forward voltage drop. The catch diode should be chosen such
that its current rating is greater than:
The reverse breakdown rating of the diode must be at least
the maximum input voltage plus appropriate margin. To im-
prove efficiency, choose a Schottky diode with a low forward
voltage drop.
OUTPUT VOLTAGE
The output voltage is set using the following equation where
R2 is connected between the FB pin and GND, and R1 is
connected between V
is 2kΩ.
OUT
I
D
= I
and the FB pin. A good value for R2
OUT
x (1-D)
IN
pin. For MLCCs it is
14
EFFICIENCY ESTIMATION
The complete LM2833 DC/DC converter efficiency can be
calculated in the following manner:
Or
Calculations for determining the most significant power loss-
es are shown below. Other losses totaling less than 2% are
not discussed.
The main power loss (P
basic types of losses: switching loss and conduction loss. In
addition, there is loss associated with the power required for
the internal circuitry of IC. Conduction losses usually domi-
nate at higher output loads, whereas switching losses domi-
nate at lower output loads. The first step in determining the
losses is to calculate the duty cycle (D):
V
it is on, and is equal to:
V
be obtained from the diode manufactures Electrical Charac-
teristics section. If the DC voltage drop across the inductor
(V
The conduction losses in the catch diode are calculated as
follows:
Often this is the single most significant power loss in the cir-
cuit. Care should be taken to choose a Schottky diode with a
low forward voltage drop.
Another significant external power loss is the conduction loss
in the output inductor. The equation can be simplified to:
The LM2833 conduction loss is mainly associated with the
internal power switch:
SW
D
DCR
is the forward voltage drop across the catch diode. It can
is the voltage drop across the internal power switch when
) is accounted for, the equation becomes:
P
DIODE
V
P
SW
IND
V
= I
= V
LOSS
REF
= I
OUT
D
OUT
= 0.60V
) in the converter includes two
x I
x R
2
OUT
x R
DS(ON)
x (1-D)
DCR